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Coaxial Feeder / RF Coax Cable Tutorial
- coax cable or RF coaxial feeder is a form of RF feeder -
it
offers a relativly low loss, while remaining rugged and
flexible.
Coax cable tutorial includes:
Coaxial feeder overview Coax specifications Coax impedance Loss
or attenuation Coax power rating Coax velocity factor Coax
environmental factors Coax types & data Coaxial installation
tips
The most common type of antenna feeder used today is undoubtedly
coaxial feeder or coax cable. Coax cable, often referred to as RF
cable, offers advantages of convenience of use while being able to
provide a good level of performance. In view of this vast amounts
of coax cable, coax feeder are manufactured each year, and it is
also available in a wide variety of forms for different
applications.
Applications of coax cable Coax cable or coaxial feeder is used
in many applications where it is necessary to transfer radio
frequency energy from one point to another. Possibly the most
obvious use of coax cable is for domestic television down-leads,
but it is widely used in many other areas as well. While it is sued
for domestic connections between receivers and aerials, it is
likewise also used for commercial and industrial transmission lines
connecting receivers and transmitters to antennas. However it is
also sued where any high frequency signals need to be carried any
distance. Its construction means that signals that the levels of
loss and stray pick-up are minimised. In view of this it is also
used in many computer applications. Coax cable was used for some
early forms of Ethernet local area networks, although now optical
fibres are used for higher data rates, or twisted pairs where
frequencies are not so high as these cables are much cheaper than
coax.
RF coax cable history RF coaxial cable is a particularly
important part of today's RF and electronics scene. It is a
component that could easily be overlooked with little thought of
how it appeared. In the late 1800s there were a huge number of
basic discoveries being made in the field of electricity. Radio, or
wireless as it was originally called was not understood well, and
the first transmissions were made in the 1890s. Some transmissions
were made earlier but not understood.
The first known implementation of coax cable was in 1884 when
Ernst von Siemens (one of the founders of the Siemens empire)
patented the idea, although there were no known applications
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at this time. It then took until 1929 before the first modern
commercial coax cables were patented by Bell Laboratories, although
its use was still relatively small. Nevertheless it was used in
1934 to relay television pictures of the Berlin Olympics to
Leipzig. Then in 1936 an a coaxial cable was installed between
London and Birmingham in the UK to carry 40 telephone calls, and in
the USA an experimental coaxial cable was installed between New
York and Philadelphia to relay television pictures.
With the commercial use of RF coax cable establishing itself,
many other used the cable for shorter runs. It quickly established
itself, and now it is widely used for both commercial and domestic
applications.
What is coax cable? - the basics Coax cable, coaxial feeder is
normally seen as a thick electrical cable. The cable is made from a
number of different elements that when together enable the coax
cable to carry the radio frequency signals with a low level of loss
from one location to another. The main elements within a coax cable
are:
1. Centre conductor
2. Insulating dielectric
3. Outer conductor
4. Outer protecting jacket or sheath
The overall construction of the coax cable or RF cable can be
seen in the diagram below and from this it can be seen that it is
built up from a number of concentric layers. Although there are
many varieties of coax cable, the basic overall construction
remains the same:
Cross section though coaxial cable
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1. Centre conductor The centre conductor of the coax is almost
universally made of copper. Sometimes it may be a single conductor
whilst in other RF cables it may consist of several strands.
2. Insulating dielectric Between the two conductors of the coax
cable 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 coax cable 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.
3. Outer conductor The outer conductor of the RF cable is
normally made from a copper braid. This enables the coax 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 coax
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.
4. Outer protecting jacket or sheath Finally there is a final
cover or outer sheath to the coax cable. 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, and prevent the coax cable from being damaged
by other mechanical means.
How RF coax cable 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 coax cable it is not affected by nearby objects.
Accordingly it is ideal for applications where the RF 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.
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Coax Cable Specifications & Parameters
- definitions and explanations of the variety of
specifications
and parameters used to define the performance of a type of
coax cable.
When choosing a type of coax cable to be used, it is necessary
to understand its performance. Coax cable specifications define the
performance so that decision can be made about which type to use
for a given application.
In order to understand the performance of the coaxial cable it
is necessary to understand the specifications for the different
parameters.
Characteristic impedance specification Possibly one of the most
defining coax cable specifications is its characteristic impedance.
This is the impedance seen looking into an infinitely long length
of cable by a signal source. The dimensions of the cable along with
the dielectric used determine the overall impedance. This
specification is measured in ohms and is resistive.
The most common impedance figures are:
50/52 ohms : This cable is the form that is generally used for
professional RF applications. 75 ohms: This impedance is more
widely used in domestic applications for television and
hi-fi RF signal leads. 93 ohms: Coax with this impedance
specification was used in many early computers,
linking the computers themselves and also monitors. It was used
because of its low capacitance level.
Other values of impedance are available although they are
considerably less widely used. Some searching may be required to
locate coaxial cable with an unusual impedance level.
Read more about characteristic impedance
Loss / attenuation specification Another major parameter for
coaxial cable is its loss or attenuation. It is found that there is
a degree of loss as a signal travels along a coax cable. This
arises from a number of factors and is present on all cables. It is
also proportional to the length.
The coax loss or attenuation parameter is specified in terms of
a loss over a given length. It is generally specified in terms of a
loss measured in decibels over a given length, e.g. 0.5dB / 10
metres.
Unfortunately not all manufacturers define the loss over the
same length and therefore comparisons take a little more
calculation to determine.
Read more about coax cable loss
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Power rating specification Although for low level signal
applications the power rating is unlikely to be important, where
higher power levels are being carried, this specification can be an
issue. Normally the limiting factor arises from the heat loss
within the cable. If the power in the RF cable is to be pulsed,
then it is necessary to check that the operating voltage is not
exceeded.
Read more about power rating
Velocity factor specification The velocity factor specifications
of a coaxial cable is the speed at which the signal travels within
the cable compared to the speed of the signal (i.e. speed of light)
in a vacuum.
In some instances, the velocity factor specification for the
coax cable may be of importance. For many areas where the coax is
simply being used for feeding signals from one point to another, it
will not be important.
For applications where the phase of the signal is of importance,
the velocity factor needs to be known.
The velocity factor specification is quoted as a figure which is
less than "1". It cannot go above unity otherwise signals would be
travelling faster than the speed of light.
It is found that cables have very similar velocity factor
figures. This is because the dielectric between the two conductors
governs the velocity factor. Cables using a solid polyethylene
dielectric will have a velocity factor around 0.66, and those using
foam polyethylene will have velocity factor figures ranging from
about 0.80 to 0.88.
Read more about velocity factor
Capacitance specification For some applications the capacitance
specification of the coax cable will be important. As can be
imagined, there is a capacitance between the inner and outer
conductors of the cable, and this is proportional to the length of
cable used as well as the dielectric constant and the inner and
outer conductor diameters.
Read more about coax capacitance
Maximum voltage In some applications the voltage may rise to
high levels. At some voltage it is possible the cable may break
down, causing damage to the cable itself.
Voltages can arise as a result of high levels of standing waves
and high power levels. Checks should be made, before selecting a
particular type of coax, that it will be able to withstand the
level of voltage anticipated.
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Coax mechanical dimensions specification The mechanical
dimensions specification of the coax is important for a variety of
reasons. The dimensions of different coax cables are obviously
often different. Larger diameter coax cables often tend to have
lower loss levels and higher power ratings.
As cable size may be important to ensure that it fits apertures
etc this may be an issue. However one of the major reasons to know
the size is to ensure that correct terminating connectors can be
used. As connectors need to have the correct dimensions to ensure
the cable will fit with the connector correctly, it is necessary to
know the dimensions of cable. Often connectors will be made
specifically for a popular size of cable.
Coax Impedance / Coaxial Cable
Impedance
- details of the calculation, application and determination
of
coax impedance - coax cable impedance, inductance and
capacitance.
All forms of feeder including coax cable have a characteristic
impedance.
The coax impedance is one of the main parameters in its
specification, one that governs which type of coax cable is
obtained.
Coax impedance / characteristic impedance All feeders possess a
characteristic impedance. For RF coax cable there are two main
standards that have been adopted over the years, namely 75 and 50
ohms.
75 ohm coax cable is used almost exclusively for domestic TV and
VHF FM applications. However for most commercial RF applications 50
ohms coax cable has been taken as the standard for many years.
The reason for the choice of these two impedance standards is
largely historical but arises from the properties provided by the
two impedance levels:
75 ohm coax cable gives the minimum weight for a given loss 50
ohm coax cable 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 RF cable is
extensively used in magnetic core broadband transformers. These
values and more are available through specialist coax cable
suppliers.
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Coax impedance background When analysed a coaxial cable can be
considered as a distributed series inductance with a distributed
capacitance between the inner and outer conductors. The levels of
inductance can be calculated as seen below.
Coax capacitance The capacitance of a coaxial 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 coax capacitance for a given length because the
conductor spacing is decreased. The coax capacitance also increases
with increasing dielectric constant, as in the case of an ordinary
capacitor.
Where: C = Capacitance in pF / metre r = Relative permeability
of the dielectric D = Inner diameter of the outer conductor d =
Diameter of the inner conductor
Coax inductance The inductance of the line can also be
calculated. Again this is proportional to the length of the
line.
However the inductance is independent of the dielectric constant
for the material between the conductors.
Where: L = Inductance in H / metre D = Inner diameter of the
outer conductor d = Diameter of the inner conductor
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Coax impedance determination The impedance of the RF coax cable
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 of the RF coax cable has a bearing. The
relationship needed to calculate the impedance is given simply by
the formula:
Where: Zo = Characteristic impedance in r = Relative
permeability of the dielectric D = Inner diameter of the outer
conductor d = Diameter of the inner conductor
Note: The units of the inner and outer diameters can be anything
provided they are the same, because the equation uses a ratio.
Coax Impedance Calculator
Enter Values: Outer diameter:
Inner diameter: same units as above.
r:
Clear Data
Results: Impedance:
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Importance of coax impedance The coax impedance is one of the
major specifications associated with any piece of coax cable. As it
will determine the matching within the system and hence the level
of standing waves and power transfer, it is a crucial element. It
is therefore necessary to ensure that the correct coax impedance is
chosen for any system.
Coax cable attenuation / loss
- an overview of the effects and causes of attenuation or
loss
in coax cable.
Attenuation is a key specification for all coax cables. The
function of a coax cable is to transfer
radio frequency power from one point to another. In doing so, in
the ideal world, the same
amount of power should exit from the remote end of the coax
cable as enters it. However in the
real world this is not so, and some power is lost along the
length of the RF cable, and less power
reaches the remote end than enters the RF cable.
Coax cable attenuation
The power loss caused by a coax cable is referred to as
attenuation. It is defined in terms of decibels per unit length,
and at a given frequency. Obviously the longer the coax cable, the
greater the loss, but it is also found that the loss is frequency
dependent, broadly rising with frequency, although the actual level
of loss is not linearly dependent upon the frequency.
For virtually all applications the minimum level of loss is
required. The power is lost in a variety of ways:
Resistive loss Dielectric loss Radiated loss
Of all these forms of loss, the radiated loss is generally the
least important as only a very small amount of power is generally
radiated from the cable. Accordingly most of the focus on reducing
loss is placed onto the conductive and dielectric losses.
Resistive loss: Resistive losses within the coax cable arise
from the resistance of the conductors and the current flowing in
the conductors results in heat being dissipated. The actual area
through which the current flows in the conductor is limited by the
skin effect, which becomes progressively more apparent as the
frequency rises. To help overcome this multi-stranded conductors
are often used. To reduce the level of loss due in the coax cable,
the conductive area must be increased and this results in low loss
coax cables being made larger. However it is found that the
resistive losses increase as the square root of the frequency.
Dielectric loss: The dielectric loss represent another of the
major losses arising in most coax cables. Again the power lost as
dielectric loss is dissipated as heat.
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It is found that the dielectric loss is independent of the size
of the RF cable, but it does increase linearly with frequency. This
means that resistive losses normally dominate at lower frequencies.
However as resistive losses increase as the square root of
frequency, and dielectric losses increase linearly, the dielectric
losses dominate at higher frequencies.
Radiated loss: The radiated loss of a coax cable is normally
much less than the resistive and dielectric losses. However some
very cheap coax cables may have a very poor outer braid and in
these cases it may represent a noticeable element of the loss.
Power radiated, or picked up by a coax cable is more of a problem
in terms of interference. Signal radiated by the coax cable may
result in high signal levels being present where they are not
wanted. For example leakage from a coax cable carrying a feed from
a high power transmitter may give rise to interference in sensitive
receivers that may be located close to the coax cable.
Alternatively a coax cable being used for receiving may pick up
interference if it passes through an electrically noisy
environment. It is normally for these reasons that additional
measures are taken in ensuring the outer screen or conductor is
effective. Double, or even triple screened coax cables are
available to reduce the levels of leakage to very low levels.
Coax cable attenuation with time
It is found that the attenuation of coax cables increases over a
period of time for a number of reasons. The main reasons are as a
result of flexing, and moisture entry into the RF cable. As the
degradation and increase in loss depends to some degree on the
construction of the coax cable, this may affect he choice of which
cable to employ.
Although many coax cables are flexible, the level of loss or
attenuation will increase, particularly if the RF cable is bent
sharply, even if within the makers recommended bend radius. This
increase in loss can arise as a result of disruption to the braid
or screen, and as a result of changes to the dielectric. At
frequencies of 1 GHz with RF cables normally exhibiting a loss of
10 dB, there could be an increase of a decibel or so.
Even if a cable is not flexed, there can be a gradual
degradation in performance over time. This can be caused by
contamination of the braid by the plastictisers in the outer
sheath. Additionally moisture penetration can affect both the braid
where it causes corrosion, and it may enter the dielectric where
the moisture will tend to absorb power.
It is found that the loss in coax cables that use either bare
copper braid, or tinned copper braid exhibit more degradation than
those with silver plated braids, although the later are more
expensive. Additionally it is found that braids using tinned copper
exhibit about 20% greater loss than those using bare copper, but
they are more stable over time.
The dielectric also has an effect. It is found that some
versions of polyethene can absorb moisture more readily than other
types. Although foam polyethene offers a lower level of loss or
attenuation when new, it absorbs moisture more readily than the
solid types. Accordingly coax cables with solid dielectric
polyethene are more suited to environments where the level of loss
needs to remain constant, or where moisture may be encountered.
Although RF cables are enclosed in a plastic sheath, many of the
plastics used will allow some moisture to pass through them. For
applications where moisture may be encountered, specialized cables
should be used otherwise the performance will degrade.
The loss introduced by a coax cable is of paramount importance.
Any power that is lost in the RF cable will degrade the performance
of the system in which it is used. However the decision of which RF
cable to use may not just rest in deciding which cable provides the
lowest loss, but in a variety of parameters including its size,
weight and also its long term stability
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RF coax cable power rating
- an overview of the maximum power levels that an RF coax
cable may be able to carry.
Although for receiver applications the level of power that coax
cable can handle is not an issue,
when it comes to medium or high power transmitters, the power
handling capacity of the coax
cable becomes very important. If the incorrect type of coax
cable is used, it can result in a failure
of the cable, and possible damage to the transmitter.
For most applications where the power is applied continuously,
the limiting factor arises from the heat loss within the cable. If
the power in the RF cable is to be pulsed, then it is necessary to
check that the operating voltage is not exceeded.
RF coax cable power loss
The major root cause for the limit in power handling capacity of
an RF cable is the level of heat caused by the power losses
occurring in the cable. If the temperature of the coax cable rises
too high, the cable my become deformed and be permanently
damaged.
For the resistive losses in the coax cable, it is found that
most of the heat is generated in the centre conductor. Additionally
any heat generated as a result of dielectric losses will be
dissipated within the dielectric. It is therefore the construction
of the dielectric that is of key importance in determining the
power handling capability of the coax cable. Its maximum operating
temperature, and its heat transfer coefficient both have a major
effect.
It can be seen that the lower the losses of the cable the
smaller the temperature rise, and the greater the power handling
capability is for the cable. As a broad rule of thumb, lower loss
cables will have a higher power rating than higher loss RF
cables.
RF coax cable derating
Although a power rating may be given for a particular coax
cable, it is often necessary to de-rate it to cater for non-optimal
operating conditions.
The temperature of the environment is one factor. If the coax
cable is operating in a high temperature environment, it will not
be able to dissipate as much heat, and therefore the operating
temperature will rise. Even at the highest foreseeable
environmental operating temperature, the RF cable must be able to
remain within its maximum internal temperature. Accordingly a
de-rating factor is normally applied if the coax cable is to be
used at high temperatures.
If the coax cable is operated under conditions where the VSWR is
high, the cable rating needs to be reduced. The reason for this is
that when there is a high level of VSWR, there are positions of
high and low current along the coax cable. These may be such that
they cause the power dissipation to rise significantly in some
areas causing higher levels of power to be dissipated locally.
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Altitude also has an effect, although at significant heights. If
the cable is to be operated at altitude and hence under reduced
pressure, the any cooling will be less effective. Therefore the
temperature rise within the cable will be greater.
High power coax cables For coax cables where high powers are
likely to be used, specially constructed cables are needed. If
cables using ordinary polyethylene were used, then the might higher
temperatures encountered would soon melt and distort the cable.
This would then lead to its destruction.
For very high power applications, typically cables are used that
possess an air dielectric and employed. The centre conductor is
then held in place by a form of coil that runs along the length of
the cable.
For medium to high power coax cables a Teflon dielectric can be
used. This can withstand high temperatures of typically up to
around 160C.
When considering which cable to use, it should be remembered
that as the frequency increases, so the skin effect becomes more
pronounced, and coupled with increased losses in the dielectric,
this limits the power handling capacity.
Although the power handling capability of RF coax cable may not
be an issue for many installations, when using medium or high power
transmitters the power rating or handling capability of RF coax
cable needs to be carefully considered.
Coax Cable Velocity Factor
- the velocity factor of a coax cable is the speed an
electromagnetic wave travels along a coax cable relative to
the
speed in a vacuum.
The speed at which a signal travels within a coax cable is not
the same as an electromagnetic
wave travelling in free space.
Instead it is affected by the dielectric that is used within the
coax cable, and this has the effect of slowing the signal down.
The velocity factor can be of great importance in some
applications, although for many purposes it does not need to be
known.
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What is velocity factor? The speed at which the signal travels
is normally given the designation Vp or Vg and this is the faction
of the speed at which the signal travels when compared to a signal
travelling in free space. Thus Vp for a signal travelling at the
speed of light would be 1.0, and for one travelling at half the
speed of light it would be 0.5.
The velocity factor of the cable is found to the reciprocal of
the square root of the dielectric constant:
Coax cable electrical length One important factor of a coax
cable in some applications is the wavelength of the signals
travelling in it. In the same way that the wavelength of a signal
is the speed of light divided by the frequency for free space, the
same is also true in any other medium. As the speed of the wave has
been reduced, so too is the wavelength reduced by the same factor.
Thus if the velocity factor of the coax cable is 0.66, then the
wavelength is 0.66 times the wavelength in free space.
In some instances lengths of coax cable are cut to a specific
length to act as an impedance transformed or a resonant circuit,
then this needs to be taken into consideration when determining the
required length of coax cable.
The advantage of using a coax cable with a low velocity factor
is that the length of coax cable required for the resonant length
is shorter than if it had a figure approaching 1. Not only does
this save on cost, but it can also be significantly more convenient
to use and house.
Dielectric materials There is a variety of materials that can be
successfully used as dielectrics in coax cables. Each has its own
dielectric constant, and as a result, coax cables that use
different dielectric materials will exhibit different velocity
factors.
Dielectric constants and velocity factors
of some common dielectric materials used in
coax cables
Material Dielectric
constant
Velocity
factor
Polyethylene 2.3 0.659
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Dielectric constants and velocity factors
of some common dielectric materials used in
coax cables
Material Dielectric
constant
Velocity
factor
Foam polyethylene 1.3 - 1.6 0.88 - 0.79
Solid PTFE 2.07 0.695
From this it can be seen that the velocity factor of coax cables
that use a polyethylene dielectric will have a velocity factor of
0.66 or thereabouts.
If resonant lengths of RF coax cable are to be used, then it is
necessary to know the velocity factor of the coax cable. It is
often possible to determine this to a sufficient degree of accuracy
from a knowledge of the dielectric material.
Coax cable environmental resistance
- an overview of the environmental elements associated with
coaxial cables and the precautions to take when using them.
Coax cable, or as it is sometimes called RF cable can be
expensive, but it is also required to
withstand some harsh environmental conditions. In view of its
cost, care needs to be taken
otherwise the performance of the coax cable will deteriorate and
the RF cable will require
replacement. By taking suitable precautions, the life of the
coax cable can be maximised and the
periodic replacement costs for RF cables can be reduced.
There are many factors that affect coax cables to greater or
lesser degrees:
Humidity and water vapour Sunlight Corrosive vapours and
liquids
Effect of humidity and water vapour on coax
cables One of the biggest enemies for coaxial cable is that of
water vapour. If it enters a coax cable then it can significantly
degrade its performance, requiring the cable to be replaced.
Moisture causes two main effects that give rise to an increase in
the level of attenuation or loss in the cable. The first is an
increase in resistive loss arising from oxidation of the braid that
gives rise to an increase in the resistance of the braid or outer
conductor in the coax cable. The second is an increase in the loss
arising in the dielectric. Water absorbed into the dielectric heats
up when power is passed along the coax cable. This heat is as a
result of power loss in the cable.
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Water vapour, or even water itself can enter the coax cable
through a number of ways:
1. Through the termination of the coax cable (i.e. through
connector or other termination method)
2. Through pin-holes in the jacket 3. By water vapour
transmission through the jacket.
1. Moisture entry through coax cable termination The most
obvious method of humidity entering a coax cable is through the
termination. One very good example is the small termination box
provided with many TV antennas. When used externally these
termination enclosures provide little protection against the
elements and the coax will quickly deteriorate. Even when a
connector is used to terminate the coax cable there will be
problems if the coax is used externally. Very few connectors are
weather proofed, and even if they are supposedly weatherproof, then
it is wise to take additional precautions.
Normally the best method is to use self-amalgamating tape. This
tape comes in the form of a roll and appears like thick PVC tape
but it has a thin paper backing on one side to keep each layer
separate and prevents it amalgamating with itself before use. It is
used in a similar way to insulating tape. The backing strip is
peeled off and then it is wrapped around whatever it is to be
waterproofed - in this case a coax cable termination - overlapping
each winding by about 50% of its width to ensure a good seal. When
applying the tape keep it stretched so that it is applied under
tension. Also it is best to start from the thinner end of the job,
i.e. where the diameter of whatever it is being applied to is
smallest. Where there is a connector on a cable, start on the cable
and work towards the connector. Also when winding ensure that there
are no holes of voids in which water could condense or enter. Keep
the self-amalgamating tape in intimate contact with whatever it is
to be waterproofed.
2. Moisture entry though pin holes in coax cable jacket Most
coax cables have some small holes in their jacket along their
length. It is therefore quite possible that moisture will enter
through any imperfection in the coax cable jacket. If the pin holes
are located externally where they can be affected by the weather
then moisture will enter. Unfortunately it is very easy for small
abrasions to occur during the installation of a cable and these can
include small pin holes right through the jacket. Great care must
therefore be taken when installing a cable, and in particular when
the coax cable is passed trough a wall or other barrier.
3. Water vapour transmission through the coax cable jacket All
materials exhibit a finite vapour transmission rate. Accordingly if
a coax cable is constantly in contact with moisture, then this will
permeate through the jacket. In view of this coax cable should
never be buried directly in the ground. Either use some external
protection such as a waterproof pipe, and ensure that no water
enters it so that small patches of water form in it. Alternatively
use the "bury direct" cables that are available. It is also found
in airborne applications that the large temperature extremes
encountered cause water condensation in the coax cables. This
moisture can collect in low areas of the cable causing local areas
of corrosion. One method of overcoming this is to fill any voids in
the aircraft where coax cable are carried with non-hardening
moisture-proof compound.
Effect of sunlight on coax cables Sunlight has an effect on many
substances, and the same is true of coax cable jackets or sheaths.
It is particularly the ultra-violet light that causes the
degradation to the cables. To increase the life of coax cables,
manufacturers use high molecular weight polythene.
Polyvinylchloride (PVC) jackets exhibit less than half the life
expectancy of the high molecular weight polythene.
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Effect of corrosive vapours on coax cables Using a coax cable in
the vicinity of corrosive liquids and vapours can reduce the life
of a cable faster than if it was used externally. Salt water is a
common problem on sea going vessels, and chemical vapours may be
present on other installations requiring coax cables. Although the
rigours of the weather can be very tough, some vapours and liquids
can speed the deterioration of the coax cable even faster. The use
of tin or silver coatings can provide some additional protection
but this is not permanent. However it is recommended that specially
environmentally hardened cables be used where extreme conditions
are anticipated.
Coax cables are normally quite tolerant to being used in a
variety of conditions. However to ensure the longest operational
life it is best to ensure that they are not exposed to
environmental conditions that would cause their performance to
deteriorate. If they are then it is necessary to adopt a few
precautions to ensure that the coax cable life is maintained for as
long as possible.
RF Coax Cable Types & Data
- table data including size, impedance, loss propagation
constant, etc for the more commonly used types of RF coax
cable.
There is a variety of different types of coax or coaxial cable
that are in widespread use. Different types of coax cable or feeder
are needed for different purposes and applications and accordingly
it is necessary have specifications and data to be able to
determine the required coax type or RF cable type easily.
While it would be possible to manufacture an infinite variety of
RF cables, standard varieties are specified. There are two basic
systems that are used for defining RF cables. One originated in the
United Kingdom and its type numbers all start with UR. The other
system is American with type numbers commencing with the letters
RG.
The RG series was originally used to specify the types of coax
cables for military use, and the specification took the form RG (RG
from Radio Guide) plus two numbers. In some instances these numbers
were followed by the letter U which indicated it was for multiple
uses. These types of coax cable were all listed in the MIL-HDBK-216
which is now obsolete. Although full MIL specifications are now
officially used for specifying most components for military use,
the RG series of RF cables continued to be used because of its
widespread acceptance. However it should be noted that the RG
specifications are no longer maintained so there is no complete
guarantee to the exact specification for the particular type of
coax cable.
A summary of data for some of the more commonly used types of
coax or coaxial cable is given below. Most of these RF cables are
easily available from RF cable stockists.:
-
TABLE OF DATA FOR COMMON COAX CABLE TYPES
COAX
TYPE
CHARACTERISTIC
IMPEDANCE
OUTSIDE
DIAMETER
VELOCITY
FACTOR
ATTEN
@ 100
MHZ
ATTEN
@
1000
MHZ
COMMENTS
RG5/U 52.5 8.4 0.66 1.0 3.8
RG6A/U 75 8.4 0.66 1.0 3.7
RG9/U 51.0 10.7 0.66 0.66 2.4
RG10A/U 50 12.1 0.66 0.66 2.6
RG11A/U 75 10.3 0.66 0.76 2.6
RG12A/U 75 12.1 0.66 0.76 2.6
RG20A/U 50 30.4 0.66 0.22 1.2
RG22 95 10.7 0.75 1.5
RG23 125 24.0 0.52 2.0
RG24 125 25.5 0.52 2.0
RG34 75 16.0 0.46 1.8
RG58C/U 50 5.0 0.66 1.8 7.6
RG59B/U 75 6.1 0.66 1.2 4.6
RG62A/U 93 6.1 0.84 0.9 2.8
RG63 125 10.3 0.6 2.1
RG79 125 12.1 0.6 2.1
RG108 78 6.0 1.1 3.8
RG111 95 12.1 0.75 2.6
RG114 185 10.3 1.1 3.8
RG119 50 11.8 0.5 1.8
-
TABLE OF DATA FOR COMMON COAX CABLE TYPES
COAX
TYPE
CHARACTERISTIC
IMPEDANCE
OUTSIDE
DIAMETER
VELOCITY
FACTOR
ATTEN
@ 100
MHZ
ATTEN
@
1000
MHZ
COMMENTS
RG120 50 13.3 0.5 1.8
RG122 50 4.1 1.7 5.5
RG213/U 50 10.3 0.66 0.62 2.6 Polythene
dielectric
RG214/U 50 10.8 0.66 0.76 2.9 Double
screened,
silver plated
copper wire
RG223/U 50 5.5 0.66 1.58 5.4
UR43 50 5 0.66 1.3 4.46
UR57 75 10.2 0.66 0.63 2.3 Similar to
RG11A/U
UR67 50 10.3 0.66 0.66 2.52 Similar to
RG213/U
UR74 51 22.1 0.66 0.33 1.4
UR76 51 5 0.66 1.7 7.3 Similar to
RG58C/U
UR77 75 22.1 0.66 0.33 1.4
UR79 50 21.7 0.96 0.17 0.6
UR90 75 6.1 0.66 1.2 4.1 Similar to
RG59B/U
Data for attenuation figures are typical figures and measured in
dB / 10 metres dimensions in mm
The RF cables described above are all flexible cable types. For
microwave applications where very low loss is needed, semi rigid
coaxial RF cable using a solid copper outer sheath may be used.
This type of coax offers superior screening compared to RF cables
with a braided outer
-
conductor, especially at microwave frequencies. As the name
implies, though, it is not particularly flexible and is not
intended to be flexed after it has been formed to the required
shape.
This RF cable data has been presented as a guide and no
liability can be taken for any errors or mistakes in the data.
Naturally every care has been taken to ensure the data concerning
these RF cables is correct.
RF Coax Cable Installation Guide
- essential points about how to install RF coax cable and
the
key points to note to ensure the best performance is
obtained
from the coaxial cable installation.
RF coax cable is widely used for a variety of professional RF
applications where RF power either from a transmitter or to a
receiver needs to be transferred from one point to another. While
RF coaxial cable is very easy to install, it is necessary to ensure
that a number of points are observed to ensure the coax cable
installation is satisfactory initially, and then lasts. This is
particularly important because many coax cable installations are
external and need to withstand the rigours of the environment.
While coax cable may perform perfectly well when first
installed, factors such as the ingress of moisture may cause the
performance to degrade over time. Accordingly this performance
reduction may pass un-noticed until the performance has reduced to
a point where it may not be usable. By adopting a few simple
precautions, the performance of the RF coaxial cable installation
can be preserved and a much slower rate of degradation seen.
Coax cable areas to address The hints and tips to help install
coax cable can be grouped into a number of categories:
1. Choosing the right coaxial cable 2. Weatherproofing the coax
3. General installation 4. Terminations / connections for the RF
cable
Choosing the right coax cable There is an enormous variety of
coax cables on the market, and at first sight the choice may not
appear easy. The first decision to make, prior to any installation
is to choose the required impedance. Domestic hi-fi and video
antenna feeds use 75 ohm coax cable. Professional, CB, and amateur
radio standardise on 50 ohm cable. Once this choice has been made
the next decision will probably be made on the level of signal loss
that is acceptable. Typically the lower the loss, the greater the
diameter of the cable, and also its cost. Typically there are
several cables with similar performance figures and often the
decision of the exact type number will
-
depend on the stock position of suppliers. Once a suitable cable
has been found then it can be purchased and installed.
Weatherproofing the coaxial cable When installing coax cable
externally it is very important to ensure the cable is adequately
weatherproofed. This is critical because any moisture entering the
RF coax cable will produce a considerable increase in the level of
loss. 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 oxidise, 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.
An additional method of preventing large amounts of moisture
entering the cable is to loop it up and down. In this way it is
more difficult for water to enter the cable and then move along it.
However if some moister enters the cable it will move into it by
capillary action, so it is always best to ensure that the ends are
properly sealed and protected.
General installation tips for RF coax cable All cables have a
bend radius. In order to prevent damage they should not be bent
into curves tighter than this. If RF coax cable 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.
In a similar line, care should be taken to ensure that the cable
is not crushed, or likely to be crushed. If the RF cable does
suffer damage in this way, the dimensions of the cable will be
changed and it will not maintain its characteristic impedance.
Additionally if the dielectric between the two concentric
conductors in the coax cable is damaged, then there is the
likelihood of an increase in the level of loss.
While on the subject of physical damage to the cable, it is
necessary to ensure that the sheath of the cable remains intact. If
it is broken in any place, then this may allow moisture to enter if
it is used externally, and this will cause oxidation and moisture
retention in the dielectric that will increase the level of
loss.
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. However ensure that the conduit does
not become water logged. Alternatively solution to using some form
of conduit is to use a form of coax cable known as "bury direct".
This is designed for being buried, and its outer sheath can
withstand these conditions.
Coax terminations / connections When installing RF coax cable,
it is important to terminate the cable correctly. In most instances
the coax cable will be physically terminated using an RF connector,
the electrical termination
-
being either at the antenna or in the receiver. Accordingly the
connections to the connectors must be made correctly and the right
quality RF connectors should be used.
Although connectors for domestic installations are often poor in
terms of their electrical radio frequency performance, there is
little alternative to using them in view of the fact that they have
to mate with the RF connectors on the equipment. For professional
applications, RF connectors can be far better, although it is
necessary to ensure that the connectors are suitable for the
frequencies used. Some cheap versions of RF connectors may not meet
the full specification and can thereby impair the performance of
the RF coax cable. It is therefore wide to always buy connectors
from reputable sources.
By correctly installing a RF coax cable it can provide many
years of satisfactory service. However wear, and exposure to the
elements will mean that after some time it may be prudent to
replace the RF coax cable. As the degradation in performance will
be slow, it may mean that this is not noticed. Only when it is
ultimately replaced will a major difference be seen.
Coaxial Feeder / RF Coax Cable Tutorial- coax cable or RF
coaxial feeder is a form of RF feeder - it offers a relativly low
loss, while remaining rugged and flexible.Applications of coax
cableRF coax cable historyWhat is coax cable? - the basicsHow RF
coax cable works
Coax Cable Specifications & Parameters- definitions and
explanations of the variety of specifications and parameters used
to define the performance of a type of coax cable.Characteristic
impedance specificationLoss / attenuation specificationPower rating
specificationVelocity factor specificationCapacitance
specificationMaximum voltageCoax mechanical dimensions
specification
Coax Impedance / Coaxial Cable Impedance- details of the
calculation, application and determination of coax impedance - coax
cable impedance, inductance and capacitance.Coax impedance /
characteristic impedanceCoax impedance backgroundCoax
capacitanceCoax inductanceCoax impedance determinationCoax
Impedance CalculatorImportance of coax impedance
Coax cable attenuation / loss- an overview of the effects and
causes of attenuation or loss in coax cable.Coax cable
attenuationCoax cable attenuation with time
RF coax cable power rating- an overview of the maximum power
levels that an RF coax cable may be able to carry.RF coax cable
power lossRF coax cable deratingHigh power coax cables
Coax Cable Velocity Factor- the velocity factor of a coax cable
is the speed an electromagnetic wave travels along a coax cable
relative to the speed in a vacuum.What is velocity factor?Coax
cable electrical lengthDielectric materials
Coax cable environmental resistance- an overview of the
environmental elements associated with coaxial cables and the
precautions to take when using them.Effect of humidity and water
vapour on coax cablesEffect of sunlight on coax cablesEffect of
corrosive vapours on coax cables
RF Coax Cable Types & Data- table data including size,
impedance, loss propagation constant, etc for the more commonly
used types of RF coax cable.
RF Coax Cable Installation Guide- essential points about how to
install RF coax cable and the key points to note to ensure the best
performance is obtained from the coaxial cable installation.Coax
cable areas to addressChoosing the right coax cableWeatherproofing
the coaxial cableGeneral installation tips for RF coax cableCoax
terminations / connections