Antenna topics
Antenna topics
http://www.radio-electronics.com/info/antennas/index.php
Antennas (or aerials) are an essential element of any radio
link, whether it is for a high power transmitter like those used
for broadcasting, or low power ones like those used wireless
technologies such as WLAN or remote control and sensing
applications. Apart from the power levels, antennas are used across
the whole radio spectrum, from ELF right up to the microwave bands.
Whatever the power, and the frequency, the basic theory remains the
same, although the practical approach has to change.
Antenna Basics
Electromagnetic waves and basic antenna operation
Polarization
Antenna feed impedance - including radiation resistance, loss
resistance and efficiency
Resonance and bandwidth
Directivity and gainFeeders
The ideal position for an antenna is rarely in the optimum
position for the transmitting or receiving equipment. As a result a
form of transmission line or feeder is required to transfer the
signals and power to and from the antenna.
Coaxial feeder
Balanced feederWaveguide
Waveguide data
The dipole antenna
The dipole antenna is one of the most basic forms of antenna
available. It is widely used on its own, and it is also used as the
"driven" element in many other types of antenna.
The dipole antenna
Folded dipole
The vertical antenna
Vertical antennas are widely used in many areas, and are
particularly widely used for mobile applications because they
radiate all around them in the horizontal plane. This means that
they do not need redirecting as the mobile station moves. The
quarter wave vertical is the simplest, but there are many other
designs that provide improved performance and gain.
Quarter wave vertical
Five eighths wavelength vertical
J pole vertical antenna Directional antennas
There is a good variety of different types of directive antenna
that can be used. Although the yagi antenna is the most popular, it
is by no means the only one, and other designs and approaches are
more applicable in many instances.
Yagi
Log periodic beam antenna
Parabolic reflector
Horn antenna
Wideband antennas
Most antennas are only able to operate over a narrow bandwidth.
There are some techniques that enable the bandwidth of an antenna
to be increased considerably, and also some designs that are able
to operate over very wide bandwidths.
Discone
Log periodic beam antennaLoop antennas
Where directivity and small size are required, loop antennas may
often provide an answer. Although different types of loop have
slightly different properties, they are able to provide a good
antenna solution in many circumstances.
Loop antenna overview
Ferrite rod antennaApplications
Antennas can be used in many applications from reception of
terrestrial and satellite television to point to point radio, short
wave radio and much more.
Satellite antennas for satellite television and other satellite
applications
Electromagnetic waves and antenna basics
Radio signals are a form of electromagnetic wave. They are the
same type of radiation as light, ultra-violet and infra red rays,
differing from them in their wavelength and frequency.
Electromagnetic waves have both electric and magnetic components
that are inseparable. The planes of these fields are at right
angles to one another and to the direction of motion of the
wave.
An electromagnetic wave
The electric field results from the voltage changes occurring in
the antenna which is radiating the signal, and the magnetic changes
result from the current flow. It is also found that the lines of
force in the electric field run along the same axis as the antenna,
but spreading out as they move away from it. This electric field is
measured in terms of the change of potential over a given distance,
e.g. volts per meter, and this is known as the field strength.
Similarly when an antenna receives a signal the magnetic changes
cause a current flow, and the electric field changes cause the
voltage changes on the antenna.
There are a number of properties of a wave. The first is its
wavelength. This is the distance between a point on one wave to the
identical point on the next. One of the most obvious points to
choose is the peak as this can be easily identified although any
point is acceptable.
Wavelength of an electromagnetic wave
The wavelength of an electromagnetic wave
The second property of the electromagnetic wave is its
frequency. This is the number of times a particular point on the
wave moves up and down in a given time (normally a second). The
unit of frequency is the Hertz and it is equal to one cycle per
second. This unit is named after the German scientist who
discovered radio waves. The frequencies used in radio are usually
very high. Accordingly the prefixes kilo, Mega, and Giga are often
seen. 1 kHz is 1000 Hz, 1 MHz is a million Hertz, and 1 GHz is a
thousand million Hertz i.e. 1000 MHz. Originally the unit of
frequency was not given a name and cycles per second (c/s) were
used. Some older books may show these units together with their
prefixes: kc/s; Mc/s etc. for higher frequencies.
The third major property of the wave is its velocity. Radio
waves travel at the same speed as light. For most practical
purposes the speed is taken to be 300 000 000 meters per second
although a more exact value is 299 792 500 meters per second.
Frequency to Wavelength Conversion
Although wavelength was used as a measure for signals,
frequencies are used exclusively today. It is very easy to relate
the frequency and wavelength as they are linked by the speed of
light as shown:
Lambda = c / f
Where lambda = the wavelength in meters
f = frequency in Hertz
c = speed of radio waves (light) taken as 300 000 000 meters per
second for all practical purposes.
Field measurements
It is also interesting to note that close to the antenna there
is also an inductive field the same as that in a transformer. This
is not part of the electromagnetic wave, but it can distort
measurements close to the antenna. It can also mean that
transmitting antennas are more likely to cause interference when
they are close to other antennas or wiring that might have the
signal induced into it. For receiving antennas they are more
susceptible to interference if they are close to house wiring and
the like. Fortunately this inductive field falls away fairly
rapidly and it is barely detectable at distances beyond about two
or three wavelengths from the antenna.
Antenna polarization
Polarization is an important factor for antennas. Both antennas
and electromagnetic waves are said to have a polarization. For the
electromagnetic wave it is effectively the plane in which the
electric vibrates. This is important when looking at antennas
because they are sensitive to polarization, and generally only
receive or transmit a signal with a particular polarization. For
most antennas it is very easy to determine the polarization. It is
simply in the same plane as the elements of the antenna. So a
vertical antenna (i.e. one with vertical elements) will receive
vertically polarized signals best and similarly a horizontal
antenna will receive horizontally polarized signals.
An electromagnetic wave
It is important to match the polarization of the antenna to that
of the incoming signal. In this way the maximum signal is obtained.
If the antenna polarization does not match that of the signal there
is a corresponding decrease in the level of the signal. It is
reduced by a factor of cosine of the angle between the polarization
of the antenna and the signal.
Accordingly the polarization of the antennas located in free
space is very important, and obviously they should be in exactly
the same plane to provide the optimum signal. If they were at right
angles to one another (i.e. cross-polarized) then in theory no
signal would be received.
For terrestrial applications it is found that once a signal has
been transmitted then its polarization will remain broadly the
same. However reflections from objects in the path can change the
polarization. As the received signal is the sum of the direct
signal plus a number of reflected signals the overall polarization
of the signal can change slightly although it remains broadly the
same.
Polarization categories
Vertical and horizontal are the simplest forms of polarization
and they both fall into a category known as linear polarization.
However it is also possible to use circular polarization. This has
a number of benefits for areas such as satellite applications where
it helps overcome the effects of propagation anomalies, ground
reflections and the effects of the spin that occur on many
satellites. Circular polarization is a little more difficult to
visualize than linear polarization. However it can be imagined by
visualizing a signal propagating from an antenna that is rotating.
The tip of the electric field vector will then be seen to trace out
a helix or corkscrew as it travels away from the antenna. Circular
polarization can be seen to be either right or left handed
dependent upon the direction of rotation as seen from the
transmitter.
Another form of polarization is known as elliptical
polarization. It occurs when there is a mix of linear and circular
polarization. This can be visualized as before by the tip of the
electric field vector tracing out an elliptically shaped
corkscrew.
However it is possible for linearly polarized antennas to
receive circularly polarized signals and vice versa. The strength
will be equal whether the linearly polarized antenna is mounted
vertically, horizontally or in any other plane but directed towards
the arriving signal. There will be some degradation because the
signal level will be 3 dB less than if a circularly polarized
antenna of the same sense was used. The same situation exists when
a circularly polarized antenna receives a linearly polarized
signal.
Applications
Different types of polarization are used in different
applications to enable their advantages to be used. Linear
polarization is by far the most widely used. Vertical polarization
is often used for mobile or point to point applications. This is
because many vertical antennas have an omni-directional radiation
pattern and it means that the antennas do not have to be
re-orientated as positions are changed if for example a moving
vehicle. For other applications the polarization is often
determined by antenna considerations. Some large multi-element
antenna arrays can be mounted in a horizontal plane more easily
than in the vertical plane. This is because the antenna elements
are at right angles to the vertical tower of pole on which they are
mounted and therefore by using an antenna with horizontal elements
there is less physical and electrical interference between the two.
This determines the standard polarization in many cases.
In some applications there are performance differences between
horizontal and vertical polarization. For example medium wave
broadcast stations generally use vertical polarization because
ground wave propagation over the earth is considerably better using
vertical polarization, whereas horizontal polarization shows a
marginal improvement for long distance communications using the
ionosphere. Circular polarization is sometimes used for satellite
communications as there are some advantages in terms of propagation
and in overcoming the fading caused if the satellite is changing
its orientation.
Antenna feed impedance
- including radiation resistance, loss resistance and
efficiency
When a signal source is applied to an antenna at its feed point,
it is found that it presents a load impedance to the source. This
is a complex impedance being made up from resistance, capacitance
and inductance. In order to ensure the optimum efficiency of the
transfer it is necessary to match the antenna to the load, and this
requires some understanding of the operation of the antenna in this
respect.
The feed impedance of the antenna results from a number of
factors including the size and shape of the antenna, the frequency
of operation and its environment. The impedance seen is normally
complex, i.e. consisting of resistive elements as well as reactive
ones. The resistive elements are made up from two constituents,
namely the "loss resistance" and secondly the "radiation
resistance."
Loss resistance
The loss resistance arises from the actual resistance of the
elements in the antenna, and power dissipated in this manner is
lost as heat. Although it may appear that the "DC" resistance is
low, at higher frequencies the skin effect is in evidence and only
the surface areas of the conductor are used. As a result the
effective resistance is higher than would be measured at DC. It is
proportional to the circumference of the conductor and to the
square root of the frequency.
The resistance can become particularly significant in high
current sections of an antenna where the effective resistance is
low. Accordingly to reduce the effect of the loss resistance it is
necessary to ensure the use of very low resistance conductors.
Radiation resistance
The other resistive element of the impedance is the "radiation
resistance". This can be thought of as virtual resistor. It arises
from the fact that power is "dissipated" when it is radiated. The
aim is to "dissipate" as much power in this way as possible. It
varies from one type of antenna to another, and from one design to
another. It is dependent upon a variety of factors. However a
typical half wave dipole operating in free space has a radiation
resistance of around 73 Ohms.
Reactive elements
There are also reactive elements to the feed impedance. These
arise from the fact that the antenna elements act as tuned circuits
that possess inductance and capacitance. At resonance where most
antennas are operated the inductance and capacitance cancel one
another out to leave only the resistance of the combined radiation
resistance and loss resistance. However either side of resonance
the feed impedance quickly becomes either inductive (if operated
below the resonant frequency) or capacitive (if operated above the
resonant frequency).
Efficiency
It is naturally important to ensure that the proportion of the
power dissipated in the loss resistance is as low as possible,
leaving the highest proportion to be dissipated in the radiation
resistance as a radiated signal. The proportion of the power
dissipated in the radiation resistance divided by the power applied
to the antenna is the efficiency.
A variety of means can be employed to ensure that the efficiency
remains as high as possible. These include the use of optimum
materials for the conductors to ensure low values of resistance,
large circumference conductors to ensure large surface area to
overcome the skin effect, and not using designs where very high
currents and low feed impedance values are present. Other
constraints may require that not all these requirements can be met,
but by using engineering judgement it is normally possible to
obtain a suitable compromise.
Antenna resonance and bandwidth
Two major factors associated with radio antennas are their
resonant or centre operating frequency and the bandwidth over which
they can operate. They naturally are very important feature of the
operation of the antenna and as such they are mentioned in
specifications for particular antennas and are a particularly
important facet. Whether the antenna is used for broadcasting,
WLAN, cellular telecommunications, PMR or any other application,
the performance of the antenna is paramount, and the resonant
frequency and bandwidth are of great importance.
Antenna resonance
An antenna is a form of tuned circuit consisting of inductance
and capacitance, and as a result it has a resonant frequency at the
frequency where the capacitive and inductive reactances cancel each
other out. At this point the antenna appears purely resistive, the
resistance being a combination of the loss resistance and the
radiation resistance.
Impedance of an antenna with frequency
The capacitance and inductance of an antenna are determined by
the physical properties of the antenna and its environment. The
major feature of the antenna is its dimensions. It is found that
the larger the antenna or more strictly the antenna elements, the
lower the resonant frequency. For example antennas for UHF
terrestrial television have relatively small elements, whilst those
for VHF broadcast sound FM have larger elements indicating a lower
frequency. Antennas for short wave applications are larger
still.
Bandwidth
Most antennas are operated around the resonant point. This means
that there is only a limited bandwidth over which it can operate
efficiently. Outside this the levels of reactance rise to levels
that may be too high for satisfactory operation. Other
characteristics of the antenna may also be impaired away from the
centre operating frequency.
The bandwidth is particularly important where transmitters are
concerned as damage may occur to the transmitter if the antenna is
operated outside its operating range and the transmitter is not
adequately protected. In addition to this the signal radiated by
the antenna may be less for a number of reasons.
For receiving purposes the performance of the antenna is less
critical in some respects. It can be operated outside its normal
bandwidth without any fear of damage to the set. Even a random
length of wire will pick up signals, and it may be possible to
receive several distant stations. However for the best reception it
is necessary to ensure that the performance of the antenna is
optimum.
Impedance bandwidth
One major feature of an antenna that does change with frequency
is its impedance. This in turn can cause the amount of reflected
power to increase. If the antenna is used for transmitting it may
be that beyond a given level of reflected power damage may be
caused to either the transmitter or the feeder, and this is quite
likely to be a factor which limits the operating bandwidth of an
antenna. Today most transmitters have some form of SWR protection
circuit that prevents damage by reducing the output power to an
acceptable level as the levels of reflected power increase. This in
turn means that the efficiency of the station is reduced outside a
given bandwidth. As far as receiving is concerned the impedance
changes of the antenna are not as critical as they will mean that
the signal transfer from the antenna itself to the feeder is
reduced and in turn the efficiency will fall. For amateur operation
the frequencies below which a maximum SWR figure of 1.5:1 is
produced is often taken as the acceptable bandwidth.
In order to increase the bandwidth of an antenna there are a
number of measures that can be taken. One is the use of thicker
conductors. Another is the actual type of antenna used. For example
a folded dipole which is described fully in Chapter 3 has a wider
bandwidth than a non-folded one. In fact looking at a standard
television antenna it is possible to see both of these features
included.
Radiation pattern
Another feature of an antenna that changes with frequency is its
radiation pattern. In the case of a beam it is particularly
noticeable. In particular the front to back ratio will fall off
rapidly outside a given bandwidth, and so will the gain. In an
antenna such as a Yagi this is caused by a reduction in the
currents in the parasitic elements as the frequency of operation is
moved away from resonance. For beam antennas such as the Yagi the
radiation pattern bandwidth is defined as the frequency range over
which the gain of the main lobe is within 1 dB of its maximum.
For many beam antennas, especially high gain ones it will be
found that the impedance bandwidth is wider than the radiation
pattern bandwidth, although the two parameters are inter-related in
many respects.
Antenna directivity and gain
Antennas (aerials) do not radiate equally in all directions. It
is found that all realizable radio antennas radiate more in some
directions than others. The actual pattern is dependent upon the
type of antenna, its size, the environment and a variety of other
factors. This directional pattern can be used to ensure that the
power radiated is radiated in the desired directions.
It is normal to refer to the directional patterns and gain in
terms of the transmitted signal. It is often easier to visualize
the antenna is terms of its radiated power, however the antenna
performs in an exactly equivalent manner for reception, having
identical figures and specifications.
In order to visualize the way in which an antenna radiates a
diagram known as a polar diagram is used. This is normally a two
dimensional plot around an antenna showing the intensity of the
radiation at each point for a particular plane. Normally the scale
that is used is logarithmic so that the differences can be
conveniently seen on the plot. Although the radiation pattern of
the antenna varies in three dimensions, it is normal to make a plot
in a particular plane, normally either horizontal or vertical as
these are the two that are most used, and it simplifies the
measurements and presentation. An example for a simple dipole
antenna is shown below.
Polar diagram of a half wave dipole in free space
Antennas are often categorized by the type of polar diagram they
exhibit. For example an omni-directional antenna is one which
radiates equally (or approximately equally) in all directions in
the plane of interest. An antenna that radiates equally in all
directions in all planes is called an isotropic antenna. As already
mentioned it is not possible to produce one of these in reality,
but it is useful as a theoretical reference for some measurements.
Other antennas exhibit highly directional patterns and these may be
utilized in a number of applications. The Yagi antenna is an
example of a directive antenna and possibly it is most widely used
for television reception.
Polar diagram for a yagi antenna
There are a number of key features that can be seen from this
polar diagram. The first is that there is a main beam or lobe and a
number of minor lobes. It is often useful to define the beam-width
of an antenna. This is taken to be angle between the two points
where the power falls to half its maximum level, and as a result it
is sometimes called the half power beam-width.
Antenna gain
An antenna radiates a given amount of power. This is the power
dissipated in the radiation resistance of the antenna. An isotropic
radiator will distribute this equally in all directions. For an
antenna with a directional pattern, less power will be radiated in
some directions and more in others. The fact that more power is
radiated in given directions implies that it can be considered to
have a gain.
The gain can be defined as a ratio of the signal transmitted in
the "maximum" direction to that of a standard or reference antenna.
This may sometimes be called the "forward gain". The figure that is
obtained is then normally expressed in decibels (dB). In theory the
standard antenna could be almost anything but two types are
generally used. The most common type is a simple dipole as it is
easily available and it is the basis of many other types of
antenna. In this case the gain is often expressed as dBd i.e. gain
expressed in decibels over a dipole. However a dipole does not
radiated equally in all directions in all planes and so an
isotropic source is sometimes used. In this case the gain may be
specified in dBi i.e. gain in decibels over an isotropic source.
The main drawback with using an isotropic source as a reference is
that it is not possible to realize them in practice and so that
figures using it can only be theoretical. However it is possible to
relate the two gains as a dipole has a gain of 2.1 dB over an
isotropic source i.e. 2.1 dBi. In other words, figures expressed as
gain over an isotropic source will be 2.1 dB higher than those
relative to a dipole. When choosing an antenna and looking at the
gain specifications, be sure to check whether the gain is relative
to a dipole or an isotropic source.
Apart from the forward gain of an antenna another parameter
which is important is the front to back ratio. This is expressed in
decibels and as the name implies it is the ratio of the maximum
signal in the forward direction to the signal in the opposite
direction. This figure is normally expressed in decibels. It is
found that the design of an antenna can be adjusted to give either
maximum forward gain of the optimum front to back ratio as the two
do not normally coincide exactly. For most VHF and UHF operation
the design is normally optimized for the optimum forward gain as
this gives the maximum radiated signal in the required
direction.
Gain / beam-width balance
It may appear that maximizing the gain of an antenna will
optimize its performance in a system. This may not always be the
case. By the very nature of gain and beam-width, increasing the
gain will result in a reduction in the beam-width. This will make
setting the direction of the antenna more critical. This may be
quite acceptable in many applications, but not in others. This
balance should be considered when designing and setting up a radio
link.
Coaxial feeder or cable (coax)
- used to feed antenna systems
The most common type of antenna feeder used today is undoubtedly
coaxial feeder or coax. Coax offers advantages of convenience of
use while being able to provide a good level of performance. As a
result coaxial antenna feeder is universally used for domestic
feeder applications for TV and Hi-Fi antenna systems. Additionally
professionals make very widespread use of coax I their antenna
systems, although for some applications other types of feeder such
as open wire feeder, or waveguides may be used.
Basics
Coaxial feeder consists of two concentric conductors. The centre
conductor is almost universally made of copper. Sometimes it may be
a single conductor whilst at other times it may consist of several
strands.
The outer conductor is normally made from a copper braid. This
enables the cable to be flexible which would not be the case if the
outer conductor was solid, although in some varieties made for
particular applications it is. To improve the screening double or
even triple screened cables are sometimes used. Normally this is
accomplished by placing one braid directly over another although in
some instances a copper foil or tape outer may be used. By using
additional layers of screening, the levels of stray pick-up and
radiation are considerably reduced. The loss is marginally
lower.
Between the two conductors there is an insulating dielectric.
This holds the two conductors apart and in an ideal world would not
introduce any loss, although it is one of the chief causes of loss
in reality. This dielectric may be solid or as in the case of many
low loss cables it may be semi-airspaced because it is the
dielectric that introduces most of the loss. This may be in the
form of long "tubes" in the dielectric, or a "foam" construction
where air forms a major part of the material.
Finally there is a final cover or outer sheath. This serves
little electrical function, but can prevent earth loops forming. It
also gives a vital protection needed to prevent dirt and moisture
attacking the cable.
Cross section though coaxial cable
How it works
A coaxial cable carries current in both the inner and the outer
conductors. These current are equal and opposite and as a result
all the fields are confined within the cable and it neither
radiates nor picks up signals.
This means that the cable operates by propagating an
electromagnetic wave inside the cable. As there are no fields
outside the cable it is not affected by nearby objects. Accordingly
it is ideal for applications where the cable has to be routed
through or around buildings or close to many other objects. This is
a particular advantage of coaxial feeder when compared with other
forms of feeder such as two wire (open wire, or twin) feeder.
Characteristic impedance
All feeders posses characteristic impedance. For coaxial cable
there are two main standards that have been adopted over the years,
namely 75 and 50 ohms.
75 ohm cable is used almost exclusively for domestic TV and VHF
FM applications. However for commercial, amateur and CB
applications 50 ohms has been taken as the standard. The reason for
the choice of these two standards is largely historical but arises
from the fact that 75 ohm coax gives the minimum weight for a given
loss, while 50 ohm coax gives the minimum loss for a given
weight.
These two standards are used for the vast majority of coax cable
which is produced but it is still possible to obtain other
impedances for specialist applications. Higher values are often
used for computer installations, but other values including 25, 95
and 125 ohms are available. 25 ohm miniature cable is extensively
used in magnetic core broadband transformers. These values and more
are available through specialist coax cable suppliers.
Impedance determination
The impedance of the coax is chiefly governed by the diameters
of the inner and outer conductors. On top of this the dielectric
constant of the material between the conductors has a bearing. The
relationship needed to calculate the impedance is given simply by
the formula:
D = Inner diameter of the outer conductor
d = Diameter of the inner conductor
Capacitance and inductance
The capacitance of a line varies with the spacing of the
conductors, the dielectric constant, and as a result the impedance
of the line. The lower the impedance, the higher the capacitance
for a given length, because the conductor spacing is decreased. The
capacitance also increases with increasing dielectric constant, as
in the case of an ordinary capacitor.
It is also often necessary to know the inductance of a line as
well.
Practical aspects
The loss introduced by a feeder is a critical element of its
operation. While the specification for a given type of cable will
state the loss it introduces, further losses can be introduced if
it is installed badly. Any moisture entering the cable will produce
a considerable increase. If any moisture passes into the dielectric
material spacing the inner and outer conductors, this will impair
the performance of the dielectric, and increase the level of loss.
Moisture will also cause the outer braid to oxidize, and reduce the
conductivity between the small conductors making up the braid.
It is therefore very important to seal the end of the cable if
it is to be used externally, and ensure that no moisture enters. It
is also necessary to ensure that the outer sheath of the cable
remains intact and is not damaged during installation or further
use.
On some occasions it is necessary to bury coaxial cable. Ideally
normal cable should not be buried directly as this relies purely on
the outer sheath for protection and it is not designed for these
conditions. Instead it can be run through buried conduit
manufactured for carrying buried cables. This has the advantage
that it is easy to replace. Alternatively a form of coax known as
"bury direct" can be used.
All cables have a bend radius. In order to prevent damage they
should not be bent into curves tighter than this. If coax is bent
beyond its limit then damage to the inner construction of the cable
may result. In turn this can lead to much higher levels of
loss.
Balanced antenna feeder
- including open wire, two-wire, twin, and ribbon feeders
Balanced feeder is a form of feeder that can be used for feeding
balanced antennas (i.e. antennas that do not have one connection
taken to ground). It is mainly used on frequencies below 30 MHz can
offer the advantage of very low levels of loss. The feeder or
transmission line is also referred to by other names including
twin, two wire, open wire, and sometimes even ribbon feeder. These
names often depend upon the type of construction of the particular
form.
It is used less than coaxial feeder or coax, although it is able
to offer some significant advantages over coax in some
applications.
Basics
A balanced or twin feeder consists of two parallel conductors
unlike coax that consists of two concentric conductors.. The
currents flowing in both wires run in opposite directions but are
equal in magnitude. As a result the fields from them cancel out and
no power is radiated or picked up. To ensure efficient operation
the spacing of the conductors is normally kept to within about 0.01
wavelengths.
The feeder exists in a variety of forms. Essentially it is just
two wires that are closely spaced in terms of the radio frequency
of operation. In practical terms manufactured feeder is available
and it consist of two wires contained within a plastic sheath that
is also used as a spacer between them to keep the spacing, and
hence the impedance constant. Another form commonly called open
wire feeder simply consists of two wires kept apart by spacers that
are present at regular intervals along the feeder. It has an
appearance a little akin to a rope ladder.
Twin feeder a form of balanced feeder
Impedance
Like coaxial cable, the impedance of twin feeder is governed by
the dimensions of the conductors, their spacing and the dielectric
constant of the material between them. The impedance can be
calculated from the formula given below.
Where
D is the distance between the two conductors
d is the outer diameter of the conductors
Epsilon is the dielectric constant of the material between the
two conductors
Types
This type of feeder can take a variety of forms. An "open wire"
feeder can be made by having two wires running parallel to one
another. Spacers are used every fifteen to thirty centimeters to
maintain the wire spacing. Usually these are made from plastic or
other insulating material. Typically this feeder may have an
impedance of around 600 ohms, although it is very dependent upon
the wire, and the spacing used.
The feeder may also be bought as flat 300 ohm ribbon feeder
consisting of two wires spaced with a clear plastic. This is the
most common form and is the type that is used for manufacturing
temporary VHF FM antennas. If used outside this type absorbs water
into the plastic dielectric. Not only does this significantly
increase the loss on damp days, but the moisture absorbed causes
the wire to oxidize which in turn leads to increased losses over
the longer term.
The feeder can also be bought with a black plastic dielectric
with oval holes spaced at intervals in spacing. This type gives far
better performance than the clear plastic varieties which absorb
water if used outside.
Waveguide
- a basic introduction to the waveguide and the theory behind
their operation
Waveguides are used in a variety of applications to carry radio
frequency energy from one pint to another. In their broadest terms
a waveguide is defined as a system of material that is designed to
confine electromagnetic waves in a direction defined by its
physical boundaries. This definition gives a very broad view of
waveguides, and indeed waveguide theory is used in a number of
applications to provide waveguide applications in a number of
areas.
Typically a waveguide is thought if as a transmission line
comprising a hollow conducting tube, which may be rectangular or
circular within which electromagnetic waves are propagated. Unlike
coaxial cable, there is no centre conductor within the waveguide.
Signals propagate within the confines of the metallic walls that
act as boundaries
Rectangular waveguide
Waveguides will only carry or propagate signals above a certain
frequency, known as the cut-off frequency. Below this the waveguide
is not able to carry the signals. The cut-off frequency of the
waveguide depends upon its dimensions. In view of the mechanical
constraints this means that waveguides are only sued for microwave
frequencies. Although it is theoretically possible to build
waveguides for lower frequencies the size would not make them
viable to contain within normal dimensions and their cost would be
prohibitive.
Connecting signals to a waveguide
A signal can be entered into the waveguide in a number of ways.
The most straightforward is to use what is known as a launcher.
This is basically a small probe which penetrates a small distance
into the centre of the waveguide itself as shown. Often this probe
may be the centre conductor of the coaxial cable connected to the
waveguide. The probe is orientated so that it is parallel to the
lines of the electric field which is to be set up in the waveguide.
An alternative method is to have a loop which is connected to the
wall of the waveguide. This encompasses the magnetic field lines
and sets up the electromagnetic wave in this way. However for most
applications it is more convenient to use the open circuit probe.
These launchers can be used for transmitting signals into the
waveguide as well as receiving them from the waveguide.
Waveguide launcher
Waveguide parameters
- data for waveguides in terms of their frequency range,
material, attenuation, dimensions etc
Waveguides used for the transmission of radio frequency energy
come in a variety of sizes. The size or more correctly the
dimensions of the waveguide determine the properties, including
parameters such as the cut-off frequency and so forth. Accordingly
waveguides come in a variety of standard sizes.
The figures given below are for rigid rectangular waveguides, as
these are the most common form of waveguide used.
Waveguide parametersRigid Rectangular WaveguidesWG DesignFreq
range*Cut-off *Theoretical attn dB / 30mMaterialBand Dimensions
(mm)
WG00 0.32 - 0.49 0.256 0.051 - 0.031 Alum B 584 x 292
WG0 0.35 - 0.53 0.281 0.054 - 0.034 Alum B,C 533 x 267
WG1 0.41 - 0.625 0.328 0.056 - 0.038 Alum B,C 457 x 229
WG2 0.49 - 0.75 0.393 0.069 - 0.050 Alum C 381 x 191
WG3 0.64 - 0.96 0.513 0.128 - 0.075 Alum C 292 x 146
WG4 0.75 - 1.12 0.605 0.137 - 0.095 Alum C,D 248 x 124
WG5 0.96 - 1.45 0.766 0.201 - 0.136 Alum D 196 x 98
WG6 1.12 - 1.70 0.908 0.317 - 0.212 Brass D 165 x 83
WG6 1.12 - 1.70 0.908 0.269 - 0.178 Alum D 165 x 83
WG7 1.45 - 2.20 1.157 D,E 131 x 65
WG8 1.70 - 2.60 1.372 0.588 - 0.385 Brass E 109 x 55
WG8 1.70 - 2.60 1.372 0.501 - 0.330 Alum E 109 x 55
WG9A 2.20 - 3.30 1.736 0.877 - 0.572 Brass E,F 86 x 43
WG9A 2.20 - 3.30 1.736 0.751 - 0.492 Alum E,F 86 x 43
WG10 2.60 - 3.95 2.078 1.102 - 0.752 Brass E,F 72 x 34
WG10 2.60 - 3.95 2.078 0.940 - 0.641 Alum E,F 72 x 34
WG11A 3.30 - 4.90 2.577 F,G 59 x 29
WG12 3.95 x 5.85 3.152 2.08 - 1.44 Brass F,G 48 x 22
WG12 3.95 x 5.85 3.152 1.77 - 1.12 Alum F,G 48 x 22
WG13 4.90 - 7.05 3.711 G,H 40 x 20
WG14 5.85 - 8.20 4.301 2.87 - 2.30 Brass H 35 x 16
WG14 5.85 - 8.20 4.301 2.45 - 1.94 Alum H 35 x 16
WG15 7.05 - 10.0 5.26 4.12 - 3.21 Brass I 29 x 13
WG15 7.05 - 10.0 5.26 3.50 - 2.74 Alum I 29 x 13
* frequency in GHz and for TE10 modeAlum = Aluminum
The dipole antenna
The dipole antenna or dipole aerial is one of the most important
and commonly used types of antenna. It is widely used on its own,
and it is also incorporated into many other antenna designs where
it forms the radiating or driven element for the antenna.
Basic facts
As the name suggests the dipole antenna consists of two
terminals or "poles" into which radio frequency current flows. This
current and the associated voltage causes and electromagnetic or
radio signal to be radiated. Being more specific, a dipole is
generally taken to be an antenna that consists of a resonant length
of conductor cut to enable it to be connected to the feeder. For
resonance the conductor is an odd number of half wavelengths long.
In most cases a single half wavelength is used, although three,
five, . wavelength antennas are equally valid.
The basic half wave dipole antenna
The current distribution along a dipole is roughly sinusoidal.
It falls to zero at the end and is at a maximum in the middle.
Conversely the voltage is low at the middle and rises to a maximum
at the ends. It is generally fed at the centre, at the point where
the current is at a maximum and the voltage a minimum. This
provides a low impedance feed point which is convenient to handle.
High voltage feed points are far less convenient and more difficult
to use.
When multiple half wavelength dipoles are used, they are
similarly normally fed in the centre. Here again the voltage is at
a minimum and the current at a maximum. Theoretically any of the
current maximum nodes could be used.
Three half wavelength wave dipole antennasFeed impedance
The feed impedance of a dipole antenna is dependent upon a
variety of factors including the length, the feed position, the
environment and the like. A half wave centre fed dipole antenna in
free space has an impedance 73.13 ohms making it ideal to feed with
75 ohm feeder.
The feed impedance of a dipole can be changed by a variety of
factors, the proximity of other objects having a marked effect. The
ground has a major effect. If the dipole antenna forms the
radiating element for a more complicated antenna, then elements of
the antenna will have an effect. Often the effect is to lower the
impedance, and when used in some antennas the feed impedance of the
dipole element may fall to ten ohms or less, and methods need to be
used to ensure a good match is maintained with the feeder.
Polar diagram
The polar diagram of a half wave dipole antenna that the
direction of maximum sensitivity or radiation is at right angles to
the axis of the antenna. The radiation falls to zero along the axis
of the antenna as might be expected.
Polar diagram of a half wave dipole in free space
If the length of the dipole antenna is changed then the
radiation pattern is altered. As the length of the antenna is
extended it can be seen that the familiar figure of eight pattern
changes to give main lobes and a few side lobes. The main lobes
move progressively towards the axis of the antenna as the length
increases.
Antenna length
The length of a dipole is the main determining factor for the
operating frequency of the dipole antenna. Although the antenna may
be an electrical half wavelength, or multiple of half wavelengths,
it is not exactly the same length as the wavelength for a signal
traveling in free space. There are a number of reasons for this and
it means that an antenna will be slightly shorter than the length
calculated for a wave traveling in free space.
For a half wave dipole the length for a wave traveling in free
space is calculated and this is multiplied by a factor "A".
Typically it is between 0.96 and 0.98 and is mainly dependent upon
the ratio of the length of the antenna to the thickness of the wire
or tube used as the element. Its value can be approximated from the
graph:
Multiplication factor "A" used for calculating the length of a
dipole
In order to calculate the length of a half wave dipole the
simple formulae given below can be used:
Length (meters) = 150 x A / frequency in MHz
Length (inches) = 5905 x A / frequency in MHz
Using these formulae it is possible to calculate the length of a
half wave dipole. Even though calculated lengths are normally quite
repeatable it is always best to make any prototype antenna slightly
longer than the calculations might indicate. This needs to be done
because changes in the thickness of wire being used etc may alter
the length slightly and it is better to make it slightly too long
than too short so that it can be trimmed so that it resonates on
the right frequency. It is best to trim the antenna length in small
steps because the wire or tube cannot be replaced very easily once
it has been removed.
The folded dipole antenna
- providing a higher impedance and greater bandwidth
The standard dipole is widely used in its basic form. However
under a number of circumstances a modification of the basic dipole,
known as a folded dipole provides a number of advantages that can
be used to advantage.
In its basic form a dipole consists of a single wire or
conductor cut in the middle to accommodate the feeder. It is found
that the feed impedance is altered by the proximity of other
objects, especially other parasitic elements that may be used in
other forms of antenna. This can cause problems with matching and
because resistance losses in the antenna system can start to become
significant.
Additionally many antennas have to be able to operate over large
bandwidths and a standard dipole may be unable to fulfill this
requirement adequately.
The basic folded half wave dipole
A variation of the dipole, known as a folded dipole provides a
solution to these problems, offering a wider bandwidth and a
considerable increase in feed impedance. The folded dipole is
formed by taking a standard dipole and then taking a second
conductor and joining the two ends. In this way a complete loop is
made as shown. If the conductors in the main dipole and the second
or "fold" conductor are the same diameter, then it is found that
there is a fourfold increase in the feed impedance. In free space,
this gives a feed impedance of around 300 ohms. Additionally the
antenna has a wider bandwidth.
Impedance increase
In a standard dipole the currents flowing along the conductors
are in phase and as a result there is no cancellation of the fields
and radiation occurs. When the second conductor is added, this can
be considered as an extension to the standard dipole with the ends
folded back to meet each other. As a result the currents in the new
section flow in the same direction as those in the original dipole.
The currents along both the half-waves are therefore in phase and
the antenna will radiate with the same radiation patterns etc as a
simple half-wave dipole.
The impedance increase can be deduced from the fact that the
power supplied to a folded dipole is evenly shared between the two
sections which make up the antenna. This means that when compared
to a standard dipole the current in each conductor is reduced to a
half. As the same power is applied, the impedance has to be raised
by a factor of four to retain balance in the equation Watts = I^2 x
R.
Applications
Folded dipoles are sometimes used on their-own, but they must be
fed with a high impedance feeder, typically 300 ohms. However they
find more uses when a dipole is incorporated in another antenna
with other elements nearby. This has the effect of reducing the
dipole impedance. To ensure that it can be fed conveniently, a
folded dipole may be used to raise the impedance again to a
suitable value.
Quarter wave vertical antenna
- including the ground plane antenna
Vertical antennas are widely used at all frequencies from MF up
to VHF and beyond. They exist in a variety of forms including the
quarter wave vertical and ground plane antennas. They possess many
advantages and are widely used for medium wave broadcasting as well
as for mobile applications in areas including private mobile
radio.
The reason for this widespread use is the omni-directional
radiation pattern that they give in the horizontal plane. This
means that the antennas do not have to be re-orientated to keep the
signals constant as the car moves it position.
Single element vertical antennas posses an omni-directional
radiation pattern (in the horizontal plane). This means that the
antennas do not have to be re-orientated when used in mobile
applications as the vehicle moves. This is obviously an essential
requirement.
A further advantage is that much of the radiation is at right
angles to the antenna element, and as a result it travels close to
the earth's surface where the receiving stations are located.
Radiation directed upwards is wasted in many instances as VHF
transmissions are normally not reflected by the ionosphere.
For medium wave broadcast stations a particular advantage is
that the radiation is vertically polarized. It is found that the
vertically polarized transmissions propagate further via the ground
wave that these transmissions use.
Basic element
Like the name suggests the antenna consists of a quarter
wavelength vertical element. The antenna is what is termed
"un-balanced" having one connection to the vertical element and
using an earth connection or simulated earth connection to provide
an image for the other connection.
The voltage and current waveforms show that at the end the
voltage rises to a maximum whereas the current falls to a minimum.
Then at the base of the antenna at the feed point, the voltage is
at a minimum and the current is at its maximum. This gives the
antenna a low feed impedance. Typically this is around 20 ohms.
A quarter wave vertical
The ground is obviously an important part of the antenna. Many
MF and HF installations use a ground connection for this. These
ground systems need to be very effective fort he antenna to perform
satisfactorily. They must obviously have a very low resistance, and
often utilize large "mats" of radials extending out from the base
of the antenna to ensure excellent RF performance.
For VHF and UHF installations, height is obviously important and
antennas need to be raised to ensure they are above the nearby
obstructions. Also for mobile installations it is clearly not
possible to use a true earth connection. In these cases a simulated
earth is used. For mobile applications this consists of the body of
the vehicle. The antenna mounting will normally enable a suitable
connection to be made to the vehicle body, sometimes using a
capacitive connection. However it is necessary to ensure that the
vehicle body is metal, and not plastic in the vicinity of the
antenna mounting.
For fixed stations a set of radials simulating a ground plane is
used. In theory the ground plane should extend out to infinity, but
in practice a number of radials a quarter wave-length long is used.
Typically for many VHF applications four radials is sufficient.
A radial system used with a quarter wave vertical
If the radials are bent downwards from the horizontal, then the
feed impedance will be raised. A 50 ohm match is achieved when the
angle between the ground plane rods and the horizontal is 42
degrees. Another solution is to include an impedance matching
element in the antenna. Normally this is in the form of a tapped
coil that can be conveniently housed in the base of the
antenna.
Folded element
In view of the low impedance presented to the feeder by the
antenna, methods must be found of presenting a good match and some
have already been outlined. Another is to use a folded element. In
the same way that a folded dipole increases the feed impedance of
the antenna, so a folded vertical element can be used. If the
diameter of both sections is the same, then an increase by a ratio
of 4:1 is achieved. This would bring the impedance to 80 ohms and
will provide an acceptable match to 75 ohm feeder. By using a
smaller diameter grounded element the feed impedance can be reduced
so that a good match to 50 ohm coax can be achieved.
Summary
The quarter wave vertical antenna is widely used in view of its
simplicity and convenience. To improve on its performance other
types of vertical are available. It is also possible to use further
verticals and feed them with different phases to provide gain to
the overall antenna system.
Five eighths wavelength vertical antenna
- providing gain by adding length
Vertical antennas find widespread use in applications where an
"all round" radiation pattern is required. In these applications it
is necessary to keep the maximum amount of radiation parallel to
the earth. It is in applications such as these that the five
eighths wavelength vertical antenna has become widely used.
Development
The most straightforward vertical antenna is the quarter
wavelength version. However it is found that by extending the
length of the vertical element, the amount of power radiated at a
low angle is increased. If a half wave dipole is extended in length
the radiation at right angles to the antenna starts to increase
before finally splitting into several lobes. The maximum level of
radiation at right angles to the antenna is achieved when the
dipole is about 1.2 times the wavelength.
Gain
When used as a vertical radiator against a ground plane this
translates to a length of 5/8 wavelength. It is found that a five
eighths vertical has a gain of close to 4 dB. To achieve this gain
the antenna must be constructed of the right materials so that
losses are reduced to the absolute minimum and the overall
performance is maintained, otherwise much of the advantage of using
the additional length will be lost.
Matching
For most applications, it is necessary to ensure that the
antenna provides a good match to 50 ohm coaxial cable. It is found
that a 3/4 wavelength vertical element provides a good match, and
therefore the solution to the 5/8 wavelength antenna is to make it
appear as a 5/8 radiator but have the electrical length of a 3/4
element. This is achieved by placing a small loading coil at the
base of the antenna to increase its electrical length.
Mechanical considerations
Five eighths wavelength vertical antennas are often used on
automobiles. Accordingly one of the main constraints is to ensure
that the coil at the base of the antenna is being kept rigid and
does not bend as the antenna flexes with the movement of the car.
If there is too much flexing then the match to the feeder will
change and the operation will be impaired.
The J or J Pole Antenna
- the J pole antenna is a vertical antenna that does not require
radials
The J or J pole antenna has found favor in many applications.
The J antenna has a number of advantages over the standard vertical
antennas such as the quarter wavelength vertical antenna and the
five eights wavelength antenna. Unlike the other vertical antennas
just mentioned, the J pole antenna does not require radials for its
operation. In applications where radials may appear unsightly or
where they may not be suitable for other reasons, the J pole
antenna provides a useful alternative. Additionally its length
means that the J pole antenna also provides some gain over a normal
quarter wavelength vertical. These two attributes make the j pole
antenna the ideal type for many applications. As a result the J
Pole antenna is finding many applications, many of which are at VHF
and above. Here it forms a compact self contained antenna that can
fit in many locations and can give a high level of performance
without a large visual impact.
Although the fact that the J antenna does not have any radials
may make it appear that it will not work, it is a well established
design. It is a form of antenna known as a Zepp or Zeppelin antenna
that found favor in the 1930s as an HF antenna. This antenna gained
its name from the fact that it was used on the Zeppelin airships.
It consists of a half wave radiating element which is end fed using
a quarter wave stub of open wire or 300 Ohm balanced feeder used to
match the impedance to the normal 50 Ohm coaxial feeder.
The development of the J or J Pole antenna
The diagram shows the development of the J pole antenna and its
operation. This shows the antenna radiating element which is a half
wavelength. Being end fed this presents a high impedance to the
feeder and this is matched using a half wave matching stub. In the
first form of the antenna, the radiating element is fed from the
source, with the other leg of the stub providing a passive balance.
It can also be seen that it is possible to feed the antenna using
the other arm of the stub.
The development of the J or J Pole antenna
The final implementation of the J pole antenna uses the stub to
provide a good match to 50 Ohm cable. The feed point is moved up or
down the stub to provide the best match, and adjustment can be made
once the antenna is in position if required. In this way any
spurious changes resulting from the position, etc can be
removed.
The J pole antenna is quite easy to construct and gives good
results. The main disadvantage is that it can be a little more
difficult to adjust than some other forms. The reason for this is
that impedance matching has to be accomplished by altering the
trimming length of the stub.
The length of the half wave radiating stub for the j pole
antenna can be determined using the same formula as used in
calculating the length of a half wave dipole. The physical length
of the balanced feeder will depend on the velocity factor of the
feeder in use. For open wire feeder the velocity factor is nearly
unity and the length will be very close to that of the free space
quarter wavelength. If 300 twin feeder is used then the length
required will be shorter because its velocity factor is about
0.85.
The Yagi antenna
The Yagi or Yagi-Uda radio antenna or aerial is one of the most
successful designs of directive radio antenna in use today. It is
used in a wide variety of applications where an antenna with gain
and directivity is required. It has become particularly popular for
television receiving applications. Here most households that use a
television have a Yagi antenna directed towards the broadcast
transmitter to give sufficient signal to provide a high quality
picture.
The full name for the antenna is the Yagi-Uda antenna. It was
derives it name from its two Japanese inventors Yagi and his
student Uda. The antenna itself was first outlined in a paper that
Yagi himself presented in 1928. Since then its use has grown
rapidly to the stage where today a television antenna is synonymous
with an antenna having a central boom with lots of elements
attached.
The antenna
The Yagi has a dipole as the main radiating or driven element.
Further "parasitic" elements are added which are not directly
connected to the driven element. Instead they pick up power from
the dipole and re-radiate it such a manner that it affects the
properties of the antenna as a whole.
Basic concept of a Yagi antenna
The parasitic elements operate by re-radiating their signals in
a slightly different phase to that of the driven element. In this
way the signal is reinforced in some directions and cancelled out
in others. It is found that the amplitude and phase of the current
that is induced in the parasitic elements is dependent upon their
length and the spacing between them and the dipole or driven
element.
Using a parasitic element it is not possible to have complete
control over both the amplitude and phase of the currents in all
the elements. This means that it is not possible to obtain complete
cancellation in one direction. Nevertheless it is still possible to
obtain a high degree of reinforcement in one direction and have a
high level of gain, and also have a high degree of cancellation in
another to provide a good front to back ratio.
To obtain the required phase shift an element can be made either
inductive or capacitive. If the parasitic element is made inductive
it is found that the induced currents are in such a phase that they
reflect the power away from the parasitic element. This causes the
antenna to radiate more power away from it. An element that does
this is called a reflector. It can be made inductive by tuning it
below resonance. This can be done by physically adding some
inductance to the element in the form of a coil, or more commonly
by making it longer than the resonant length. Generally it is made
about 5% longer than the driven element.
If the parasitic element is made capacitive it will be found
that the induced currents are in such a phase that they direct the
power radiated by the whole antenna in the direction of the
parasitic element. An element which does this is called a director.
It can be made capacitive tuning it above resonance. This can be
done by physically adding some capacitance to the element in the
form of a capacitor, or more commonly by making it about 5% shorter
than the driven element.
It is found that the addition of further directors increases the
directivity of the antenna, increasing the gain and reducing the
beam-width. The addition of further reflectors makes no noticeable
difference.
The antenna exhibits a directional pattern consisting of a main
forward lobe and a number of spurious side lobes. The main one of
these is the reverse lobe caused by radiation in the direction of
the reflector. The antenna can be optimized to either reduce this
or produce the maximum level of forward gain. Unfortunately the two
do not coincide exactly and a compromise on the performance has to
be made depending upon the application.
Polar diagram of the Yagi antenna
Gain
The gain of a Yagi antenna is governed mainly by the number of
elements in the antennas. However the spacing between the elements
also has an effect. As the overall performance of the antenna has
so many inter-related variables, many early designs were not able
to realize their full performance. Today computer programs are used
to optimize designs before they are even manufactured and as a
result the performance of antennas has been improved somewhat.
Feed impedance
It is possible to vary the feed impedance of a Yagi over a wide
range. Although the impedance of the dipole itself would be 73 ohms
in free space, this is altered considerably by the proximity of the
parasitic elements. The spacing, their length and a variety of
other factors all affect the feed impedance presented by the dipole
to the feeder. In fact altering the element spacing has a greater
effect on the impedance than it does the gain, and accordingly
setting the required spacing can be used as one design technique to
fine tune the required feed impedance. Nevertheless the proximity
of the parasitic elements usually reduces the impedance below the
50 ohm level normally required. It is found that for element
spacing distances less than 0.2 wavelengths the impedance falls
rapidly away.
To overcome this, a variety of techniques can be sued. One is to
use a folded dipole for the driven element. This provides an
increase in impedance of around four times dependent upon the ratio
of the thicknesses of the basic dipole conductor and the "fold"
conductor. Other techniques involve using gamma matches, delta
matches, baluns and the like. Delta matches can be very convenient.
They involve "fanning out" the connection to the driven element.
This method has the advantage that the driven element does not need
to be broken to apply the feed as shown. As this is really
applicable to a balanced feeder, a balun is required if coaxial
cable is to be used.
A gamma match is another alternative that is often used. The
outer or braid of the coax feeder is connected directly to the
centre of the driven element. This can be done because the RF
voltage at the centre is zero at this point. The inner conductor of
the feeder carrying the RF current is taken out along the driven
element. The inductance of the arm is then tuned out by the
variable capacitor. When adjusting the antenna design, both the
variable capacitor and the point at which the arm contacts the
driven element are adjusted. Once a value has been ascertained for
the variable capacitor, its value can be measured and a fixed
component inserted if required.
The log periodic antenna
One of the major drawbacks with many antennas is that they have
a relatively small bandwidth. This is particularly true of the Yagi
beam antenna. One design named the log periodic is able to provide
directivity and gain while being able to operate over a wide
bandwidth.
The log periodic antenna is used in a number of applications
where a wide bandwidth is required along with directivity and a
modest level of gain. It is sometimes used on the HF portion of the
spectrum where operation is required on a number of frequencies to
enable communication to be maintained. It is also used at VHF and
UHF for a variety of applications, including some uses as a
television antenna.
Capabilities
The log periodic antenna was originally designed at the
University of Illinois in the USA in 1955.
The antenna is directional and is normally capable of operating
over a frequency range of about 2:1. It has many similarities to
the more familiar Yagi because it exhibits forward gain and has a
significant front to back ratio. In addition to this the radiation
pattern stays broadly the same over the whole of the operating band
as do parameters like the radiation resistance and the standing
wave ratio. However it offers less gain for its size than does the
more conventional Yagi.
Basics
The log periodic antenna can exist in a number of forms. The
most common is the log periodic dipole array (LPDA). It basically
consists of a number of dipole elements. These diminish in size
from the back towards the front. The main beam of the antenna
coming from the smaller front. The element at the back of the array
where the elements are the largest is a half wavelength at the
lowest frequency of operation. The element spacings also decrease
towards the front of the array where the smallest elements are
located. In operation, as the frequency changes there is a smooth
transition along the array of the elements that form the active
region. To ensure that the phasing of the different elements is
correct, the feed phase is reversed from one element to the
next.
A log periodic dipole array
It is possible to explain the operation of a log periodic array
in straightforward terms. The feeder polarity is reversed between
successive elements. Take the condition when the antenna is
approximately in the middle of its operating range. When the signal
meets the first few elements it will be found that they are spaced
quite close together in terms of the operating wavelength. This
means that the fields from these elements will cancel one another
out as the feeder sense is reversed between the elements. Then as
the signal progresses down the antenna a point is reached where the
feeder reversal and the distance between the elements gives a total
phase shift of about 360 degrees. At this point the effect which is
seen is that of two phased dipoles. The region in which this occurs
is called the active region of the antenna. Although the example of
only two dipoles is given, in reality the active region can consist
of more elements. The actual number depends upon the angle [Greek
letter alpha] and a design constant.
The elements outside the active region receive little direct
power. Despite this it is found that the larger elements are
resonant below the operational frequency and appear inductive.
Those in front resonate above the operational frequency and are
capacitive. These are exactly the same criteria that are found in
the Yagi. Accordingly the element immediately behind the active
region acts as a reflector and those in front act as directors.
This means that the direction of maximum radiation is towards the
feed point.
Feed arrangements
The log periodic dipole antenna presents a number of
difficulties if it is to be fed properly. The feed impedance is
dependent upon a number of factors. However it is possible to
control this by altering the spacing, and hence the impedance for
the feeder that connects each of the dipole elements together.
Despite this the impedance varies with frequency, but this can be
overcome to a large extent by making the longer elements out of a
larger diameter rod. Even so the final feed impedance does not
normally match to 50 ohms on its own. It is normal for a further
form of impedance matching to be required. This may be in the form
of a stub or even a transformer. The actual method employed will
depend to a large degree on the application of the antenna and its
frequency range.
Overview
The log periodic antenna is a particularly useful design when
modest levels of gain are required, combined with wideband
operation. A typical antenna will provide between 4 and 6 dB gain
over a bandwidth of 2:1 while retaining an SWR level of better than
1.3:1. With this level of performance it is ideal for many
applications, although a log periodic antenna will be much larger
than a Yagi that will produce equivalent gain. However the Yagi is
unable to operate over such a wide bandwidth.
The parabolic reflector antenna
- also widely termed the dish antenna
The parabolic reflector or "dish" antenna has been used far more
widely in recent years with advent of satellite television (TV).
However this antenna finds uses in many radio and wireless
applications at frequencies usually above about 1GHz where very
high levels of antenna gain are required along with narrow
beam-widths. In many professional applications these parabolic
reflectors are used for satellite as well as for radio astronomy
and it is used in many microwave links, often being seen on radio
relay towers and mobile phone antenna masts. In all these
applications very high levels of gain are required to receive the
incoming signals that are often at a very low level. For
transmitting they are able to concentrate the available radiated
power into a narrow beam-width, ensuring all the available power is
radiated in the required direction.
The Goldstone parabolic reflector antenna
Image courtesy NASA
Basics
The antenna consists of a radiating system that is used to
illuminate a reflector that is curved in the form of a paraboloid.
This shape enables a very accurate beam to be obtained. The antenna
exists in two basic forms. These are termed the focal feed
reflector where source of radiation is placed at the focal point of
the parabola and this is used to illuminate the reflector.
An alternative form of feeding the antenna is known as a
Cassegrain reflector system. Here the radiation is fed through the
centre of the reflector towards a hyperbolic reflector which
reflects the radiation back onto the parabolic reflector. In this
way it is possible to control the radiation more accurately.
Diagram of a focal feed parabolic reflector antenna
The gain is a function of the diameter of the reflecting
surface, the surface accuracy, and the quality of the illumination
from the radiator. Despite these factors it is possible to estimate
the gain of the antenna which can be deduced from the following
formula:
G = 10 log10 k (pi D)^2 / lambda^2
Where
G is the gain over an isotropic source
k is the efficiency factor which is generally about 50%
D is the diameter of the parabolic reflector in meters
Lambda is the wavelength of the signal in meters
From this it can be seen that very large gains can be achieved
if sufficiently large reflectors are used. However when the antenna
has a very large gain, the beam-width is also very small and the
antenna requires very careful control over its position. In
professional systems electrical servo systems are used to provide
very precise positioning.
To provide the optimum illumination of the reflecting surface,
the level of illumination should be greater in the centre than at
the sides. It can be shown that the optimum situation occurs when
the centre is around 10 to 11 dB greater than the illumination at
the edge. Lower levels of edge illumination result in lower levels
of side lobes.
The reflecting surface antenna forms a major part of the whole
system. In many respects it is not as critical as may be thought at
first. Often a wire mesh may be used. Provided that the pitch of
the mesh is small compared to a wavelength it will be seen as a
continuous surface by the radio signals. If a mesh is used then the
wind resistance will be reduced, and this provides significant
advantages.
Focal feed system
The antenna consists of a radiating element which may be a
simple dipole or a waveguide horn antenna. This is placed at the
focal point of the parabolic reflecting surface. The energy from
the radiating element is arranged so that it illuminates the
reflecting surface. Once the energy is reflected it leaves the
antenna system in a narrow beam. As a result considerable levels of
gain can be achieved.
Achieving this is not always easy because it is dependent upon
the radiator that is used. For lower frequencies a dipole element
is often employed whereas at higher frequencies a circular
waveguide may be used. In fact the circular waveguide provides one
of the optimum sources of illumination.
Cassegrain feed system
The Cassegrain feed system, although requiring a second
reflecting surface has the advantage that the overall length of the
antenna between the two reflectors is shorter than the length
between the radiating element and the parabolic reflector. This is
because there is a reflection in the focusing of the signal which
shortens the physical length. This can be an advantage in some
systems.
Diagram of a focal feed parabolic reflector antenna with a
Cassegrain feed
Summary
For most domestic systems a small reflector combined with a
focal point feed are used, providing the simplest and most
economical form of construction. This is the form that is most
widely sued for satellite television applications. These antennas
may not always look exactly like the traditional full dish antenna.
For mechanical and production reasons the feed is often offset from
the centre and a portion of the paraboloid used, again offset from
the centre. This provides mechanical advantage. Nevertheless the
principles are exactly the same.
Horn antenna
- an overview of the horn antenna used in microwave
applications
The horn antenna is used in the transmission and reception of
microwave signals, and the antenna is normally used in conjunction
with waveguide feeds. The horn antenna gains its name from its
appearance. The waveguide can be considered to open out or to be
flared, launching the signal towards the receiving antenna.
Horn antennas are often used as gain standards, and as feeds for
parabolic or 'dish' antennas, as well as being used as antennas in
their own right. One particular use of horn antennas themselves is
for short range radar systems, such as those used for automotive
speed enforcement.
When used as part of a parabolic reflector, the horn is
orientated towards the reflector surface, and is able to give a
reasonably even illumination of the surface without allowing
radiation to miss the reflector. In this way it is able to maximize
the efficiency of the overall antenna. The use of the horn antenna
also minimizes the spurious responses of the parabolic reflector
antenna to signals that are not in the main lobe.
Horn antenna
Basic concept
The horn antenna may be considered as an RF transformer or
impedance match between the waveguide feeder and free space which
has an impedance of 377 ohms. By having a tapered or having a
flared end to the waveguide the horn antenna is formed and this
enables the impedance to be matched. Although the waveguide will
radiate without a horn antenna, this provides a far more efficient
match.
In addition to the improved match provided by the horn antenna,
it also helps suppress signals traveling via unwanted modes in the
waveguide from being radiated.
However the main advantage of the horn antenna is that it
provides a significant level of directivity and gain. For greater
levels of gain the horn antenna should have a large aperture. Also
to achieve the maximum gain for a given aperture size, the taper
should be long so that the phase of the wave-front is as nearly
constant as possible across the aperture. However there comes a
point where to provide even small increases in gain, the increase
in length becomes too large to make it sensible. Thus gain levels
are a balance between aperture size and length. However gain levels
for a horn antenna may be up to 20 dB in some instances.
Horn antenna types
There are two basic types of horn antenna: pyramid and conical.
The pyramid ones, as the name suggests are rectangular whereas the
corrugated ones are usually circular. The corrugated horn provides
a pattern that is nearly symmetrical, with the E and H plane
beam-widths being nearly the same. Additionally it is possible to
control the side lobes better with a conical or corrugated horn
antenna.
The discone antenna
- for wide band applications
The discone antenna is widely used where an omni-directional
wide band or bandwidth antenna is needed. It finds many uses,
particularly for all type of radio scanning and monitoring
applications from the commercial or military monitoring services to
the home scanner enthusiast for frequencies above 30 MHz.
Overview
The discone antenna receives its name from its distinctive
shape. The antenna consists of a top "disc" formulated from a
number of elements arranged in a disc at the top, and further
elements pointing downwards in the shape of a cone. Although the
antenna could be made as a full disc and a cone, this would
considerably increases its weight and wind loading, which would not
be advisable from mechanical considerations.
This type of antenna can operate over frequency ranges of up to
10:1 dependent upon the particular design, and it also offers a
relatively low angle of radiation (and reception). This makes it
ideal for VHF / UHF applications as its greatest sensitivity is
parallel or almost parallel to the Earth. However towards the top
of its frequency range it is found that the angle of radiation
increases somewhat.
Although it is widely used for receiving applications, it is
less commonly used for transmitting. There are several reasons for
this. Although it offers a wide bandwidth, it is not optimized for
a particular band of frequencies and is less efficient than many
other designs that are available. Additionally the wideband with of
the antenna means that spurious signals can be radiated more easily
and the level of reflected power will vary over the operating range
and may rise above acceptable limits in some areas.
Physical aspects
The antenna consists of three main components: the insulator,
the cone elements and the disc elements.
Of the antenna components the insulator size governs a number of
factors of the performance of the antenna. It is made from
insulating material and acts to hold the disc and cone elements in
place, keeping them a fixed distance apart. In fact this distance
is one of the factors that determine the overall frequency range of
the antenna.
Secondly, the cone elements should be a quarter wave-length at
the minimum operating frequency. This can be calculated from the
formula A = 75000 / frequency (MHz) millimeters where A is the
length of the cone elements.
Thirdly the disc elements should be made to have an overall
length of 0.7 of a quarter wave-length. This can be calculated from
the formula B = 52550 / frequency (MHz) millimeters. The diameter
of the top of the cone is mainly dependent upon the diameter of the
coaxial cable being used. This determines the upper frequency limit
of the antenna. The smaller the diameter the higher the frequency.
For many designs operating in the VHF / UHF region of the radio
spectrum it is around 15 millimeters. The spacing between the cone
and the disc should be about a quarter of the inner diameter of the
cone, i.e. around three of four millimeters.
Operation
The way in which the discone operates is relatively complicated,
but it can be envisaged in a simplified manner. The disc and cone
elements sufficiently simulate an electrically complete disc and
cone from which the energy is radiated. As a result the greater the
number of elements, the better the simulation, although in reality
there is a balance between performance, cost and wind resistance.
Often around six elements are used, but the number is not
critical.
In operation energy from the feeder meets the antenna and
spreads over the surface of the cone from the apex towards the base
until the vertical distance between the point on the cone and the
disc is a quarter wavelength. In this way it is possible for the
energy to be radiated or received efficiently.
The antenna radiates and receives energy that is vertically
polarized, and the radiation pattern is omni-directional in the
horizontal plane. The antenna radiates most of the energy at a low
angle which it maintains over the most of the operating range.
Typically there is little change over a range of 5:1 and above this
a slight increase in the angle.
With the feed point at the top of the antenna the current
maximum point is also at the top. It is also found that below the
minimum frequency the antenna presents a very bad mismatch to the
feeder. However once the frequency rises above this point then a
reasonable match to 50 ohm coax is maintained over virtually the
whole of the band.
The log periodic antenna
One of the major drawbacks with many antennas is that they have
a relatively small bandwidth. This is particularly true of the Yagi
beam antenna. One design named the log periodic is able to provide
directivity and gain while being able to operate over a wide
bandwidth.
The log periodic antenna is used in a number of applications
where a wide bandwidth is required along with directivity and a
modest level of gain. It is sometimes used on the HF portion of the
spectrum where operation is required on a number of frequencies to
enable communication to be maintained. It is also used at VHF and
UHF for a variety of applications, including some uses as a
television antenna.
Capabilities
The log periodic antenna was originally designed at the
University of Illinois in the USA in 1955.
The antenna is directional and is normally capable of operating
over a frequency range of about 2:1. It has many similarities to
the more familiar Yagi because it exhibits forward gain and has a
significant front to back ratio. In addition to this the radiation
pattern stays broadly the same over the whole of the operating band
as do parameters like the radiation resistance and the standing
wave ratio. However it offers less gain for its size than does the
more conventional Yagi.
Basics
The log periodic antenna can exist in a number of forms. The
most common is the log periodic dipole array (LPDA). It basically
consists of a number of dipole elements. These diminish in size
from the back towards the front. The main beam of the antenna
coming from the smaller front. The element at the back of the array
where the elements are the largest is a half wavelength at the
lowest frequency of operation. The element spacings also decrease
towards the front of the array where the smallest elements are
located. In operation, as the frequency changes there is a smooth
transition along the array of the elements that form the active
region. To ensure that the phasing of the different elements is
correct, the feed phase is reversed from one element to the
next.
A log periodic dipole array
It is possible to explain the operation of a log periodic array
in straightforward terms. The feeder polarity is reversed between
successive elements. Take the condition when the antenna is
approximately in the middle of its operating range. When the signal
meets the first few elements it will be found that they are spaced
quite close together in terms of the operating wavelength. This
means that the fields from these elements will cancel one another
out as the feeder sense is reversed between the elements. Then as
the signal progresses down the antenna a point is reached where the
feeder reversal and the distance between the elements gives a total
phase shift of about 360 degrees. At this point the effect which is
seen is that of two phased dipoles. The region in which this occurs
is called the active region of the antenna. Although the example of
only two dipoles is given, in reality the active region can consist
of more elements. The actual number depends upon the angle [Greek
letter alpha] and a design constant.
The elements outside the active region receive little direct
power. Despite this it is found that the larger elements are
resonant below the operational frequency and appear inductive.
Those in front resonate above the operational frequency and are
capacitive. These are exactly the same criteria that are found in
the Yagi. Accordingly the element immediately behind the active
region acts as a reflector and those in front act as directors.
This means that the direction of maximum radiation is towards the
feed point.
Feed arrangements
The log periodic dipole antenna presents a number of
difficulties if it is to be fed properly. The feed impedance is
dependent upon a number of factors. However it is possible to
control this by altering the spacing, and hence the impedance for
the feeder that connects each of the dipole elements together.
Despite this the impedance varies with frequency, but this can be
overcome to a large extent by making the longer elements out of a
larger diameter rod. Even so the final feed impedance does not
normally match to 50 ohms on its own. It is normal for a further
form of impedance matching to be required. This may be in the form
of a stub or even a transformer. The actual method employed will
depend to a large degree on the application of the antenna and its
frequency range.,/p>
Overview
The log periodic antenna is a particularly useful design when
modest levels of gain are required, combined with wideband
operation. A typical antenna will provide between 4 and 6 dB gain
over a bandwidth of 2:1 while retaining an SWR level of better than
1.3:1. With this level of performance it is ideal for many
applications, although a log periodic antenna will be much larger
than a Yagi that will produce equivalent gain. However the Yagi is
unable to operate over such a wide bandwidth.
Loop antenna
- an overview of the basics of the different types of loop
antennas.
Loop antennas, or more correctly, closed loop antennas are
widely used in many applications, often providing advantages over
other types of antenna. Loop antennas can be placed into two
categories, namely small loops and large loops. The terms refer to
their size when compared to a wavelength of the frequency in
use.
Small loop antennas
Small loop antennas can be likened to coils, as they have the
same current distribution as ordinary 'circuit' coils, having the
same phase and amplitude through the whole coil. To achieve this,
the total length of the conductor used in the loop antenna must be
no more than about 0.1 wavelengths long. Any longer than this and
the current phase and amplitude will start to vary over the length
of the conductor and some of the properties start to change.
Small loop antennas may also be split into those that use a
single turn, and those that have a multi-turn loop, as in the case
of a coil. One common form of multi-turn small loop antenna is the
popular ferrite rod antenna that is used in many domestic portable
radios and is also starting to be used in applications such as RFID
devices. Another form of this antenna was the frame antenna or
aerial found in many domestic radio sets of the 1940s and 1950s.
Here a multi-turn coil about 30 centimeters or more square was
built into the set to act as the antenna.
Multi-turn loop antennas are nor normally used for transmitting
because the losses are high and the level of heat dissipated can
give rise to rapid temperature increases. Instead single turn loop
antennas may be used if a loop antenna is needed. These antennas
have a number of advantages and disadvantages.
The main advantages of loop antennas are their size and
directivity. Often a single turn small loop antenna is much smaller
than a wavelength by its definition. They are also quite directive,
and this can be used to direct the radiated power in the required
direction. Both these advantages can be very useful in many
applications. They find uses for transmitting and receiving,
particularly on the MF and HF or short wave bands. Here they
provide very compact antennas for applications such as amateur
radio and shipping, etc. as well as receiving antennas for MF or
medium wave receivers.
There are naturally disadvantages to thes