50 GPS WORLD July 1998

A Primer onGPS AntennasRichard B. Langley

University of New Brunswick

The GPS receiver is a marvel of modernelectronic engineering. By processing thesignals transmitted by the constellation oforbiting Navstar satellites, its sophisticatedcircuitry can deliver position, velocity, andtime information to a user anywhere on ornear the earth’s surface, 24 hours a day, every day. But before the receiver can use the signals, they must first be captured. This is the task of the receiver’s antenna.

GPS signals are relatively weak comparedwith the signals from broadcasting stationsand terrestrial communications services, and a GPS antenna is specially designed towork with these feeble signals — a coat hangerwill not do! In this month’s column, we’ll take a look at the GPS antenna. This will only be an introduction to the complex subject of antenna design and construction,but it should enable you to better understandantenna specifications and how yourreceiver’s antenna works.

“Innovation” is a regular column featuringdiscussions about recent advances in GPStechnology and its applications as well as thefundamentals of GPS positioning. The columnis coordinated by Richard Langley of theDepartment of Geodesy and GeomaticsEngineering at the University of NewBrunswick, who appreciates receiving yourcomments as well as topic suggestions forfuture columns. To contact him, see the“Columnists”section on page 4 of this issue.

How does a GPS receiver’s antenna, or anyantenna for that matter, manage to snareradio signals out of the ether and pass themon to the receiver? And what are the impor-

and magnetic fields are transverse to thedirection of propagation, and the fields aremutually perpendicular. If we introduce acoordinate system whose positive z-axis isaligned with the direction of propagation ofthe wave, then the electric and magnetic fieldvectors lie in the x-y plane.

One can decompose the vector describingthe electric field into two orthogonal vectors,one parallel to the positive x-axis and oneparallel to the positive y-axis. If the x- and y-components have the same phase (or are dif-ferent by an integer multiple of π), the waveis said to be linearly polarized, as the electricfield vector is always directed along a fixedline. Most terrestrial radio signals are linearlypolarized with the electric field orientedeither horizontally or vertically. If the twocomponents differ in phase, their sumdescribes an ellipse about the z-axis. This isan elliptically polarized wave. If the twocomponents have the same amplitude but areπ/2 (or an odd multiple of π/2) out of phase,the ellipse becomes a circle and the wave issaid to be circularly polarized (see Figure 1).

Circular polarization implies a “handed-ness.” If, at a fixed point in space, the electric(and magnetic) field vectors rotate clockwise(counterclockwise) for an observer lookingfrom the source toward the direction of wavepropagation, the polarization is right-handed(left-handed). (This is the electrical engi-neer’s’ convention; in classical physics literature, we sometimes find the oppositeconvention.) The signals emitted by GPSsatellites are right-hand circularly polarized(RHCP). Some other satellites, notably spin-stabilized ones, also use circular polar-ization. For maximum signal strength, thepolarization of the receiving antenna mustmatch the polarization of the signals.

The antenna described in our openingparagraph is perhaps the simplest kind ofantenna — the dipole. At its microscopiclimit, it consists of a pair of spaced charges ofequal magnitude and of opposite signs. Whenthis antenna is driven, the charges oscillate inmagnitude and polarity, generating a uniformcurrent between the charges and radiating anelectromagnetic wave. On the macroscopicscale, one can approximate such an idealizedantenna using a center-driven piece of wirewhose length is a fraction of the wavelengthof its radiation, with metal plates or spheresattached to the ends of the dipole that in-crease the antenna’s capacitance. The capac-itive ends allow the antenna to store moreenergy, which helps maintain a nearly uni-form charging current in the wire.

We can easily calculate the radiating prop-erties of this so-called Hertzian dipole. We

I N N O V A T I O N

tant characteristics of antennas? These arequestions that we will attempt to answer inthis month’s column. Before we can under-stand how antennas work, however, we needa quick refresher on electromagnetic waves.

FIELDS AND WAVESAn electromagnetic wave is a self-propagat-ing wave with both electric and magneticfield components generated by the accelera-tion of a charged particle. If we connect anoscillator — a generator of alternating volt-age — to the center of a piece of wire, theouter electrons of the atoms making up thewire move back and forth, generating analternating current and an oscillating electricfield. The oscillating electric field producesan oscillating magnetic field, which, in turn,gives rise to a new oscillating electric field,and so on. These coupled fields form a uni-fied electromagnetic field that continuallyspreads ever outward from the antenna in theform of an electromagnetic wave.

Unlike sound waves, electromagneticwaves do not require a medium for theirtransmission. The ether, the medium oncethought to permeate all of space and to permitthe waves to propagate, is now consideredimaginary. When a receiving antenna inter-cepts an electromagnetic wave, the wave’sassociated field will induce currents in theantenna that are then fed to the receiverthrough a transmission line.

Although it is the wave’s electric field thatstimulates the currents in most receivingantennas, the current in some antennas —notably loop antennas of the kind used in AMradios and some antennas used to receivemarine differential GPS radiobeacons, forexample — comes primarily from the mag-netic field.

In many practical situations, an antennaused for reception has the same properties asthe identical antenna when used for transmis-sion. This characteristic of reciprocity provesuseful when describing how antennas work.

It can be shown that in free space (a vac-uum) — or in any homogeneous, isotropic,linear, and stationary medium — the electric

July 1998 GPS WORLD 51

can consider a real antenna to be equivalentto a number of Hertzian dipoles in series, andwe can deduce its properties by superimpos-ing the effects of the individual Hertziandipoles.

ANTENNA CHARACTERISTICSThe GPS antenna’s job is to convert theenergy in the electromagnetic waves arrivingfrom the satellites into an electric current thatcan be processed by the electronics in thereceiver. The antenna’s size and shape arevery important, as these characteristics gov-ern, in part, the antenna’s ability to pick upand pass on to the receiver the very weakGPS signals. The antenna may be required tooperate at just the L1 frequency or, for dual-frequency receivers, at both the L1 and L2frequencies. Also, because the GPS signalsare RHCP, all GPS antennas must be RHCPas well. Despite these restrictions, severaldifferent types of antennas have been and arecurrently used with GPS receivers. Theseinclude monopole and dipole configurations,quadrifilar helices (also known as volutes),spiral helices, slots, and microstrips.

Perhaps the most common GPS antenna isthe microstrip because of its ruggedness andrelative ease of construction. It can be circu-lar or rectangular in shape and is roughlysimilar in appearance to a small piece of cop-per-clad printed circuit board. Made up ofone or more patches of metal separated froma ground plane by a dielectric sheet (referredto as the substrate), microstrips are oftenreferred to as patch antennas. They may haveeither single- or dual-frequency capability,and their exceptionally low profile makesthem ideal for many applications.

Axial Ratio. To be maximally sensitive toGPS signals, the ideal GPS antenna should beperfectly right-hand circularly polarized.However, a real antenna will actually beelliptically polarized. The more elliptically

same frequency traveling in opposite direc-tions. In the ideal half-wavelength dipole, forexample, the current traveling from the feedpoint undergoes a reflection at the ends of thewire. The reflected wave, when combinedwith the incident wave, creates a sinusoidalstanding wave that has a constant zero ampli-tude at each end — a null or node — and amaximum amplitude in the center — a loop.

An important consideration is theantenna–receiver connection. This isachieved with a transmission line, usually acoaxial cable. To maximize signal transferfrom the antenna to the receiver, we mustminimize power loss. Power may be lost ifthe coupling between the antenna and thecable is imperfect and also within the cableitself. (We’ll discuss cable loss later.) To pre-vent power loss at the interface between thecable and the antenna, the impedances of thecable and the antenna must be the same.

Antennas are designed to be coupled to acoaxial cable with a certain impedance. If theantenna’s characteristic impedance is differ-ent from that of the cable, then one mustincorporate a matching circuit of some kindwithin the antenna so that the cable andantenna impedances match at the antennaconnection terminals. As we’ve mentioned,most GPS antennas have a 50-ohm imped-ance because they are designed to work with50-ohm cable. If there is a mismatch betweenantenna and matching circuit or matching cir-cuit and cable, signal reflections can occur,which give rise to standing waves in thecable.

The impedance mismatch can be quanti-fied by measuring the peak signal voltagesalong the cable. The ratio of the maximum tominimum peak voltages is called the voltagestanding wave ratio (VSWR). (We can alsomeasure the VSWR by expressing it as afunction of the forward and reflected signalpowers.) In the absence of reflections (inother words, a perfect match), the VSWR is1:1. Such an ideal VSWR is essentiallyimpossible to obtain in practice, and mostconsider a value of 1.5:1 to be quite good.The corresponding signal loss for a VSWR of1.5:1 is only 0.18 dB.

How an antenna is fed can also determineits polarization. Several techniques exist forfeeding microstrip patch antennas, for exam-ple, to produce circular polarization.

Bandwidth. Another important characteristicof an antenna is its bandwidth. This is the frequency band over which the antenna’sperformance, as measured by one or moreparameters (such as input impedance, pat-tern, polarization, and so on) is acceptablygood. The bandwidth needs to be large

polarized it is, the lower its RHCP sensitiv-ity, or gain. The degree of ellipticity is givenby the antenna’s axial ratio (essentially theratio of the axes of the polarization ellipse).An axial ratio of unity, or 0 decibels (dB),implies circular polarization. Good GPSantennas have an axial ratio in the zenithdirection of 2 dB or better.

Impedance. An antenna will have some resis-tance to current flowing in it. In addition, itmay have some reactance because of antennacapacitance or inductance, which also affectthe alternating current flow. The combinedopposition to current is known as impedance.It can be expressed as a complex numberwhose real part is the resistance and theimaginary part is the reactance. The relation-ship between impedance and the voltage andcurrent in an antenna or any circuit element isgiven by the extension of Ohm’s law to alter-nating current: It is the ratio of the voltage tothe current and is measured in ohms.

For antennas, impedance is typically mea-sured at the feed point where the antenna isconnected to the transmission line. Anantenna’s impedance depends on many factors: how it is constructed, how it is fed, and, to some degree, the surroundingenvironment. For example, an isolated cen-ter-fed dipole with a length equal to one-halfits operating free-space wavelength has acharacteristic impedance of 73 + j42.5 ohms.By making the antenna shorter by about 5percent to account for the current speed in the antenna being slightly smaller than thevacuum speed of light, the reactance disap-pears and the antenna becomes resonant atthe free-space wavelength.

The folded dipole, commonly used as thedriving element in multi-element Yagi anten-nas (often used for point-to-point signaltransmission and reception at very- and ultra-high frequencies, for example), has a reso-nant impedance of about 300 ohms. MostGPS antennas are designed with a character-istic impedance of 50 ohms.

If one places an antenna inside an enclo-sure, its impedance and resonant frequencymay change. A microstrip patch antennaplaced in a plastic enclosure, for example,can have its resonant frequency shifteddownward by several megahertz (MHz),depending on the thickness of the plastic andits dielectric constant. Antenna manufactur-ers, therefore, purposely design their patchantennas to resonate at a higher frequencythan the actual operating value.

Standing Wave Ratio. In a resonant antenna, thecurrent and voltage distribution is a standingor stationary wave. A standing wave isformed by the superposition of waves of the

I N N O V A T I O N

Figure 1. At a fixed point in space, theelectric field vector of a right-handcircularly polarized wave rotates clock-wise as seen from the wave’s source.

x

y

t

52 GPS WORLD July 1998

enough so that the antenna functions wellover the range of frequencies for which it isintended.

Antennas may be narrowband or broad-band. Resonant antennas are characteristi-cally narrowband, although their bandwidthcan be increased through certain constructiontechniques. If one uses the antenna for a sys-tem that employs only a narrow frequencyband, then keeping the antenna’s bandwidthnarrow is advantageous, as potentially inter-fering signals on adjacent frequencies willthen be attenuated somewhat. Most single-frequency GPS antennas are narrowbanddevices.

A microstrip patch antenna designed foruse with a standard C/A-code receiver mighthave a quoted bandwidth of only ±2 MHz.Antennas for receivers that use more than thecentral lobe of the C/A-code spectrum or thatuse the encrypted P-code — whether fullP(Y)-code or codeless or semicodeless track-ing — need to have a wider bandwidth, say±10 MHz. Dual-frequency, L1/L2 antennastypically have two patches, one for each fre-quency, each one of which has a bandwidthof about ±10 MHz. Back in GPS’s early days,some dual-frequency receivers used conicalspiral antennas whose bandwidths stretchedall the way from the L2 to the L1 frequency.

Gain Pattern. Other important characteristicsof a GPS antenna are its gain pattern, whichdescribes its sensitivity over some range ofelevation and azimuth angles; its ability todiscriminate against multipath signals, thatis, signals arriving at the antenna after beingreflected off nearby objects; and, for anten-nas used in very precise positioning applica-tions, the stability of its phase center — theantenna’s electrical center, to which the posi-tion given by a GPS receiver actually refers.

A GPS antenna is typically omnidirec-tional. Such an antenna has an essentiallynondirectional pattern in azimuth and a direc-tional pattern in elevation angle. At thezenith, a flat microstrip patch, for example,might have a few dB of gain with respect to acircularly polarized isotropic radiator (dBic),a hypothetical ideal reference antenna. Thegain gradually drops down to a few dB belowthat of a circularly polarized isotropic radia-tor at an elevation angle of 5 degrees or so. (Itis possible to improve the microstrip patch’slow-elevation-angle response by bendingdown the apexes of a polygonally-shapedpatch to produce a three-dimensional dome-like structure.)

Ground Planes. Some antennas, such as themicrostrip patch, require a ground plane tomake them work properly. This is usually aflat or shaped piece of metal on which the

their effects in relative positioning can beminimized by orienting antennas on shortbaselines to the same direction, say magneticnorth. For a well-designed antenna, the phasecenter’s mean horizontal position usuallycoincides with the antenna’s physical center.The phase center’s vertical position withrespect to an accessible physical planethrough the antenna must be established byanechoic chamber measurements. Note thatthe L1 and L2 phase centers of dual-fre-quency antennas may be different.

Now, as long as one is using the samemake and model of antenna at both ends of abaseline, the phase center’s actual position isusually unimportant; only the vertical heightsof a specific point on the exterior of theantennas (say on the preamplifier housing’sbase) above the geodetic markers must bemeasured. If one employs a mixture of anten-nas of different make and/or model on a baseline or in a network, however, then thedata-processing software must know theheights of the antennas’ mean phase centerswith respect to the physical reference pointson the antennas so that it can make the appro-priate corrections.

The effects of the variation in phase centerposition on some geodetic positioning can beimportant. Observation site, length of obser-vation session, use of ground planes, choiceof elevation cut-off angle, antenna orienta-tion, and frequency all can affect the an-tenna’s estimated coordinates. The maxi-mum sizes of the effects can range from afew millimeters to more than a centimeter.

actual antenna element sits. Up to a certainsize, the larger the ground plane, the higherthe antenna’s zenith gain. For example, amicrostrip patch element might have a zenithgain of only 0.75 dBic. By placing the ele-ment on a 10 3 10-centimeter ground plane,one can increase the gain to about 5 dBic.

In geodetic surveying, a metal plate orplates often further extend the antenna’sground plane to enhance its performance inthe presence of multipath. This is donethrough beam shaping (reducing the device’sgain at low elevation angles) and enhancingthe attenuation of LHCP signals. On reflec-tion, the polarization of RHCP signalschanges. Depending on the nature of thereflector, the reflected signals can be linearlypolarized or LHCP. An antenna designed to have a low sensitivity to LHCP signalsoffers some protection from single-bouncereflections.

One very useful form of ground plane isthe choke ring. It consists of several concen-tric metal hoops, or thin-walled hollow cylin-ders, mounted on a circular base at the centerof which is placed a microstrip patchantenna. Choke rings have proven particu-larly effective in reducing the effects ofground-bounce multipath.

One antenna not requiring a ground planeis the quadrifilar helix. This antenna is used,for example, with some popular handheldGPS receivers. It consists of two bifilar heli-cal loops, orthogonally oriented on a com-mon axis. As with the microstrip patch, this isan RHCP antenna with an omnidirectionalpattern.

Depending on its construction, the quadri-filar helix can have a gain pattern similar tothat of a flat microstrip patch or its low- tomedium-elevation-angle gain can be en-hanced at the expense of its zenith gain. This can be particularly useful in acquiringand tracking low-elevation-angle satellites,although the susceptibility to mulitpath is increased.

Sometimes, one wants the antenna gainpattern to have a null in a particular direction,say, in the direction of a jammer. Antennashave been developed that can be electroni-cally steered to afford them an antijammingcapability.

Phase-Center Variation. Ideally, a GPS an-tenna’s electrical phase center is independentof a signal’s direction of arrival. In practice,however, small (subcentimeter in the case ofwell-designed, geodetic-quality antennas)displacements of the phase center may occurwith changing azimuth or elevation angle.

Antennas of the same make and modelwill typically show similar variations so that

I N N O V A T I O N

This (top) 6.2 3 6.2–centimeter micro-strip patch GPS antenna is diagonallyfed and housed in a protective radome.

This (bottom) x-ray of a quadrifilar helixGPS antenna shows its four, half-turnhelices and feed structure, which areprinted on flexible foil material. Theantenna boresight, or zenith direction, isto the right.

TE

RR

Y A

RS

EN

AU

LTG

ER

HA

RD

WO

LTE

RS

, DJ6

YB

54 GPS WORLD July 1998

Some users are applying azimuth and/orelevation angle–dependent phase center cor-rections in processing GPS data using differ-ent or widely spaced antennas.

For some applications, users may alsoneed to consider the carrier-phase windupcaused by rotating antennas.

Other Factors. There are a number of environ-mental factors that can affect a GPS an-tenna’s performance and may need to beconsidered, depending on application. Theseinclude the effects of temperature, moisture(including high relative humidity), salt,vibration, and mechanical shock.

LOW NOISE PREAMPUsually, a plastic housing (radome) — whichis designed to minimally attenuate the signals— protects GPS antennas from possible dam-age by the elements or other means. As weknow, GPS signals are very weak. The reasona GPS receiver does not need a large antennahas to do with the GPS signal’s structure andthe GPS receiver’s ability to de-spread it.

So, the power to extract a GPS signal outof the ether’s general background is concen-trated in the receiver rather than the antenna.Nevertheless, one must generally combine aGPS antenna with a low-noise preamplifierthat boosts the signal’s level before feeding itto the receiver itself. In systems where theantenna is a separate unit, the preamplifier ishoused in the base of the antenna andreceives power from the same coaxial cablealong which the signal travels to the receiver.Such devices are called active antennas.

The gain needed by a GPS antenna pre-amplifier depends on several factors, includ-ing the gain of the antenna element itself, cable-run length, and the requirements of thereceiver’s front end. Active antennas areavailable with gains of around 20, 26, 40, andeven 50 dB, among others.

The preamplifier should have a low noisefigure as well as high gain, as its noise figurehas a dominating effect on the overall signal-to-noise level performance of the completeGPS receiving apparatus: Subsequent stagesin the receiver will amplify the preamplifiernoise. Typical noise figures range from about1.2 to 2.5 dB. Although a GPS antenna’s nar-rowband design helps to protect the pream-plifier and the receiver from interferingsignals, such as those from a nearby cellulartelephone, the system can be further aided byemploying filters either before or after thepreamplifier.

Placing a filter after the preamplifier helpspreserve the antenna assembly’s low noisecharacteristics. Placing a filter ahead of thepreamplifier, while likely increasing the

in the window recesses of aircraft, for exam-ple, but it is generally recommended thatantennas be mounted in the open air and witha clear view of the satellites. Even outdoors,dense foliage, particularly when it is wet, canattenuate the GPS signals sufficiently thatreceivers may have difficulty tracking them.

Two or more GPS receivers can share thesame antenna if an antenna splitter is used.The splitter must block the preamplifier DCvoltage supplied by all but one of thereceivers. The splitter should provide adegree of isolation between the receiver portsso that no mutual interference betweenreceivers occurs. Unless the splitter containsan active preamplifier, there will be at least a3 dB loss each time the signal from theantenna is split.

CONCLUSIONAlthough the GPS receiver derives most ofits amazing capabilities from its digital circuitry and firmware, it cannot begin tofunction until its antenna picks up the GPSsignals. In this brief article, we haveoverviewed most of the important character-istics of this critical component of a GPSreceiving system. Armed with this informa-tion, you should be better able to interpretantenna specifications and be better armednext time you go antenna shopping. ■

noise figure of the antenna assembly, helps toprotect the preamplifier from potentiallyoverloading interference by preselecting thefrequency band to be amplified.

Caution should be taken when using aGPS receiver with an active antenna not sup-plied by the receiver manufacturer. Not onlymust its noise figure and gain be within anacceptable range for proper receiver opera-tion, but the voltage (including its polarity)and current supplied by the receiver to theantenna’s preamplifier must be compatiblewith the antenna’s requirements. An activeantenna should include diode protection toprevent damage to the preamplifier fromreverse or overvoltage connections.

TRANSMISSION LINESAs mentioned, the signals received by theantenna are typically passed to the receiveralong a coaxial transmission line. The signalsare attenuated with the degree of attenuation,referred to as insertion loss, dependent on thetype and length of coaxial cable used. RG-58C cable has an insertion loss of about 0.8dB per meter at a frequency of 1575 MHz.The thicker Belden 9913, on the other hand,has an insertion loss of only 0.2 dB per meter.Even lower loss cables are available. Forlong cable runs, one should use low-losscable or place an additional preamplifier inline between the antenna and cable. Theseinsertion losses assume a perfect matchbetween the cable and antenna. As previouslynoted, a mismatch produces reflections in thecable, which increases signal loss.

A minor complication in connecting anantenna to a receiver is matching the cableconnectors to the antenna and receiver con-nectors. A variety of connector types exist,including BNC, F, MCX, Type N, OSX,SMA, SMB, and TNC in both male andfemale varieties.

The signals traveling from the antenna tothe receiver experience a small delay. Thisdelay, however, is the same for the signalssimultaneously received from different satel-lites and so acts like a receiver clock offset.In positioning applications, therefore, thisdelay is immaterial and is absorbed in theestimate of the clock offset or differencedaway in between-receiver relative measure-ments. In timing applications, on the otherhand, this delay must be carefully calibrated.

LOOSE ENDSGPS signals suffer attenuation when theypass through most structures. Some antenna/receiver combinations are sensitive enoughto work with signals received inside wooden-frame houses, on automobile dashboards, and

Further ReadingFor a general introduction to the interesting butsometimes arcane subject of antennas by theAmerican dean of antenna engineering, see

n “Antennas: Our Electronic Eyes and Ears,”by J.D. Kraus, in Microwave Journal, Vol. 32,No. 1, 1989, pp. 77–92.

For the arguably most-thorough discussionabout the theory of antenna design andoperation, see

n Antennas, 2nd edition, by J.D. Kraus, pub-lished by McGraw-Hill, Inc., New York, 1988.

For a review of antenna design, includingmicrostrip patch antennas, see

n Antenna Engineering Handbook, 3rdedition, edited by R.C. Johnson and publishedby McGraw-Hill, Inc., New York, 1993.

For discussions about how antennas affectGPS observations, see

n “How Different Antennas Affect the GPSObservable,” by B.R. Schupler and T.A. Clark, inGPS World, Vol. 2, No. 10, 1991, pp. 32–36.

n “Characterizations of GPS User Antennas:Reanalysis and New Results,” by B.R. Schupler,T.A. Clark, and R.L. Allshouse, in GPS Trends inPrecise Terrestrial, Airborne, and SpaceborneApplications, the Proceedings of the Interna-tional Association of Geodesy Symposium No.115, pp. 328–332, Springer-Verlag, Berlin, 1996.

For an overview of some low-cost GPSantennas, see

n <http://callisto.worldonline.nl/~samsvl/>For the National Geodetic Survey’s measure-

ments of the phase-center variations ofcommercially available, survey-quality GPSantennas, see

n <http://www.grdl.noaa.gov/GRD/GPS/Projects/ANTCAL/>

I N N O V A T I O N