Antenna

ANTENNASANTENNAS

ANTENNA

A structure that is generally a metallic object, often a wire or group of wires, used

to convert high frequency signals into electromagnetic waves and vice versa.

A device whose function is to radiate electromagnetic energy and/or

intercept electromagnetic radiation

ANTENNA

Basic Consideration:

Maximum Power

Transfer

FIELDS OF AN

ANTENNA

INDUCTION FIELD

Considered to extend out from the antenna to a

distance of πD2/8λ

Near Field or Fresnel Region

RADIATION FIELD

Considered to extend out from a distance of

2D2/λ

Far Field or Fraunhoffer Region

TRANSITION ZONE

Zone between the two(2) regions

BASIC FORMULAS

T = 1/f

Where: T – time

F - frequency

λ = c/f

Where: λ – wavelength

F – frequency

C – velocity in free space

CHARACTERISTICS OF CHARACTERISTICS OF

ANTENNASANTENNAS

RADIATION PATTERN

A line drawn to join points in space which have equal field intensity due to the source.

Major Lobe – the direction of maximum radiation

Minor Lobe – the direction of minimum radiation

Null – the direction with radiation intensity equal

to zero.

a. Antenna height

b. Power losses

d. Thickness of the antenna wire

FACTORS WHICH DETERMINE THE

RADIATION PATTERN OF AN

ANTENNAc. Terminations

at its end

PRINCIPLE OF RECIPROCITY

The characteristics of antennas, such as impedance and radiation pattern are identical, regardless of use for

reception or transmission.

ISOTROPIC ANTENNA

An antenna that radiates uniformly in all directions in space

DIRECTIVE GAIN The ratio of the power density in a particular direction of one antenna to

the power density that would be radiated by an isotropic antenna.

HERTZIAN DIPOLE: 1.5 : 1 (1.76 dB)

HALF WAVE DIPOLE: 1.64 : 1 (2.15 dB)

NOTES:

The longer the antenna, the higher the directive gain

Non resonant antennas have higher directive gain than resonant

antennas

The directive gain of all practical antennas is greater than unity

DIRECTIVITY, D

Maximum directive gain

The gain in the direction of one of the major lobes in the antenna’s radiation pattern.

POWER GAIN

Overall gain considering losses and efficiency

Ap = %D

Where: % - antenna efficiency

D - directivity

ANTENNA RESISTANCE

The ratio of the applied voltage to

the flowing current

AC resistance

The ratio of the power radiated by the

antenna to the square of the current at the

feedpoint.

1. RADIATION RESISTANCE, Rr

Antenna and ground resistance

Discharge or corona effects

Losses in imperfect dielectric very near

the antenna

Eddy current loss

2. LOSS RESISTANCE, Rd

ANTENNA EFFICIENCY

The ratio of the power radiated by the antenna to the power delivered at the feedpoint

The ratio of radiation resistance to the total system resistance

% = Rr / Rr + Rd

An antenna has a radiation resistance of 72 ohms, a loss resistance of 8 ohms, and a power gain of 16. What efficiency and directivity does it have.a. 90% and 17.78b. 10% and 17.78c. 90% and 14.4d. 10% and 14.4

To produce a power density of 1 mw/m2 in a given direction, at a distance of 2 km, an antenna radiates a total of180 w. An isotropic antenna would have to radiate 2400 w to produce the same power density at that distance. What, in dB, is the directive gain of the practical antenna?a. 11.25 dBb. 13.21 dBc. 10 dBd. 6 dB

EFFECTIVE RADIATED POWER (ERP)

The product of the power fed to an antenna and its power gain.

ERP = Total Radiated Power x Power Gain

The power radiated by an antenna in its favored direction, taking the gain of the antenna

into account as referenced to an isotropic radiator

EFFECTIVE ISOTROPIC RADIATED POWER (EIRP)

FRONT TO BACK RATIO

Ratio of the power at the optimum direction of the antenna to that

of the power 180 degrees from the

optimum direction

The operating frequency range

of an antenna

BANDWIDTH

BEAMWIDTH

Angular separation between two half power points in a major lobeof an antenna radiation pattern

The degree of concentration of the antenna’s radiation

POLARIZATION

Space orientation of the waves that the antenna

radiates

The electric field vector is always parallel to the

antenna elements.

PHYSICAL LENGTH

Actual length of the antenna

λ/2 is the shortest length of a conductor which will

resonate at a given frequency

L =λ/2; λ= c/f

ELECTRICAL LENGTH

Dependent upon the velocity coefficient or velocity factor

where k is the dielectric constant

L = k λ

Ex. What is the wavelength of a 500 MHz signal?a. 60 cmb. 6 mc. 0.06 md. 60 m

Ex. What is the electrical length of an antenna operating at a frequency of 500 kHz?a. 500 mb. 570 mc. 600 md. 630 m

GROUNDING SYSTEMSGROUNDING SYSTEMS

EFFECTS OF GROUND ON ANTENNAS

Whereas an ungrounded antenna with its image forms an antenna

array, the bottom of the grounded antenna is joined to the top of the

image; the system acts as an antenna of double size.

EFFECTS OF GROUND ON ANTENNAS

Whereas an ungrounded antenna with its image forms an antenna array, the bottom of the grounded antenna is joined to the top of the image; the system acts as an antenna of

double size.

GROUND SCREEN

A network of buried wires directly under the antenna, consisting of a large number of radials extending

from the base of the tower, like spokes on a wheel, and placed 15 and 30 cm

below the ground.

COUNTERPOISE

A substitute for ground screen in areas of low conductivity, i.e. rock, mountains and antennas on top of

buildings

ANTENNA HEIGHTANTENNA HEIGHT

ANTENNA HEIGHT

The actual antenna height should at least be λ/4, but where this is not

possible, the effective height should correspond to λ/4.

TOP LOADING

A good method of increasing radiation resistance by having a horizontal portion at the top of the antenna

Effect: to increase the current at the base of the antenna and to make the current distribution more uniform

EFFECTIVE LENGTH

Antennas behave as though (electrically) they were longer than

their physical length

The result of physical antennas having finite thickness, instead of being

infinitely thin.

END EFFECT

ANTENNA COUPLING AND ANTENNA COUPLING AND

IMPEDANCE MATCHINGIMPEDANCE MATCHING

ANTENNA COUPLING

A network composed of reactances and transformers, which may be lumped or distributed, to provide

impedance matching

To tune out the reactive component of the antenna impedance

REASONS FOR

COUPLING To provide the transmitter with the correct value of load resistance

To prevent illegal radiation of spurious frequencies

ANTENNA COUPLERS

The antennas are coupled directly to their transmitters

Affords a wider reactance range, giving adequate harmonic suppression

Used for balanced lines

Direct Coupler

π Coupler

Symmetrical πCoupler

IMPEDANCE MATCHING

Accomplished by connecting the coax or twin lead to the stub and sliding the connections up or down the stub until

the proper SWR is indicated by a meter connected in the system.

Stub Matching

IMPEDANCE MATCHING

Accomplished by spreading the ends of the feedline and adjusting

the spacing until optimum performance is reached.

Delta MatchingDelta Matching

A sliding clamp is included in the assembly to permit fine

tuning for minimum SWR at the time of installation

Gamma Matching

IMPEDANCE MATCHING

A section of transmission line one quarter wavelength

long placed between the load and the line

Quarter Wave

Matching

BALUN Used to connect an unbalanced (coaxial line) to a balanced antenna

IMPEDANCE MATCHING

CURRENT FED (LOW Z FEED)

An antenna is said to be current fed if it is fed at the point of current maximum

Includes all feed point impedances below 600 ohms

Ex. Center fed half wave dipole or Marconi antenna

VOLTAGE FED (HIGH Z FEED)

An antenna is said to be voltage fed if it is fed at the point of voltage maximum

Includes all feed point impedances in excess of 600 ohms

Ex. Center fed full wave dipole

REVIEW QUESTIONSREVIEW QUESTIONS

1. Device that converts high frequency current into electromagnetic waves.

a. antennab. loudspeakerc. microphoned. lightning arrester

2. A polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna.

a. Venn Diagramb. Figure 8 patternc. Lissajous figured. Radiation Pattern

3. Refers to the orientation of the electric field radiated from an antenna.

a. radiationb. polarizationc. beamwidthd. bandwidth

4. Pertains to a wire structure placed below the antenna and erected above the ground which is a form of capacitive grounding system.

a. imageb. counterpoisec. antenna orientationd. polarization

5. What is the technique used to electrically increase the antenna length?

a. loadingb. using image antennac. using antenna arraysd. increasing antenna height

6. Antenna supported by insulators seems electrically longer than its physical length due to

a. imageb. reflectionc. end effectd. broadside effect

7. The ratio of the power radiated by the antenna to the total input power.

a. power gainb. directive gainc. antenna efficiencyd. radiation efficiency

8. The ratio of the front lobe power to the back lobe power

a. front to side ratiob. front to back ratioc. back to front ratiod. minor to major ratio

9. The standard reference antenna for directive gaina. infinitesimal dipoleb. isotropic antennac. elementary doubletd. half wave dipole

10. The gain of a hertzian dipole with respect to an isotropic antenna

a. 1.76 dBb. 2.15 dBc. 1.5 dBd. 1.64 dB

11. A half wave dipole antenna is capable of radiating 2000 watts and has a 2.15 dB gain over an isotropic antenna. How much power must be delivered to the isotropic antenna to match the field strength of the directional antenna?a. 1640 wattsb. 3280 wattsc. 4300 wattsd. 3520 watts

12. An ungrounded antenna near the grounda. acts as a single antenna of twice the heightb. is unlikely to need a ground screenc. acts as an antenna arrayd. must be horizontally polarized

13. Top loading is sometimes used with an antenna in order to increase itsa. effective heightb. bandwidthc. beamwidthd. input capacitance

14. Very low signal strength in an antenna

a. minor lobesb. nullsc. antenna patternsd. major lobes

15. A horizontal antenna is ______ polarized.a. verticallyb. horizontallyc. centrallyd. circularly

16. An antenna with unity gain.a. rhombicb. half wave dipolec. isotropicd. whip

17. What is the front to back ratio of an antenna which radiates 500 watts in a northernly direction and 50 watts in a southernly direction?a. 25000 dBb. 10 dBc. 100 dBd. 20 dB

18. Good grounding is important fora. Horizontal antennasb. Broadside arrayc. Vertical antennasd. Yagi Uda Antennas

19. If the radiated power increases 10.89 times, the antenna current increases by

a. 3.3 timesb. 6.6 timesc. 1.82 timesd. 10.89 times

20. Shortening effect of an antenna that makes it appear as if it were 5% longer

a. end effectb. flywheel effectc. skin effectd. capture effect

21. If an antenna is too short for the wavelength being used, the effective length can be increased by adding

a. capacitance in seriesb. inductance in seriesc. resistance in paralleld. resistance in series

22. Actual height of an antenna should be at leasta. 1λb. λ/2c. λ/4d. ¾ λ

23. The directivity pattern of an isotropic radiatora. figure 8b. a spherec. unidirectional cardioidd. parabola

24. A Hertz antenna is operating on a frequency of 2182 kHz and consists of a horizontal wire that is hanged between two towers. What is the frequency of its third harmonic?a. 727 kHzb. 6546 kHzc. 436 kHzd. 6.546 kHz

25. What is the gain of an antenna over a half wavelength dipole when it has 6 dB gain over an isotropic radiator?a. 6 dBb. 8.1 dBc. 3.9 dBd. 10 d

BASIC TYPES OF BASIC TYPES OF

ANTENNASANTENNAS

a standard reference antenna , radiating equally in all directions,

so that the radiation pattern is spherical.

ISOTROPIC ANTENNA ELEMENTARY DOUBLET

A theoretical antenna shorter than a wavelength used as a standard to

which all other antenna characteristics can be compared

9 = 60π le I sin θ / λr

Where

N – antenna length

r – antenna length

Le – antenna length

I – antenna current

θ – angle of axis and point of maximum radiation

Ex. An elementary doublet is 10 cm long. If the 10 MHz current flowing through it is 2 A, what is the field strength 20km away from the doublet in a direction of maximum radiation?a. 6.28 uV/mb. 62.83 uV/mc. 15.92 uV/md. 1.59 uV/m

An antenna made up of two wires bent at 90

degrees to each other so as to be in the same line and signal is fed at the center

DIPOLE

HALF WAVE DIPOLE

Length is λ/2 and radiation pattern is a toroid (bidirectional)

VOLTAGE AND CURRENT CHARACTERISTICS

NON-RESONANT ANTENNA

One in which there are no standing waves

Radiation pattern is directional

Standing waves are suppressed by the use of a correct termination to ensure that no power is reflected, so that only a forward traveling

wave will exist.

LONG WIRE ANTENNA

Lengths in the order of several wavelengths

When an antenna is 2 or more wavelengths long, it provides gain and a multilobe radiation

pattern.

When terminated at one end, it becomes unidirectional.

RHOMBIC ANTENNA

Consists of non-resonant antenna elements arranged differently, i.e. planar rhombus

Length of equal radiators = 2 to 8 ‘s

Angle of tilt: 40 to 75˚

Rt = 800 ohms

Rin = 650 to 700 ohms

Non-resonant antenna used for long distance sky wave transmission or reception of horizontally polarized

waves over distances from 200 to over 3000 miles at frequencies from 4 to 22

MHz.

RHOMBIC ANTENNA

RESONANT ANTENNA

Standing waves exist, caused by the presence of both a reflected traveling

wave and the forward wave.

Antenna whose length is a multiple of λ/4’s

HERTZ ANTENNA

An antenna system in which the ground is not an essential part

Half Wave Dipole

Half wave antenna used for frequencies above 2 MHz

MARCONI ANTENNA

Grounded Quarter Wavelength antenna

Vertical Monopole

Quarter Wavelength antenna used for frequencies below 2

MHz; omnidirectional

ANTENNA ARRAYSANTENNA ARRAYS

ANTENNA ARRAY

A radiating system consisting of individual

radiators or elements placed close together so

as to be within each other’s induction field

DRIVEN ELEMENT

Element of an array connected to the output

of the transmitter

Radiation not directly connected to the output of the

transmitter

PARASITIC ELEMENT

Receives energy through the induction field of a driven

element

REFLECTOR

A parasitic element longer than the driven element and close to it reduces signal strength in its own direction and

increases it in the opposite direction.

DIRECTOR

A parasitic element shorter than the driven

one from which it receives energy; tends to increase radiation in its

own direction

BROADSIDE ARRAY

Simplest array which consists of a number of dipoles of equal size, equally spaced along a straight line with all dipoles fed in the same phase from the same source.

Typical antenna length: 2 to 10 ‘s

Typical spacing: λ/2 or λ/4

Number of elements: dozens

ENDFIRE ARRAY

Physical arrangement is the same as that of the broadside array

The magnitude of the current in each element is still the same as in every other element, there is now a phase difference between these currents.

TURNSTILE ARRAY

Consists of two horizontal, half wave antennas mounted at right angles to each

other

YAGI UDA ANTENNA

An array consisting of a driven element and one or more parasitic elements arranged collinearly

and close together.

FOLDED DIPOLE

Single antenna which consists of 2 elements, one is fed directly and

the other coupled conductively at the

ends.

LOG PERIODIC ANTENNA

Main feature is frequency independence for both radiation resistance and pattern

Bandwidths of 10:1 are achievable with ease

Radiation patterns: uni and bidirectional

Main feature is frequency independence for both

radiation resistance and pattern

LOG PERIODIC ANTENNA

PYRAMIDAL ANTENNA

a type of log periodic antenna

It looks and works in much the same way as a standard log periodic

antenna, with one big difference: the two halves of the transmission line are

separated and positioned as a V, so each half of the transmission line is in effect a single wire transmission line.

UHF AND MICROWAVE UHF AND MICROWAVE

ANTENNASANTENNAS

PARABOLIC ANTENNA

Parabola – a plane curve defined as the locus of a point which moves so that its distance from another point (called the focus) plus its distance from a straight

line (directrix) is constant.

Works on the principle of a parabola

PARABOLIC ANTENNA

All waves coming from the source and reflected by the

parabola will travel in the same distance by the time they reach

the directrix, no matter from what point on the parabola they

are reflected.

CASSEGRAIN ANTENNA

an antenna in which the radiator is mounted at or near the surface of a

concave main reflector and is aimed at a convex secondary reflector slightly inside

the focus of the main reflector.

BEAMWIDTH

Ф = 70λ / D

Ф0 = 2 Ф

Where:

Ф0 = beamwidth between nulls, degrees

Ф = beamwidth between half power points, degrees

λ = wavelength, meters

D= mouth diameter, meters

GAIN OF A PARABOLIC ANTENNA

Ap = 6 ( D/λ)2

Where:

Ap = Power Gain

λ = wavelength, meters

D= mouth diameter, meters

Ex. Calculate the beamwidth between nulls of a 1m paraboloid reflector used at 6GHz.

a. 3.5˚b. 7˚c. 1.75˚d. 14˚

Ex. Calculate the gain of the paraboloid reflector in the previous problem.

a. 9600b. 2400c. 1600d. 2800

HORN ANTENNA

Ideal as primary feed antenna for parabolic reflectors and lenses

LENS ANTENNA

Used as a collimator of frequencies in excess of 3 GHz

HELICAL ANTENNA

Broadband VHF and UHF antenna which is used when it is desired to provide circular polarization characteristics

Consists of a loosely wound helix, backed up by a ground plane, which is simply a screen made of chicken wire

DISCONE ANTENNA

A combination of a disk and a cone in close proximity

Characterized by an enormous bandwidth for both input impedance and

radiation pattern

A constant angle, low gain antenna; omnidirectional

LOOP ANTENNA

Used for direction finding, because they do not radiate in a direction at right angles to the

plane of the loop.

For portable domestic receivers Circular or square shaped

PHASED ARRAY

Group of antennas, connected to one

transmitter or receiver, whose radiation beam can be adjusted electronically

without physically moving parts; used in radars.

WHIP ANTENNA

the most common example of a monopole antenna, an antenna with a single driven

element and a ground plane.

The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end

adjacent to a ground plane.

SLOT ANTENNA

When the plate is driven as an antenna by a driving frequency, the

slot radiates electromagnetic waves in similar way to a dipole antenna.

consists of a metal surface, usually a flat plate, with a hole or slot cut out.

NOTCH ANTENNA

An open ended slot antenna

Since the currents are zero at the middle of the slot, we may cut the ground plane here to

make a notch antenna.

ANTENNA SUPPORT ANTENNA SUPPORT

STRUCTURESSTRUCTURES

POLE

Used to signify one piece of structure similar to the

common telephone pole

ANTENNA SUPPORTING STRUCTURES

Used to designate a structure made of

metal or wood which may be either in a form

of a one piece or sectionalized structure

MAST

Applied to a very large, high

structure, which in most cases, is constructed of

metal

TOWER

REVIEW QUESTIONSREVIEW QUESTIONS

1. A non-resonant antenna that is capable of operating satisfactorily over a relatively wide bandwidth, making it ideally suited for HF transmission.

a. end-fire arrayb. rhombicc. broadside arrayd. log periodic

2. Antenna that is independent of their radiation resistance and radiation pattern to frequency. It has bandwidth ratios of 10:1 or greater.

a. loop antennab. helicalc. Yagi Uda antennad. Log periodic antenna

3. A half wave antennaa. Hertzb. Marconic. Parabolicd. Vertical Monopole

4. Antenna which is used very frequently but almost entirely as a reception antenna and is usually found at the back of table radios.

a. loop antennab. folded antennac. rhombicd. log periodic

5. One of the special purpose antennas which has broadband VHF and UHF that is ideally suited for applications for which radiating circular rather than horizontal or vertical polarized electromagnetic waves are required.

a. loop antennab. phased arrayc. folded dipoled. helical

6. What are the two types of antenna elements?a. driven and reflectorb. director and reflectorc. parasitic and directord. driven and parasitic

7. Which one of the following terms does not apply to the Yagi Uda array?

a. good bandwidthb. parasitic elementsc. folded dipoled. high gain

8. Indicate the antenna that is not wideband.a. disconeb. folded dipolec. helicald. Marconi

9. One of the following is not an omnidirectional antennaa. halfwave dipoleb. log periodic antennac. disconed. Marconi

10. One of the following consists of nonresonant antennasa. rhombicb. folded dipolec. end fire arrayd. broadside array

11. Which of the following is best excited from a waveguide?

a. biconicalb. hornc. helicald. discone

12. An antenna that is circularly polarizeda. parabolic reflectorb. Yagi Udac. Helicald. Circular loop

13. What is the polarization of a discone antenna?a. verticalb. horizontalc. circulard. spiral

14. When speaking of antennas, ____ is a section which would be a complete antenna by itself.a. imageb. top loadingc. bayd. quarterwave

15. ______ is an antenna with a number of half wave antennas in it.

a. antenna arrayb. towerc. omnidirectionald. rhombic

16. Which antenna radiates an omnidirectional pattern in the horizontal plane with vertical polarization?a. Marconib. Disconec. Hornd. Helical

17. An antenna with very high gain and very narrow beamwidth.a. helicalb. disconec. hornd. parabolic dish

18. An open ended slot antennaa. helicalb. rhombicc. notchd. cassegrain

19. Which antenna is properly terminated?a. Marconib. Rhombicc. Dipoled. Yagi Uda

20. What is the radiation characteristic of a dipole antenna?a. omnidirectionalb. bidirectionalc. unidirectionald. hemispherical

21. An antenna which is one tenth wavelength long.a. hertz antennab. loop antennac. Marconi antennad. Elementary doublet

22. What is the minimum number of turns a helical antenna must have?a. 3b. 4c. 5d. 6

23. An antenna made up of a number of full wavelengthsa. elementary doubletb. log periodicc. long wired. whip

24. Which of the following improves antenna directivity?a. driven elementb. reflectorc. directord. parasitic element

25. The frequency of operation of a dipole antenna cut to a length of 3.4 ma. 88 MHzb. 44 MHzc. 22 MHzd. 11 MHz

26. Where are the voltage nodes in a half wave antenna?a. at the endsb. three quarters of the way from the feedpoint towards

the endc. one half of the way from the feedpoint towards the endd. at the feedpoint

27. A simple half wavelength antenna radiates the strongest signala. at 45 degrees to its axisb. parallel to its axisc. at right angles to its axisd. at 60 degrees to its axis

28. An antenna array which is highly directional at right angles to the plane of the arraya. Broadsideb. End firec. Turnstiled. Log periodic

29. What is the usual electrical length of a driven element in an HF beam antenna?a. λ/4b. λ/2c. 3/4 λd. 1 λ

30. The input terminal impedance at the center of a folded dipole antennaa. 72 Ωb. 300 Ωc. 50 Ωd. 73 Ω

31. The length of a Marconi antenna to operate with 985 kHz isa. 200 ftb. 500 ftc. 250 ftd. 999 ft

32. The purpose of stacking elements on an antennaa. sharper directional patternb. increased gainc. improved bandpassd. all of these

33. Which of the following antennas is used for testing and adjusting a transmitter for proper modulation, amplifier operation and frequency accuracy?a. elementary doubletb. realc. isotropicd. dummy