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RADAR CIRCUIT ANA . YSIS 12-1

C H A P T E R 1 2

This chapter deals with the basic principlesof antennas. It discusses antennas in general,the principles of electromagnetic radiation andits application to radar antennas, variousantenna arrays, and typical airborne radar an-tenna systems.

FUNCTION OF THE ANTENNAAn antenna is an electronic device that is used

either for radiating electromagnetic energy intospace or for collecting electromagnetic energyfrom space. In the radar transmitter, the mag-netron generates the high frequency signal, butan antenna is needed to change this signal intoelectromagnetic fields which are suitable forpropagation into space. The radar receiverwill amplify any signal that appears at its inputterminals, but an antenna is required to inter-cept the electromagnetic fields that are in spaceand to change these fields into a voltage whichthe receiver can interpret.

ANTENNA RECIPROCITYFortunately, separate antennas seldom are

required for transmitting and receiving radioenergy, for any antenna transfers energy fromspace to its input terminals with the same effi-ciency with which it transfers energy from theoutput terminals into space, assuming, of course,that the frequency is the same. This propertyof interchangeability of the same antenna fortransmitting and receiving operations is knownas antenna reciprocity. Antenna reciprocity ispossible chiefly because antenna characteristicsare essentially the same regardless of whetheran antenna is sending or receiving electro-magnetic energy. Because of antenna reci-procity, most radar sets installed in aircraft

employ the same antenna both for receiving andtransmitting. An automatic switch in the radiofrequency line first connects the single antennato the transmitter, then to the receiver, depend-ing upon the sequence of operation. Because ofreciprocity of radar antennas, this chaptertreats antennas from the viewpoint of the trans-mitting antenna with the understanding that thesame principles apply equally well when theantennas are used for receiving electromagneticenergy.

DIRECTIONAL PROPERTIESUsually, the most important characteristic ofa radar antenna is its directional property or

simply its directivity. Directivity means thatan antenna radiates more energy in one direc-tion than in another. For that matter, all an-tennas are directional, some slightly; others, al-most entirely. In radar operations, someantennasare required to send all energy in one directionin order that as much as possible of the electro-magnetic energy generated by the transmitterwill strike an object in a given direction. Inother systems, it is desirable for the energy tobe radiated equally well in all directions fromthe source. An example of an antenna system inradar which radiates energy in a given directionis the airborne navigation and bombing set.In this set, there is only a limited amount ofpower available at the transmitter. In order toachieve maximum benefit from this minimumpower, all of it is sent in the same direction.Since the antenna in this set is also used forreception, it likewise receives electromagneticenergy only from one direction. Because ofdesign features, it is possible to tell the directionof an object at which this directional type an-

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RADAR CIRCUIT ANALYSIS 12-2

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but now add and consequently create strongfields about the antenna, as you can see indiagrams C and D. Later you shall see that oneof these fields is responsible for electromagneticradiation.

The distribution of current (I) and voltage(E) about an antenna is shown by curves shownin the illustration. The dotted line representscurrent distribution, and the full line, voltagedistribution. The current curve at B shows thatmost of the current flows at the center and noneof it at the ends of the antenna. The voltage, onthe other hand, is maximum at the ends andminimum at the center. The magnetic field isgreatest where the current is greatest, as youcan see in the diagram at C, and the electricfield is strongest at the outer ends and weakestat the center. Note that, as in the case of a reso-nant circuit where voltage is maximum, thatsimilarly about the antenna the E-field is maxi-mum at the time the H-field is minimum. Bothof these fields vary at a sinusoidal rate with atime differenceof one-fourth cycle or a differenceof 90 between them.

The fields associated with the energy stored inthe resonant circuit (antenna) are called theinduction fields. These fields decrease with thesquare of the distance from the antenna. Theyare only local in effect and play no part in thetransmission of electromagnetic energy. Theyrepresent only the stored energy in the antennaand are responsible only for the resonant effectswhich the antenna reflects to the generator.The fieldsset up in the transfer of energy throughspace are known as the radiation fields. Al-though these fields decrease with distance fromthe antenna, this decrease is much less rapidthan the decrease of the induction fields. This is


because it is linear and is not according to thesquare rule. Therefore, the radiation fields reachgreat distance from the antenna. It is the radia-tion fields that are responsible for electro-magnetic radiation.Radiation of Fields

The exact cause and the mechanism for thephenomenon of radiation is not known to theelectronic scientist. However, you can get apretty good idea of what takes place by study-ing the illustration below showing a simple pic-ture of an E-field detaching itself from an an-tenna. At A in this illustration, the E-field ismaximum. Notice that the outer e-lines arestretched away from the inner ones. This isdue to the repelling force between the linesof force. As the voltage decreases toward zero,the radiation field decreases and the e-Iinesretract back into the conductor. If the voltagedecreases slowly, the entire field will collapseback into the conductor. On the other hand, ifthe decrease of voltage is rapid, the outer partsof the field cannot move in very fast and whenthe voltage is almost zero, as shown at B, arelatively large E-field will still exist aroundthe conductor. At C an exact zero voltage con-dition is shown. The E-field in this case doesnot reach zero, and it is left with no voltageto support it. When the next E-field developsaround, it will be pushed away by the pre-ceding E-field in the manner shown. The actionof one field pushing away the preceding fieldis analogous to the snapping of a whip, in thata part of the E-field is snapped off the antennawith each cycle. The snapped offfieldis projectedinto space and moves away from the antennaat a constant speed of 186,000miles per second.A similar action projects H-fields into space.

___ AN TENN A

A B c D

o80E-Field Detaching Itself from an Antenna

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CU;:{VES Of VARIATIONOf fLUX DENSITY

eUNES ORELECTRIC fLUX

HLlNES ORt-~AGNETIC fLUX

Fields in Space Around a Half- Wave AntennaAnother factor to remember about radiation

is that the ease with which it may occur, varieswith frequency. At lower frequencies, suchas 60 cps power frequency, for example, voltageon the antenna changes so slowly that thecomponent of energy radiated is so extremelysmall that it is of no practical value. At higherfrequencies, such as 50,000 cps and up, theradiated energy is great enough to meet com-munications requirements.Fields in Space

The above illustration shows the manner inwhich radiation fields are propagated from theantenna. The E-field and the H-field are shownas separate sets of flux lines about the antenna.The electric flux lines are the closed loops oneach side of the antenna. The magnetic fluxlinesare closed circular loops which have their axisaround the antenna, or in other words, theantenna is the center of each loop. They arerepresented as dots and crosses. The sine waveswhichare labeled the curves ofradial variation of

nux density indicate the relative field strengthat various distances and anglesfromthe antenna.Since the field configuration is not a standingwave, but a traveling wave phenomenon, themagnetic and electric fields are in phase andthus the sine waves apply to the flux density ofeither field.

In the direction perpendicular to the antenna,both the H-field and E-field are strong, for thisis the direction where both fields originallyformed. In the direction parallel to the antenna,or off the ends, no H-field forms at all and onlya very small E-field. The flux density, therefore,is small in these two directions. In other words,due to the directional properties of the half-waveantenna, most of the radiated energy travels inthe direction perpendicular to the antenna, butvery little energy in the direction along itsaxis.

As previously stated in the discussionofwaveguides, no moving E-field can exist without anH-field, and no H-field can be propagated with-out an associated E-field. Similarly, with the

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propagation of. electromagnetic energy intospace, no moving electrostatic forces can existwithout magnetic stresses existing in space andno magnetic force in motion can exist without anelectrostatic stress. Each creates the other andone cannot exist without the other. The direc-tion of motion is the travel of the fields awayfrom the antenna. The current associated withthe magnetic field does not flow because inspace there is no conductor to carry current, butthe field exists, nevertheless. To visualize afield existing without current flow, think of amoving magnetic field cutting a glass rod.A voltage (electrostatic stress) is induced in therod, but there is little actual electron movementbecause the rod is a good insulator. Magneticlines move in space, and set up electric stressesin space in a similar manner.

Propagation into SpaceAny part of the electromagnetic field set upby an antenna travels away from the antennain a straight line. There are many parts to thisfieldand many directions in which energy travels.In the illustration below are four of the largenumber of paths which the energy can take.These and other paths which energy takesaffect reception of radiated energy. If thereis nothing between the emitting antenna andreceiving station (Rj) some energy will traveldirectly to it via path 3. Receiving stationRz cannot receive energy because the earthis between the two points and because theenergy cannot go through the earth for any


distance. Some of the energy will follow path2 out into space. At a height of some 60 milesabove the earth, there is a heavily ionizedlayer of atmosphere, called the ionosphere. Thisconstitutes a change of media through which theenergy must travel, and is sufficient to refracta wave. In the case of path 2, the refraction issufficient to bend the energy wave out of theionosphere back to the earth. Therefore, receiv-ing station R 2, which is beyond the horizonand not located for receiving a direct ray, re-ceives the reflected ray from the ionosphere.

It is the ionosphere that makes possibleround-the-world communications. If the angle of inci-dence of an energy wave with the ionosphere istoo great, the angle refraction may not be suffi-cient to bend the energy back toward the earthand it may go on through the ionosphere andbe lost, as shown by path 1. Paths 3 and 4follow along the surface of the earth and arecalled ground waves. The waves bent back to theearth by the ionosphere are called sky waves.

Generally, medium and high frequency wavesare reflectedreadily, whereas ultra high and superhigh frequencies are more likely to be lostthrough penetration of the ionosphere. Airborneradar operates in the super high-frequency(3000 me to 30,000 me) part of the spectrum.Irregularity of the sky waves makes them un-reliable for radar use. Therefore, radar uses onlythe direct ground wave. Path 4, for example,where the ray goes directly to some objectand returns by a direct path is a direct groundWRVe. _ . .- ----- -""""

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THE HALF-WAVE ANTENNAAi3 stated previously in the discussionof elec-

tromagnetic radiation, a half-wave length con-ductor is the simplest of the radiating elements.Considerable radiation occurs in this elementbecause of its resonant characteristics and itsability to store large amounts ofenergy in induc-tion fields. Resonance causes high voltages andhigh circulating currents and they in turn pro-duce strong fieldsaround the antenna.

-.------- "12---------VOlTAGE DISTRIBUTION

+o(\"'~_-------V2--------_.

CURRENT DISTRIBUTION

Current and Voltage Distribution inHa/f- Wave Antenna

Current and Voltage DistributionAccording to the above illustration showing

voltage distribution in a half wave antenna, volt-age standing wave is high at the ends of theantenna and low at the center. Ai3 previouslyexplained, this is also the case with the quarterwave open circuit two-wire line from which thehalf-wave antenna is developed.

An examination of the current distributioncurve shows that the current standing wavereaches maximum a quarter cycle after thevoltage reaches maximum at which time thecurrent is maximum at the center and zero atthe ends. At the ends where there is no placefor electrons to go, the current is zero. In prac-tical applications the endsofa half-wave antennamust be insulated due to the high voltagesthere and the center of the antenna must havelow resistance in order to minimize the PRlossesdue to the high current there.

RESONANCE AND DIMENSIONSElectromagnetic waves travel through space

at a speed of 300million meters per second. Thelength of one cycle in space depends upon fre-quency and is called the wave length. Mathe-

matically, the length of an electromagneticwave is expressedby the formula,

A = 3XIO'fwhere A is the wave length in meters, and f isthe frequency in cyclesper second.

Since an electromagnetic wave travels on anantenna, the antenna, too, has wave length.But, due to the resistance of the wire, the move-ment of waves along a wire (antenna) is some-what slower than wave movement in space.Wave length on a wire, therefore, is slightly lessthan that of a wave traveling in space and isexpressed by the equation,

3 XIOx.94l = f metersPhysically, an antenna is about 6% shorter

than a half wave traveling in space. (This ac-counts for the fact that it is necessary to mul-tiply the wavelength in the antenna formulaby .94.) In this manual, assume that the correc-tion in antenna length has been made wheneverthe length of the antenna is given in wavelength.Effect of Length on Antenna Impedance

An antenna of the correct length acts like aresonant circuit and presents pure resistance tothe excitation circuit. (SeeAin the illustration onthe next page). An antenna having other thanthe correct length displays both resistance andreactance to the excitation circuit. An an-tenna slightly longer than a half wave, for ex-ample, acts like an inductive circuit. This isunderstandable if you think of the antenna interms of a quarter wave RF line. When theantenna is excited at the center, it is equivalentto a Uwave RF line, looking at it from thegenerator end. Any two-wire line, longer thana quarter wave, is like a quarter wave sec-tion with a short, open-circuited section addedto it. The open section, which is capacitive, isinverted to inductance at the input terminalsby the quarter wave. In the same manner aslightly long antenna looks inductive.

According to the equivalent lumped circuitat B, the inductance is not entirely balanced bythe capacitance. The remedy for correcting thelength is either cutting the conductor shorter,or tuning out the inductive reactance by addingcapacity in series.

If the antenna is physically shorter than itsresonant length, the input impedance becomescapacitively reactive. The two-wire line shorter

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A/2~, - > . / 4 - ,-* : :GHALF WAVE ANTENNA y . , WAVE OPEN RF LINE SERIES RESONANT CURRENT INPUT IS RESISTIVE ANTENNA OF CORRECT LENGTH IS A RESISTIVE LOAD TO GENERATOR

C

I t ~ ~ ~ ~ - - - - - - ~ ( S ~ l - - - - ~ ~ ~ ~LONG ANT. LONG RF LINE SERIES CIRCUIT WITH

PREDOMINANT x. INDUCTIVE CIRCUIT

ANTENNA LONGER THAN A HALF WAVE IS AN INDUCTIVE LOAD TO GENERATORf--A/4~~:=::J

~==:J2 WIRE LINE LESS THAN A/4ONG = SERIES CIRCUITWHERE x, IS PREDOMINANT ANTENNA SHORTER THAN A HALF WAVE IS A CAPACITIVE LOAD TO GENERATOR

I~~a- ~~~I ~~~~~SHORT ANTENNA = CAPACITIVE CIRCUIT

@DD SERIES INDUCTANCE TO MAKE ANTENNA ELECTRICAllY LONGER ADD SERIES CAPACITY TO MAKE ANTENNA ELECTRICALLY SHORTEREffect of Length on Antenna Impedance

Adding capacity to the line gives the same re-sult as adding length. Therefore, the effectivelength of the antenna in this case is equal toone-quarter wavelength.

With radar equipment, a wide band of fre-quencies-rather than a single frequency-must be handled by an antenna. This means thatan antenna which is designed for the center fre-quency is short for frequencies in the lower (lowfrequencies) sidebands and long for frequenciesin the upper (high frequency) sidebands. Theantenna must be corrected for both long andshort conditions at the same time.

Note in the chart at the bottom of the nextpage showing the variations in the reactance

than a quarter wavelength displays capacity atits input terminals. (See diagram C.) The cor-rection in this case is either to add inductancein series with the antenna to bring it back toresonance, or to add physical length to theantenna.Increasing Antenna Length

Apractical method for increasing the length ofan antenna which is physically short is shownat A in the illustration at the top of page 12-8.The spheres fitted at the ends add to the capac-ity from end to end. As shown at B, this isequivalent to adding capacity at the end of thetwo-wire line. The additional capacity comesfrom the closer spacing between conductors.

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1 '1 I/ o.-DIAMffiR= .\j50,OOO f'I I/ / I/./\ DIAMEiER= \ 500/ /\ I

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---11--- ..../"-,, "A ADDING SPHERESAT ENDSd '0OF ANTENNAS TO INCREASE\S I I 0 ELECTRICALLENGTH

B BALLS REDUCESPACING,INCREASING CAPACITYBEiWEEN ENDS

(?SA llS iNCR .EASE EFF ECT IV ELENGT H

In cre as in g E lect rica l Length of Antenna

of an antenna that each side of an antennamust be less than a quarter wave (in space)in order to present zero reactance. This agreeswith the previous statement that the physicallength of the half-wave element must be 6%shorter than the half wave in space. Twocurves are shown. One is for a large diameterantenna, and the other for a small diameterantenna. Note that the larger diameter displaysless reactance at lengths offresonance.

Antenna diameters as great as 1/20 wavelength are not uncommon in radar equipment.A large diameter increases the capacity of theantenna. The inductance is decreased for thesame resonant frequency. Lower inductancewith the same resistance lowers the XL!R ratio,or Q of the antenna. A lower Q causes the re-sonance curve to be broader and gives theantenna a more uniform response to a greaterband of frequencies.

,0004000300020001000

o- 1000- 2000- 3000- 4000

- 5000\ / 2

INPEDANCE AT INPUT TERMINALSIn practice an antenna acts like a resonant or

near resonant circuit and, theoretically, like aperfectly tuned circuit. When it looks likea seriesresonant circuit, its input terminals must dis-play zero impedance, and when it is connectedlike a parallel resonant circuit, it must displayinfinite impedance. Further, if it is not of thecorrect length, it must display reactance as wellas resistance,

Practical antennas, like practical resonant cir-cuits, have losseswhich must be replaced by thesource. These losses make the input impedanceof the antenna somewhat resistive. This resist-ance is a combination of two resistances-theresistance of the conductors themselves, which isincreased by the skin effect at high frequencies,and the radiation resistance.

Radiation resistance is a fictitious resistancewhich dissipates the same amount of power inthe form of heat that is actually dissipated asradiated energy. Because of radiation resistance,an antenna allowspart of its fields to escape intospace. This makes it necessary for additionalcurrent to flowinto the antenna from the sourceof power to replace the part of the field thatescaped. Therefore, the SOUTcemust provide theenergy lost by radiation. The energy lost ispower (P) and its ratio to the square of thecurrent (I) is the radiation resistance, RI"'Mathematically, it is expressed,

pR =-r I~

Radiation resistance may be defined as theratio of the power radiated to the square of thecurrent in the antenna. Radiation resistance andconductor resistance constitute the total inputresistance to an antenna. It is possible to makethe conductor resistance small in comparison

I . ---I 1 - " - - --+1;1C : : : : : = = = = = = l 1 = = = = = = : : : : : : : 1

Rea ctance at Input.of a Cen ter-Fed Antenn a of Arbitrary Lengt h

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n1 1

VDIAMETER~>./50,000

1 \ /AMETER~ >./500 1 \~ )-J -

10,000

8000

6000

4000

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Resistance at Input of a Center-Fed Antenna of Arbitrary Lengthto the radiation resistance by using large diame-ter low resistance conductors. It is desirablethat a major portion of the input resistancebecomes the radiation resistance. In practiceradiation ratios as high as 9:1 are obtainable.This means that an antenna can be 90% efficientas a radiator.

Above is the graph showing the input resis-tance at the center ofantennas ofvarious lengths.For the half-wave dipole (L= .. /4 ) , the inputis 73 ohms. The input resistance of a full waveantenna is (L=r .. / 2 ) is as high as 9000 ohms,depending on the diameter of the conductor.

For the lengths of antennas in which the re-actance is zero, the input resistance is the inputimpedance of the antenna. However, for lengthsin which the reactance is not zero, the inputimpedance is the vector sum of the resistance(graph above) and the reactance at the input ofthe antenna, graph page 12-8.

Other factors which affect the input impedanceof an antenna are nearby conducting objects,suchas the earth, and other antennas or the skin ofthe aircraft. The graphs above and on thepreceding page apply only to a center fedantenna which is located in free space andnot close to any conductor.POLARIZA TION OF AN ELECTROMAGNETIC

WAVEElectromagnetic fields in space are said to

be polarized and the direction of the electricfield is considered the direction of polarization.As the electric field is parallel to the axis of ahalf-wave dipole, the antenna is in the plane of

RADAR CIRCUIT ANALYSIS 12.'

f--l-----l I-----l1 ! I5Aj4

polarization. When the half-wave dipole ishorizontally orientated, the emitted wave ishorizontally polarized. A vertically polarizedwave is emitted when the antenna is erectedvertically.

For maximum absorption of energy from theelectromagnetic fields, it is necessary that ahalf-wave dipole be located in the plane ofpolarization. This places the conductor at rightangles to the magnetic lines of force that aremoving through the antenna, and parallel tothe electric lines.

In general, the polarization of a wave does notchange over short distances. Therefore, trans-mitting and receiving antennas are orientatedalike, especially if short distances separate them.

Over long distances, the polarization changes.The change is usually small at low frequencies.At high frequencies, the change is quite rapid.

With radar transmissions, a received signalis a reflected wave from some object. As thepolarization of the reflected signal varies with thetype of object, no set position of the receivingantenna is correct for all returning signals.Generally, the receiving antenna is polarizedin the same direction as the transmitting antenna.

When the transmitting antenna is close tothe ground insofar as propagation is concerned,vertically polarized waves cause a greater signalstrength along the earth's surface. On the otherhand, antennas high above the ground should behorizontally polarized to get the greatest signalstrength possible to the earth's surface. For thisreason most airborne radar systems emit hori-zontally polarized waves.

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POLAR DIAGRAM FOR HALF WAVE ANTENNA IN ANY PLANECONTAINING THE ANTENNA

POLAR DIAGRAM FOR HALF WAVE ANTENNAIN PLANE PERPENDICULAR TO ANTENNA

Polar Diagrams of Half- Wave DipoleThe other method consists ofmoving around theantenna in all directions, finding the points wherethe field strength is the same, and plotting theangle against distance on polar coordinate paper.Either type of measurement will produce iden-tical polar diagrams.

Above note the polar diagrams for the half-wave dipole. The left diagram holds good forany plane containing the antenna. For the planeperpendicular to the half-wave antenna, theantenna forms the singlepoint whichwasderivedpreviously. The polar diagram for this point be-comes a circle with the point at the centeras in the right diagram. Thus, the simple half-wave dipole is bidirectional in any plane con-taining the antenna, but nondirectional in theplane perpendicular to the antenna.

The strength of a radiation field is called fieldstrength and is measured in units called voltsper meter. Oneunit is definedas the fieldstrengthwhich will induce one volt in a conductor onemeter long. As fieldstrengths of a volt per meterare seldom encountered, weaker fields are meas-ured in microbolts per meter or millivolts per meter.

IMAGE ANTENNASThe preceding discussion dealt with an-

tennas that are isolated from any conductor.However, in all practical cases, usually there are

conductors somewhere near the antenna. Theeffect of the conductors is often undesirable butcan sometimes provide advantages.

The effect of a nearby conductor is illustrateddiagrammatically below at A. A real antenna isshown perpendicular to a horizontal perfectlyconducting plane. When a fieldis radiated, energywill be sent out in all directions. There are two

ACTUAL ANTENNA

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TIIII\CURRENT\\\\\, I I

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IMAGE ANTENNA

A THE QUARTER WAVE GROUNDED ANTENNA IS CAllED THE "MARCONI."

IMPERFECTCONDUCTOR PERFECTCONDUCTOR J---NTENNA ,. - "'" "" \~' \

\I i ! .

---CONDUCTORIi ! .

B VERTICAL FIELDSTRENGTH PATTERN

ANTENNA

C HORIZONTAL POLAR DIAGRAM OF MARCONI

Marconi Antennapossible paths for the field to take. Path onestarts out directly toward a distant point. Theenergy in path two starts out toward the con-ducting plane. Upon striking the perfect con-ductor, .it is reflected in reverse phase. (Thisis analogous to an incident wave being re:B.ectedin reverse phase from the shorted end of an RFline.) As with light waves, the angle of re-flection is the same as the angle of incidence.The reflected wave also proceeds toward somepoint in space. The sum of the two wavesmake up the total wave at any point in space.So far as the action in the conducting planeis concerned, it can be replaced by anotherantenna, which is a mirror image of the ac-tual antenna. The reflected wave can be as-sumed to have originated at point P' on theimage antenna.

In general, current in vertical image antennasflowsin the same direction as in actual antennas,while in horizontal antennas, the current flowsin opposite directions. Note at B on page 12-11that the combination of the real and image an-tennas for vertical quarter wave makes a half-wave dipole.Marconi Antenna

Aquarter wave grounded antenna is a commontype of grounded antenna. This type is oftencalled a Marconi antenna, as contrasted withthe half-wave (ungrounded) dipole, which iscalled a Hertz antenna, Note the standing waveamplitude of current and voltage on the MaT-coni antenna shown to the left. Note also thesimilarity to the half wave element when theimage is included.

In the Marconi antenna the vertical fieldstrength pattern (polar diagram) shown at Bis the same as that of the half-wave element, ex-cept that the conducting plane cuts it off at thecenter. The image is only effective above theplane because no energy penetrates the con-ducting plane there.

In the horizontal polar diagram at C, the ver-tical field strength pattern can be rotated withthe antenna as the axis to form the horizontalpolar diagram of a Marconi. It is non-directionalin a plane perpendicular to the length of theantenna.

The input impedance to the Marconi is ap-proximately 37 ohms when the antenna is fedat its base as illustrated at A. In addition aquarter-wave Marconi is resonant and displayszero reactance just like a half-wave antenna.

This discussionhas assumed that the conduct-ing plane is a perfect conductor. If it is not a per-fect conductor, as is the case in practice, someof the conditions just discussed must be altered.The principal alteration is the reduction in sizeof the polar diagram. This results in decreasedsignal strength from the antenna as shownat B. The conducting plane is usually the skinof the aircraft where airborne equipment is con-cerned and with ground equipment, it is theearth's surface.

Other types of conductors that might be nearan antenna are the aircraft tail pieces, the air-plane wing, or steel antenna towers. As theradiation field passes any conductor, currents areinduced in them. These currents vary at the sameradio frequency and make the conductor itselfa radiator. In other words, when the conductor

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