The Quadrifilar Helix Antenna - it

The Quadrifilar Helix Antenna 22-1

Sec 22.1 Introduction land or sea.One reason for presenting details of

the quadrifilar in this book is that it is aninherently excellent antenna for groundstation use in the amateur satellite ser-vice. Once you study the following engi-neering aspects of the quadrifilar, you’lldiscover its advantages over other circu-larly polarized antennas, such as crossedYagis. It is useful at 144 MHz, and up,and all the critical dimensions are in-cluded in this chapter. The dimensions arelisted in wavelengths, so all you have todo is convert them to the desired fre-quency. The details of feeding, phasing,and infinite balun construction are givenin the drawings and photos, long withsufficient explanation for you to constructyour own quadrifilar.

Radiation from the quadrifilar helixantenna is circularly polarized and of thesame screw sense everywhere throughoutthe radiation sphere. The antenna embod-ies a unique configuration and method offeeding loop elements to produce radia-tion having a controllable pattern shape.Refer again to Fig 22-1. The quadrifilarantenna comprises two bifilar helicalloops oriented in a mutually orthogonalrelation on a common axis. The terminalsof each loop are fed 180° out of phase, andthe currents in the two loops are in phasequadrature (90° out of phase).

By selecting the appropriate configu-ration of the loops, a wide range of radia-tion pattern shapes is available, with ex-

Chapter 22

The Quadrifilar Helix Antenna

T he quadrifilar helix antenna,shown in Fig 22-1, is seeing useon commercial and military, aswell as amateur spacecraft.

For amateur applications it first saw useon AMSAT-OSCAR 7 in November 1974.I prepared the basic information in thischapter while I was employed as an an-tenna engineer at RCA’s Astro-Electron-ics Division in Princeton, New Jersey. In-formation also appears in Chapter 20 ofThe ARRL Antenna Book, 15th edition(Ref 71). While working at RCA, I assistedin the development of the quadrifilar he-lix.1 We consider this antenna to be anoutstanding contribution to practicalspacecraft antenna engineering from theviewpoint of flexibility in design. Thequadrifilar helix provides a large rangeof radiation characteristics from a radia-tor of extremely small size and weight,from which we can select a pretested de-sign to match the radiation requirementsof any particular spacecraft.

The quadrifilars shown in Fig 22-1areflight-model units for operation at 1800and 2200 MHz. I also assisted in the de-sign of quadrifilars for 121.5, 243, and1600 MHz, which are flying on TIROS-N(NOAA 9 through 14) weather spacecraftin the worldwide Search and Rescue(SAR) Emergency Locator Transmitter(ELT) service. The service is primarily fordowned or stricken aircraft, over either

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cellent axial ratio appearing over a largevolume of the pattern. The basic form ofthe quadrifilar antenna was developed byDr C. C. Kilgus of the Applied PhysicsLaboratory, Johns Hopkins University,who has published several papers thatestablish the theoretical basis for its op-eration. (See Refs 88 through 96). TheBrown and Woodward citation (Ref 90) isespecially helpful in understanding theradiation characteristics of the quadri-filar. That paper describes radiation fromthe combination of a dipole and a circularloop sharing a common axis, and fed inphase, which produces radiation charac-teristics similar to the quadrifilar. Thisversion of the quadrifilar helix uses a

novel feed system that includes the infi-nite balun. The feed system also includesa means for attaining the required 90° dif-ferential current phase relationship with-out requiring additional components toachieve separate differential-phase exci-tations.

The quadrifilar antenna is applicablefor general use in the frequency rangeabove 30 MHz. It is especially attractivefor certain spacecraft applications becauseit can provide omnidirectional radiationin a single hemisphere (a cardioid volumeof revolution) without requiring a groundplane. This no-ground-plane feature af-fords a dramatic saving in weight andspace. In addition, the inherent cardioid

Fig 22-1 — Two flight-model quadrifilar helix antennas. The larger is for use around 1800MHz, and the smaller around 2200 MHz. Radiation from each antenna is circularlypolarized and of the same screw sense everywhere throughout its radiation sphere.


radiation characteristic that eliminatesthe need for the ground plane also affordsfreedom in the choice of a mounting posi-tion on a spacecraft. The profile of thevolume beneath the antenna has rela-tively little effect on the radiation pattern,provided that the quadrifilar is mountedat least λ/4 above conducting surfaceswhich would form a reflective plane.

Quadrifilars designed to operate inthe frequency range of 1800 and 2200MHz can be extremely light, weighingonly about 0.7 ounce. Developed by RCAAstro-Electronics, several of these anten-nas are flying on U.S. Air Force satellitesof the Defense Meteorological Supportprogram, also built by RCA. A slightlysmaller version of similar design (donatedby RCA Astro-Electronics) is the2304.1-MHz beacon antenna, which flewon AMSAT-OSCAR 7.

Quadrifilars having a somewhat dif-ferent configuration from that shown inFig 22-1 have also been developed by RCAfor use on RCA-built NOAA TIROS-Nweather satellites. The design differs be-cause the radiation pattern requirementsare different. The design of the quadri-filars for transmitting the principal videosignals from the TIROS-N spacecraft isnoteworthy because the radiation patternhas been shaped to maintain a nearly con-stant signal level versus slant-range dis-tance between the spacecraft and theground station during the in-view portionof the spacecraft orbit. (RCA-built TIROS-N, or NOAA spacecraft launched around1979 are still in service today, in April1999.)

Sec 22.2 PhysicalCharacteristics

sult, the radiation characteristics of thequadrifilar antenna are sometimes mis-understood. Although the term quadrifilaris not incorrect, the term itself often failsto conjure up a true picture of the physi-cal characteristics. Therefore, since sev-eral characteristics of the quadrifilar an-tenna differ radically from those of theconventional helical antenna (for ex-ample, opposite screw sense of circularpolarization), the following paragraphsexamine some aspects of both the physi-cal and the radiation characteristics of thequadrifilar to clarify the confusion.

In describing the physical character-istics of the quadrifilar, I will concentrateon the simple half-turn λ/2 configuration.As I stated earlier, the quadrifilar helixis a combination of two bifilar helicalsarranged in a mutually orthogonal rela-tionship along a common axis, and envel-oping a common volume.

What is a bifilar helix? One way ofvisualizing the half-turn λ/2 bifilar helixis to develop it from a continuous squareloop of wire one wavelength in perimeter,as shown in Fig 22-2. As in the driven el-ement of the conventional cubical quadantenna, each side of this square loop isλ/4, and the feed terminals are formed byopening the loop at the midpoint of thebottom side. We call this square-loop con-figuration a zero-turn, λ/2 bifilar loop.There is a half wavelength of wire radia-tor extending away from each feed termi-nal around the loop to the antipodal pointon the opposite side of the square (loophalf-length lenE° equals λ/2). The draw-ing shows length len° with a script letterL. This loop is a balanced input device,requiring a balanced, two-wire feed linewith push-pull currents.

To visualize the development of thequadrifilar helix, imagine inserting animaginary cylinder of diameter D = λ/4inside the loop. Then, while holding the

There is a tendency to confuse thequadrifilar with the conventional helicalantenna, probably because portions of thequadrifilar are helically shaped. As a re-

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bottom side of the loop fixed, grasp thetop side and give it a half turn of rotationwith respect to the bottom. As a result,shown in Fig 22-3, each of the two verti-cal sides of the square loop becomes ahalf-turn helix as it curves around thesurface of the imaginary cylinder. How-ever, because of the curved paths of theonce-straight vertical sides, the distancebetween the top and the bottom hasshrunk, and the axial length, lenp°, is lessthan the λ/4 diameter.

With these particular proportions of

the 1/2-turn, λ/2 bifilar, some of the ra-diation characteristics are not particu-larly attractive. However, some physicalparameters of the loop may be selected toobtain characteristics that make this an-tenna especially attractive. Such param-eters include the electrical length of theconductors, the number of turns, the cy-lindrical diameter D, and length lenp°. Inaddition, for a given conductor length, thelength-to-diameter ratio, abbreviated len/diam, of the cylinder is also an importantvariable, controlled by the diameter and

Fig 22-2—Horizontally polarized square loop radiator. The small arrows represent thedirection of current flow.


number of turns. For example, the an-tennas shown in Fig 22-1 are 1/2-turn, λ/2 quadrifilars. But by reducing the diam-eter from 0.25 λ to 0.18 λ for this model,the axial length is inherently increasedto 0.27 λ. This results in vastly improvedradiation characteristics.

Sec 22.3 ElectricalCharacteristics

Let’s take a short qualitative look atsome basic changes in the radiation pat-tern that result from the 1/2-turn twist ofthe square loop. First, recall that thesquare loop having a perimeter P = I λ isbasically a broadside array of two dipole

elements, with the top element being volt-age fed from the bottom element. In thisloop the currents in the top and bottomsides flow in the same direction, as shownin Fig 22-2. The horizontally polarizedfields produced by the currents in both topand bottom sides are therefore in phase.The two fields add to form the conven-tional figure-8 broadside radiation pat-tern. The bidirectional lobes in the pat-tern are at right angles to the plane ofthe loop. The null in the pattern appearsbidirectionally on a horizontal line thatis in the plane of the loop midway betweenthe top and bottom sides.

In the vertical sides of the loop, the

Fig 22-3—Half-turn bifilar helix loop. Distance len E° equals the loop half-length.

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current in the top half of each side flowsin the direction opposite to that in thecorresponding bottom half. Consequently,the vertically polarized fields produced byboth halves of each vertical side are mu-tually out of phase, and add to zero in alldirections. This cancellation of the verti-cal fields results in zero net vertically po-larized radiation.

On the other hand, in the bifilar he-lix loop having the half-turn twist, thecurrents in the top and bottom sides flowin opposite directions because of the physi-cal half-turn rotation of the top side. Thiscurrent relationship is shown in Fig 22-3.Thus, the fields produced by the currentsin the top and bottom sides are now outof phase with each other, forming anend-fire array relationship.

In the direction broadside to the planeformed by the top and bottom sides, thefields now cancel completely. As in theordinary end-fire array, this results in zeroradiation in the broadside direction,where maximum radiation appeared withthe square loop. The conventional lobesof end-fire radiation contributed by the topand bottom sides now occur toward thetop and bottom of the antenna as drawnin Fig 22-3. This radiation is horizontallypolarized.

The currents flowing in the helicalportions of the loop retain the samecurrent-flow pattern as in Fig 22-2. How-ever, the physical positions of each cur-rent segment, or element, in the twistedvertical wires have now been shifted to anew position, and to a new orientation inthe helical paths. This results in a corre-sponding different position and vector di-rection for each elemental field producedby the helical current elements. As wouldbe expected, the addition of all these el-emental fields now results in a compositefield consisting of both horizontally andvertically polarized fields.

Sec 22.4 Difference BetweenQuadrifilar and ConventionalHelix

Before considering the radiation char-acteristics of the bifilar helix further, itmay be helpful to mention some criticaldistinctions between the Kilgus bifilarhelix and the conventional helical antennaconfigurations. In the conventional heli-cal antenna having more than one radi-ating element, the elements are generallyfed in phase. However, Kilgus found thatinteresting results were obtained with ahelical antenna that has two elementsspaced radially at 180° when the two ele-ments are fed 180° out of phase. This find-ing led Kilgus to his further investigationof the bifilar helical antenna havingout-of-phase feed, and still further to thequadrifilar configuration. Thus, an alter-native way of visualizing the bifilar helixis as a conventional helical antenna withtwo elements radially spaced at 180°, butwith the outer ends radially connected toeach other, and with the elements fed outof phase.

Dr. Kilgus’ analyses provide a morecomplete theoretical basis for the func-tioning of both the bifilar and thequadrifilar helix. He shows the currentdistribution and radiation characteristicsof the half-turn, λ/2 bifilar helix to be simi-lar to those of a loop-dipole combinationdescribed by Brown and Woodward (Ref90). Brown and Woodward analyzed acombination horizontal loop and verticaldipole sharing a common axis. They showthat while both a loop and a dipole pro-duce identical toroid shaped radiationpatterns, the electric field produced by thedipole current in this arrangement is ver-tical, and the electric field produced by theloop current is horizontal. When the cur-rents in the loop and dipole are in phase,their fields are in phase quadrature, a


requirement for circularly polarized radia-tion. When the vertical and horizontalfields are also equal in magnitude, theresulting radiation is circularly polarized,with the same screw sense everywherethroughout the radiation sphere.

Experimental data obtained byKilgus supports his theoretical analysis,and confirms the similarity of radiationcharacteristics between the half-turn, λ/2 bifilar helix and the loop-dipole combi-nation. For the purpose of obtaining ad-ditional data pertinent to the developmentof quadrifilar antennas for use on theTIROS-N spacecraft, I performed exten-sive measurements of pattern shape, po-larization, axial ratio, and terminal im-pedance on more than a thousand differ-ent bifilar and quadrifilar helix configu-rations. The data obtained from thesemeasurements agree with the Kilgusdata, adding further validity to the ear-

lier findings as well as extending them.

Sec 22.5 Development of theQuadrifilar Helix Antenna

Fig 22-4—Half-turnquadrifilar (two half-turnbifilar loops).

In this section we examine the intrin-sic cardioid, or omnidirectional hemi-spheric radiation characteristic of thequadrifilar. In doing so, the toroidal ra-diation pattern of the bifilar helix becomesof initial primary interest. Recall that inthe toroidal radiation pattern of theloop-dipole, maximum radiation occursbroadside to the axis, with the null on theaxis. On the other hand, as mentionedearlier, maximum radiation from the bifi-lar loop occurs bidirectionally along theaxis of the loop, while the null appears inthe direction perpendicular to the planeformed by the top and bottom sides, orradials of the loop. Except for this 90° ro-tation of the axes, the radiation charac-teristics of the two antennas are similar.

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We may develop a quadrifilar helixfrom the bifilar (which I will call bifilarA) by adding a second bifilar, B, on thesame axis and enclosing the same space,but rotated 90° relative to bifilar A. Thisarrangement is shown in Fig 22-4. Thefields radiated by bifilar B are identicalto those of bifilar A, except that the en-tire radiation pattern of bifilar B is rotated90° relative to that of bifilar A. Conse-quently, the null of bifilar A and the maxi-mum radiation of bifilar B appear at thesame point in space, and vice versa. Onthe other hand, in the axial directions theradiation fields from both bifilars areequal.

This field relationship is the key tothe cardioid radiation characteristic of thequadrifilar. When both bifilars are fed ina 0-180° and 90-270°-phase relationship,respectively, (excited simultaneously witha mutual 90° current-phase relationship),the cardioid radiation pattern appears inthe far field. This is because the fields ofboth bifilars are in phase in one axial di-rection. In this direction, the two fieldsadd, while in the opposite direction, thefields are out of phase and cancel. As thecancellation is not perfect off axis, the re-sult is a cardioid-shaped pattern of revo-lution about the axis. In the broadsidedirection normal to the axis, the respec-

Fig 22-5—Radiation pattern of the half-turn, λ/2 quadrifilar antenna when illuminated witha circularly polarized source.


tive nulls and maxima of the two indi-vidual bifilars compensate each other toproduce a uniform omnidirectional radia-tion pattern around the axis.

See 22.6 The Quadrifilar ShapeFactor

ranges of the parameter values. However,let us now examine how the Air Force 5D,the amateur AMSAT-OSCAR 7 and theTIROS-N quadrifilar designs were tai-lored to obtain the radiation characteris-tics required for their particular missions.

When using bifilars having a diam-eter D = 0.25 λ, formed by placing a halftwist in the square loop of Fig 2, the len/diam ratio is less than one, which yieldspoor front-to-back and axial ratios. Bydecreasing the diameter to D = 0.18 λwhile retaining the half-turn, λ/2 loop, theaxial length increases to lenp° = 0.27 λ.The resulting ratio, len/diam. = 1.5, yields

Fig 22-6—Quadrifilar radiation pattern when illuminated with a spinning dipole. Thisillumination adds axial-ratio data, represented as the difference between adjacentmaximum and minimum values of a ripple wave.

As I stated earlier, the radiation pat-tern shape and the axial ratio of the po-larization can be controlled. This is doneby tailoring both the length of wire andnumber of turns in the loop, and thelength-to-diameter ratio, len/diam, of theformed cylinder. Kilgus’ data shows theserelationships graphically for certain

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a vast improvement in both front-to-backand axial ratios. These are the design val-ues used in the Air Force 5D and AMSAT-OSCAR 7 quadrifilars. The radiation pat-terns obtained with this design are shownin Figs 22-5 and 22-6. The patterns arepresented in the standard IEEEspherical-coordinate system of notation,and the orientation of the antenna rela-tive to the coordinate axes is shown in Fig22-4.

The θ radiation pattern shown in Fig22-5 was measured while using circularlypolarized illumination of the quadrifilar.This pattern shows an on-axis gain of 5dBic (decibels relative to isotropic, circu-lar), and a front-to-back ratio greater than

20 dB. The radiation at any angle θ is uni-form for all values of φ around the Z axis.Thus, the single θ pattern (at any valueof φ) represents the shape of the envelopeof the volume of revolution about the Zaxis, which defines the solid radiationpattern in all directions.

The pattern shown in Fig 22-6 wasobtained by using a spinning dipole to il-luminate the quadrifilar, to obtainaxial-ratio information. The periodicripple appearing in the pattern resultsfrom the rotation of the dipole. Themaxima and minima, respectively, corre-spond to the times when the dipole is par-allel to the major and minor axes of thepolarization ellipse. The axial ratio may

Fig 22-7—Radiation pattern of the 1 1/2 turn, 1.25- λ quadrifilar antenna.


be determined from the difference be-tween adjacent maximum and minimumvalues. Thus, the axial ratio of this de-sign is seen to be less than 2 dB over abeamwidth of ±30°.

As mentioned earlier, theshape-factor parameters for the TIROS-Nconfiguration were determined empiri-cally from the results of my research mea-surements. I took measurements on morethan a thousand combinations ofquadrifilar configurations in which eachphysical parameter was separately var-ied in small increments while holding theother variables constant. Hundreds ofshape-factor combinations were mea-sured, which provided families of patternsfrom which to select the parameters forthe desired pattern shape. The TIROS-Nmission required a radiation-patternshape which provides a nearly constantsignal level to the ground station duringin-view time. This pattern shape is shownin Fig 22-7. The configuration that yieldsthis radiation pattern was one of the hun-dreds measured during my research. All

that was required to determine the con-figuration that would satisfy the radia-tion-pattern requirements was to searchthrough the hundreds of patterns ob-tained during the research to find the onethat fit the requirements. The bifilar pa-rameters which yield this pattern shapewere found in to be 11/2 turns, 1.25-λ loop,D = 0.1 λ lenp° = 1.0 λ, and len/diam =10.0. These quadrifilars, flying onTIROS-N, are used for transmittingweather and other data in the 1600-MHzregion, completely separate from the121.5, 243, and 1600-MHz SAR-ELTquadrifilars flying on the same spacecraft.

See 22.7 Methods of Feedingthe Quadrifilar Antenna

Fig 22-8 — Half-turnbifilar loop with infinitebalun ffed.

Feeding the quadrifilar with a single,unbalanced coaxial line requires specialattention. Because the individual bifilarloops are balanced-input devices, someform of balun is required to provide bal-anced push-pull currents to the terminalsof each bifilar. In addition, to obtain theunidirectional radiation characteristic of

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the quadrifilar, the two bifilar loops re-quire separate excitations having a rela-tive phase difference of 90°. Further, thesense of the 90° phase relationship deter-mines from which end the quadrifilar ra-diates.

Several different balun andquadrature-phase circuit arrangementsare available for feeding the quadrifilar,such as the folded balun, the split-sheathbalun, or a combination of 90° and 180°hybrids, and so forth, as described byBricker and Rickert (Refs 88, 89). How-ever, to save weight and to effect simplic-ity in the construction, a unique feedingarrangement is used. This technique fea-tures the infinite, or inherent balun, com-bined with a novel method of self-phasingthe two bifilar loops to achieve the 90° dif-ferential current relationship between theloops. The constructional simplicity isapparent in Fig 22-1, which illustrates thequadrifilar configuration used on both theTIROS-N and Air Force 5D satellites, andon the amateur satellite, AMSAT-OSCAR7.

Sec 22.8 The Infinite orInherent Balun

end of the second half of the loop is con-nected to the outside surface of the coaxfeed line. Thus, the loop is closed at theantipodal point of the loop, where the feedline enters. The second half of the loop isa solid wire of the same diameter as theouter diameter of the coax.

In operation, current flowing on theinner conductor of the coax emerges at thefeed point to flow onto the second half ofthe loop. For current flowing on the in-side of the outer conductor of the coax, onits arrival at the end of the coax, the onlypath for current flow is around the endand onto the outside of the outer conduc-tor. Now such external feed-line currentis the desired antenna current, becausethe outside portion of coax extending fromthe feed point to the antipodal point is theradiator. Externally, the antipodal pointdemarks the end of the feed line and thebeginning of the loop radiator. Because ofskin effect, the transmission line currentsflowing inside the coax portion of the loopare completely divorced from the antennacurrents flowing externally on the loop.Their only relationship is that the inter-nal transmission-line currents emerge atthe feed point, where they become theexternal antenna currents.

As the feed line is dressed away fromthe antipodal point symmetrically relativeto the loop, currents induced on the feedline because of coupling from each half ofthe loop are equal and flow in oppositedirections. The opposing currents on the

Fig 22-9 — Feed arrangement for 90 ° self-phasing of loops.

In the infinite balun, shown in Fig22-9, we see the coaxial feed line extend-ing into the loop and shaped to form thefirst half of a bifilar loop. At the end ofthe coax, the inner conductor is connectedto the opposite or second half of the bifi-lar loop to form the feed point. The other


line thus cancel each other, decoupling thefeed line from the loop. In other words,from the external viewpoint of the loopradiator, the source generator can be con-sidered to exist directly between the twoinput terminals of the loop at the feedpoint, and the feed line effectively disap-pears. Thus, the current-mode transitionin the coaxial-line portion of the loop —from an internal unbalanced mode enter-ing at the antipodal point, to an externalbalanced mode emerging at the feed point— constitutes an inherent balun device.

Such a device is called an “infinite balun.”

Sec 22.9 Self-PhasedQuadrature Feed

Fig 22-10 — Impedance versus frequency plot of self-phased quadrifilar antenna.

As stated, a quadrature-phase cur-rent relationship is required between thetwo bifilar loops of the quadrifilar array.This requirement is met by using theself-phasing method. The orthogonal bifi-lar loops are designed such that one loopis larger relative to the desired resonantfrequency length and therefore inductive,while the other loop is smaller and there-

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fore capacitive. Using this method, thetwo loops are fed in parallel by connect-ing the terminals of both loops togetherat the feed point, as shown in Fig 22-9.This self-phasing method requires onlyone coaxial feed line, and any of the balunarrangements mentioned earlier (shownby Bricker and Rickert, Ref 88) may beused. It is evident that one loop half isthe coax feed line, while the other threehalf loops are simply solid wire.

The larger inductive loop is designedsuch that, at the operating frequency, thereactive component XL of the loop termi-nal impedance is equal to the resistivecomponent, R. Similarly, the smaller ca-pacitive loop is designed so its reactivecomponent XC equals R at the operatingfrequency. The ± X = R relationship isimportant, because to obtain a relativecurrent phase of 90° between the twoloops, the larger loop current must lag by45° and the smaller must lead by 45°.

For the current phase of the largerloop to lag, or the smaller loop to lead by45°, their phase angles must be ±45°, orhave an arc tangent of ±1. This occurs onlywhen ± X = R. When the two loops are fedin parallel, the relative lag and lead cur-rents in the loops differ in phase by 90ºwithout requiring any additional compo-nents to obtain separate differential phaseexcitations. Dimensions that yield thecorrect phase relationship with a loop wirediameter of 0.0088 λ are as follows.

Smaller loop:D = 0.156 λlenP° = 0.238 λPerimeter= 1.016 λ

Larger loop:D = 0.173 λlenP° = 0.260 λPerimeter = 1.120 λ

whereD = diameter of imaginary cylinder

on which bifilar is wound lenP° = axiallength, as shown in Fig 22-3

The resultant impedance versus fre-quency response of the parallel-loop com-bination is shown in the Smith Chart, Fig22-10. The formation of the cusp in theimpedance locus is the design goal thatsignifies that the 90° phase relationshipexists between the loops.

See 22.10 The Quadrifilar inSpace

As I’ve indicated above, the quadri-filar antenna was developed for spacecraftuse. Fig 22-11 shows the comparative sizebetween an experimental S-band quadri-filar and the flight-model system of acircularly polarized crossed-dipole an-tenna over a ground plane, which thequadrifilar replaced. The two antennashave identical radiation characteristics.Four of the crossed-dipole ground-planeunits flew on ITOS (TIROS-M). They werereplaced with quadrifilars on the newerTIROS-N to save space and weight, butaccomplished the same communicationstasks on the same frequencies. Using thesame technique as described in Sec 22.9for the quadrifilar, the required 90° dipolephasing to obtain circular polarization inthe crossed-dipole array was accom-plished by using the short and long dipoleelements shown in the picture. The shortelements are capacitive, causing the di-pole currents to lead by 45°, and the longelements are inductive, causing the cur-rents to lag 45°, resulting in a differen-tial phase of 90°. The balun is a split tube.

Before the launch of OSCAR 7,AMSAT publicized the quadrifilar an-tenna in the AMSAT Newsletter for March1975 (Ref 131). The cover picture for thatissue is included here as Fig 22-12. The


quadrifilar helix antenna is shownmounted on an OSCAR 7 test model atthe RCA Space Center, attached to anITOS weather satellite (TIROS-M) in thepiggyback launch configuration. Fig 22-13shows another view of the test model,while Figs 22-14 and 22-15 show thequadrifilar mounted on the OSCAR 7spacecraft as it is being readied for flight.In spite of its small size (0.7 ounce), thequadrifilar antenna boasts tremendousperformance for spacecraft use. RCA Engi-neers Randy Bricker, Herb Rickert, andmyself developed the original design, foruse on the highly successful USAF space-craft, the Block 5-D Meteorological Satel-lite, which was built by the RCA

Astro-Electronics Division of Princeton,New Jersey.

The OSCAR 7 antenna is circularlypolarized, radiates hemispherically with-out requiring a ground plane, and has again of 5 dBic on axis going down smoothlyto 0.0 dBic 90° off axis over the entire 360°around the edge of the hemisphere, withgood polarization circularity at the hemi-sphere edges. It is fed with an infinitebalun, and has elements phased at 90°with no phasing line. With a modificationI devised to operate at the 2304.1MHzbeacon frequency, this antenna was fab-ricated under the direction of Bricker andmyself specifically for the OSCAR 7 ama-teur spacecraft, and presented to AMSAT

Fig 22-11 — Comparing the size of a quadrifilar antenna and a flight model, circularlypolarized crossed dipole over a ground plane system, which it replaced. The twoantennas have identical radiation characteristics, but the quadrifilar obviously requiresless space and is lighter.

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Fig 22-12 — This photo was used as the cover picture for the March 1975 issue of theAMSAT Newsletter (Ref 131) . Shown clockwise from the upper left are Walt Maxwell(W2DU), Walt Ozman (W2WGH), OSCAR 7 Project Manager and NASA engineer Jan King(W3GEY), and Randy Bricker, who is pointing out the radiation characteristics of the2304-MHz OSCAR 7 beacon antenna.


Fig 22-13 — The OSCAR 7 quadrifilar on its test model in its piggyback launchconfiguration alongside the flight-model ITOS spacecraft with which it flew. Left to rightare Bricker, Maxwell (W2DU), and Ozman (W2WGH).

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Fig 22-14 — The OSCAR 7 quadrifilar, lower right, mounted on the OSCAR 7 spacecraftduring prelaunch rediness operations.


by RCA, which designed and built theTIROS-ESSA-ITOS-TIROS-N weathersatellite series. (After being accepted bythe U.S. Weather Bureau these RCA-builtspacecraft became known as NOAA, witha number designation to determine thespecific spacecraft of the series.) I alsoserved as consulting engineer for all an-tenna systems on AMSAT OSCAR 7. RCATechnicians Walt Ozmon, W2WGH, andJoe Rovenski, W2HLO, performed manyof the impedance and radiation patternmeasurements during the final testing ofthe OSCAR 7 quadrifilar antenna.

Scaled up for lower frequencies, thequadrifilar has similar performance char-acteristics. At 146 MHz, the quadrifilar

would form a cylinder approximately 13inches in diameter and 20 inches high,while at 432 MHz, the diameter andheight would be 4.4 inches and 6.8 inches.At 29 MHz the dimensions would be 5.5feet and 8.4 feet, respectively. The circu-lar polarization reduces polarization fad-ing caused by Faraday rotation of the elec-tric field vector, especially on the 10-meterdownlink. The quadrifilar scaled for useat 137 MHz is used quite extensively byweather buffs for receiving APT weatherpictures, with excellent results on both137.5 and 137.62 MHz. One note is of par-ticular importance for those intending tobuild the quadrifilar. The diameter of theconductors in the bifilar loops is especially

Fig 22-15 — A close-up view of the OSCAR 7 quadrifilar during prelaunch readiness.

22-20 Chapter 22

critical. The reason is that the ±45° lead-ing and lagging currents in the respectivebifilar loops, which obtains the required90° differential phasing between the twoloops, is derived by the corresponding ca-pacitive and inductive reactances in theloops. The distributed inductance in eachloop determines the reactance in the loop.When the length of the loop is less thanthe resonant length the reactance is ca-pacitive, causing the current to lead thevoltage. When the loop length is greaterthan the resonant length the reactance isinductive. Now the critical point: To makethe leading current precisely + 45° andthe lagging current precisely –45°, thedistributed inductance of the conductorsmust also be of precisely the correct value.To obtain the correct value of distributedinductance the length-to-diameter ratio ofthe conductor also must be correct. Theoptimum radiation pattern and gain of theantenna requires the leading and laggingcurrents to be ±45°. It therefore behoovesthe builder of the quadrifilar to carefullyobserve the conductor-diameter require-ment to obtain successful operation of theantenna. Using the λ dimension datalisted earlier, it may be helpful for the 137MHz builders to know that 3/4 inch softcopper tubing works very well. Generally,the 137 MHz will allow access of a NOAAweather spacecraft signal at between 5°and 10° in elevation with no nulls during

an entire pass, with continued contactuntil between 5° and 10° in elevation atthe end of the pass. The beauty of the per-formance of this antenna is that it needsto be no more than λ/4 above the ground,requires no manipulation or positioningto follow the orbital path of the spacecraft,but it does appreciate a clear shot to theorbital path of the spacecraft. NOAAspacecraft 9 through 13 are presentlyavailable for receiving APT pictures in the137 MHz band.

In summary, the quadrifilar helixantenna differs significantly from the con-ventional helical antenna. Control of pat-tern shape and other radiation character-istics are available by selecting appropri-ate dimensional parameters. A novelbalun for feeding this balanced-input de-vice from coaxial feed line is used, as is amethod of self-phasing the loop radiatingelements to obtain a quadrature currentrelationship between the loops. Thequadrifilar helix is a durable antenna, onethat is seeing use on both amateur andcommercial satellites, as well as onground stations for receiving APT weatherinformation transmissions on 137.5 and137.62 MHz from the TIROS-NOAAweather satellites. The satellites pres-ently transmitting on these APT frequen-cies are TIROS-N NOAA 9 through NOAA14.

1 Among other tasks on the quadrifilar project, I performed the R & D research onmore than 1000 different configurations of the quadrifilar helix, which led to the de-velopment of the six quadrifilar antennas flying on the polar-orbiting TIROS-N NOAAweather spacecraft. A copy of my research report appears in the Appendix.

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The Quadrifilar Helix Antenna - it