Measurement of the Antenna Factor of Diverse Antennas Using a Modified Standard Site Method

James McLean and Gentry Crook Tactical Systems Research, Inc. 3207 Yellowpine Austin, TX 78757

Zhong Chen EMC Test Systems, L.P. P.O. Box 80589 Austin, Texas, 78708

Heinrich Foltz The University of Texas-Pan American Department of Electrical Engineering Edinburg, TX 78539

Abs&act: The standard site method given in ANSI down to the absorber.Thus at 20 MHz, an absolute mini- C63.5-1988 is a convenient, widely used technique for mum acceptableheight would be & = 2.38 metersabove determining the antenna factors of EMC testing antennas the top of the absorber. However, even this arrangement using open area test sites. The technique works well for is questionable as the reflectivity of most absorberat ex- dipoles and dipole-like antennasincluding broadbandbi- tremely oblique (grazing) anglesis large. Thus, in order to conical antennas. However, for some types of antennas obtain accurate direct measurementsof free spacequan- including somelog-periodic dipole arrays as well as some tities using an OATS, large antenna heights are required new types of compact,broadband radiators, this technique even with broadbandabsorbers in place. In any case,it is can result in significant measurementerror as given since very desirableto be able to obtain accurateantenna factor the gain and phasepatterns of the antennasare not taken data without going to such lengths. A technique for ex- into account. The method can give accurateresults if it is tracting free spacedata from measurementsmade over a modified to include both the gain and phasepatterns of the conducting ground plane was first given by Smith 12,31 antennas. Here we describe this new technique and then and later incorporated in the ANSI [4] and CISPR [5] demonstratethe effect it has on the accuracy of antenna standards. The technique has since been scrutinized ex- factor measurements. tensively [6, 7, 8, 9, lo]. Much effort has been invested in examining the effects of ground plane proximity on the INTRODUCTION antennasas well as mutual impedancecoupling between the antennasdue to near field interaction. The problem The antennafactor [l] has hecomea standarddescriptor of of ground plane proximity effect on the input impedance EMC metrology antennas. The ability to determine free- and hence reflection loss of the antennascan be ameho- spaceantenna factor from measurementsconducted on an rated to some degreeby simply increasing the heights of open areatest site (OATS) with a conducting ground plane the transmit and receive antennas. is crucial becauseanechoic chamberswith operating frequency rangesextending down to 20 MHz (and below) are Nevertheless,perfect correlation between antenna factor prohibitively expensive. Accurate measurementscan be data extracted from measurementsmade in OATS envi- made using an OATS covered with broadband absorber. ronments and free spaceantenna factors has been difficult However, to obtain accurate results, the antennas (both to obtain for some types of antennas. Experimentation transmit and receive) must be elevatedhigh enough off of has shown that this problem occurs with antennaswhich the absorberso that their reactive near fields do not extend exhibit radiation patterns significantly different from iso-

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lated dipole antennas. This includes log-periodic dipole computed in a straightforward manner using geometrical arrays (LJ?DAs) as well as several new compact radiat- optics (GO). It has been pointed out that this is only valid ing elements such as the one described in another paper when the separationdistance places the antennasin each submitted to this conference [ I1 1. Essentially all of the other’s far field region. This is generally the case for a previous work in this areahas focusedon dipoles (electric spacingof 10 metersfor frequenciesof about 30 MHz and or magnetic) and dipole-like antennas(such as biconical above. However, it is not for the commonly-usedtest dis- antennas). tanceof 3 meters(at 30 MHz). Analytical expressionsfor Eom,~,which are accuratein the nearfield will be derived STANDARDSITEMETHOD in this paper. A crucial part of the determination of antennafactor from In general, the geometrical optics 2-ray model gives the measurementsmade on an OATS is the extraction of the following expression for the electric field generatedby a effects of the ground-reflected wave from the measured simple source situated over a ground plane as shown in data. The standard site method incorporated in ANSI Figure 1: StandardC63.51988 and describedin detail by Smith [3] involves three site attenuation measurementsin which the -jPd2 ,j$ receiving antennais scannedover a range of heights above E= j/3- d@%+ -jPdl + IPl dqq d2 (4) the ground plane. The geometry of this method is illus- trated in Figure 1. where

Direct Ray (length dl) Observation Point dl = dR2t- (hl -h2)2,

Transmit Antenna 42 = j/iqiz$

Prti is the radiated power, G is the gain of the antenna, and pL$ is the complex reflection coefficient of the ground plane. With a radiated power of one picowatt and half-wave dipolegain of 1.64the expressionfor ED~(= for horizontal dipoles becomes:

lx102

Image 3/ -7 where IphI & is the complex reflection coefficient for horizontal polar&&ion impinging on an imperfect ground Figure 1: StandardSite Method for Determination of An- with relative permittivity &Rand conductivity CT: tenna Factor Ph = k’hl @h If the three measuredsite attentuations are AI, AZ, and As, the antennafactors are given by

AFl = 10logJn - 24.46 For vertical dipoles

+ ;[E;;UX+&+&-As] (l) Ef;.vwi=&i%R2 AF2 = 10logfm - 24.46 . ~~+d~lpvlz+2d:~lp~l~o~(O~--BId2--ci~I)]1’2pv,m t6j 44 + ;[E~+&+&-AZ] (2) AF3 = lOlogf,-24.46 where lpVlL@” is the complex reflection coefficient for vertical polarization impinging on an imperfect ground + $[Egm+A2+As-A~] (3) with relative permittivity &Rand conductivity CT: where all quantities are in dB. Here EEK is the maximum Pv = IPJ Lb electric field strength in the receiving antenna height scan (&R-j6Ob)Siny- (&R~-j60bCOS2y)1'2 range for an ideal losslessdipole. This parametercan be = (&R- j6Ob) Siny+ (&R- j6Ohc - COS2 y) ‘I2 .

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It is important to note that these expressionsfor Er This equation is exact in the sensethat, in its derivation, apply strictly only to dipoles and dipole-like antennas(an- the convolution of the assumedcurrent with the Green’s tennaswith dipolar fields). When the method is used with function is obtained without approximation. The only as- other antenna types, these expressionsimplicitly assume sumption is the sinusoidal shape of the current distribu- that the relative contributions of the line-of-sight (LOS) tion; this is known to be quite accurate for electrically- and ground-reflected rays are the same as for a dipole. short dipoles. This solution may be used to compute the Smith noted that with some high gain antennas(such as electric field due to a horizontal dipole situatedover a conducting ground plane by invoking an image as was done horns), it might be necessaryto incorporate the far-field in references[2,3] using GO fields. The modeling of the gain into the derivation of these expressions. He further effect of the ground plane with an image dipole is exact noted that it did not appearnecessary to do so with mod- even in the near field. Thus, the electric field due to a hor- erate gain antennassuch as LPDAs. We disagreewith this izontal dipole situated over a conducting ground plane is point. Moreover, the crucial information that apparently given by: has been left out is the phase diKerence of the two rays (the line-of-sight and the ground bounce rays) in the geometrical optics model. This phase difference arises from E = the phasevariation of the farfield electric field of somean- 2eP4 pw e-iWl --2cos(2)- tennas with elevation or azimuth. Of course, for a dipole d'1 4 (regardlessof whether it is horizontally or vertically po- . { larized), the phaseis constant with elevation angle, 8 (see Figure 1). This is definitely not the case for many other types of antennas. where Rrd is the radiation resistanceof the dipole. Note LL%IITATIONSOFTHE that, in order to couch the equation in terms of radiated GEOMETRICALOPTICSAPPROACH: power as was done in ref. [2, 31, the radiation resistance NONLINEARRETARDATIONOFFHASEINNEAR (however, not the reactance)must be d&et-mined. For an FIELD electrically short dipole, the radiation resistance is to a very good approximation: The geometrical optics approach used to obtain Equa- tion 4 models the field retardation of the dipole as a lin- R,.(& = BOB Sk (11) ear function of distance. In the nearfield of any antenna, However, the radiation resistance of a dipole in close the phase retardation with distance deviates significantly proximity to a conducting ground plane deviates signifi- from a linear function. For dipoles one-half wavelength cantly from this. Nevertheless,analytical expressionsfor and shorter for which a sinusoidal current distribution can this quantity can be obtained [ 141: be assumedthe induced EMF technique provides an ele- gant, analytical solution which includes the effects of de- Rrad = w3~)2 viation from linear phasein the nearfield [12, 13, 141.For - 30(/%42 an x-directed short wire dipole of length w centeredat the ~-sin(2/%) sin(2j3h) -2flcos(2/3h) n (12) origin, the current distribution can be assumedto be of the . [ 2i3h (2Ph)31 form: On the other hand, the fields of an electrically-short dipole Z(x) = I,sin(P(w/2- Ix]). (7) are known to be those of the lowest-orderTM-to-R spherical mode [ 16, 171Thus, we can write: The electric field everywhere is then given by: Ex (x, y, z) = -j301, . [j~e-‘pdl(j&-+&+&) ,-.Nh,--jpR, -+- (8) + jpe-jPd21pl$jLP Rl RI . ( 1 1 -1+7 - (13) where . ( 8d2 (AW2' (AW3 This expression should yield more accurate predictions RI =Jx-,v/2)Z+yZ+zZ of electric field for dipoles shorter than one-half wave- R2 = (x+w/2)2+y~+22 length when the source and observation point are electrically close. However,the expressiongiven by Smith is ex- I = @GG? (9) act for horizontally polarized dipoles in the farfield limit

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and actually yields more accurate results for horizontal dipoles with low heights. Finally, we note that the an- polarization when half-wave dipoles are used. alytical approach to obtaining the electric field given in In Figure 2, the electric field of a 1.0 meter long hori- Equation 10 can be extended to obtain results which are zontal wire dipole operating at 25 MHz and situated 1.5 accurate for half-wave dipoles. However, this is beyond meters over a conducting ground plane is plotted as a the scope of this paper. Here we have only shown that function of height at a distanceof 3 metersfrom the dipole ignoring nonlinear phase retardation in the nearheld is a on the dipole’s principal axis. The electric held was com- source of error in the ANSI standard(even for measure- puted using three approaches: first, the familiar GO ex- ment distancesspecified by the standard). pression given in Equation 4, second,using Equation 10 derived from the assumptionof a sinusoidal current distri- ANTENNASWHICHDONOTEXBIBITDIPOLAR bution, third, using Equation 13 derived using the lowest- RADIATIONPATTERNS order TM spherical mode, and finally a numerical com- Of the group of antennaswhich do not exhibit a dipolar putation using the ‘WE” (nearl?eld)card in NEC-4. The radiation pattern, it is useful to divide the group into two NEC ‘WE” card computes electric field using a numeri- categories:antennas with a well-defined phasecenter, and cal convolution of the computed current distribution with antennaswithout a well-defined phase center. The phase the Green’s function. From Figure 2, it can be seen that center as defined in IEBE Standardsfor Antennas [ 151is: the GO approximation agreeswith the NECY-4predictions “The location of a point associatedwith an antenna such only to within about 20 %. Equation 10 gives essentially that, if taken as the center of a sphere whose radius ex- identical results to thosecomputed with NEC-4. It is quite tends into the far field, the phase of a given field compo- interesting to note that Equation 13 also gives nearly iden- nent over the surface of the radiation sphereis essentially tical results as the numerical simulation. Thus, one can constant,at least over the portion of the spherewhere ra- conclude that the nonlinear phase terms in Equation 13 diation is significant.” Thus, the radiation pattern of an an- are the source of the deviation from the GO predictions. tenna with a well-defined phasecenter is completely spec- Finally, we note that this discrepancy occurs only when ified with only magnitude information. On the other hand, the source/receiverdistance is small. However, we note the radiation pattern of an antennawithout a well-defined that a 3 meter distance does separatethe source and the phase center exhibits a radiation pattern which must be receiver by one-quarter wavelength and that the reactive specified using amplitude and phase information. Unfor- near field of a short dipole only extends to about one-sixth tunately, in this case,the farfield phasepattern is strongly of a wavelength. dependentupon the origin chosenfor the spherical coordi- nate representationof the radiation pattern. Log-periodic dipole arrays fall into this class of antennas. A remedy r has been proposedfor LPDAs in the form of a phasecenter which moves as a function of frequency. In Figure 3, the simulated variation of the farfield gain and phase (ar- bitrary reference) of a typical LPDA (z = -88, d = -05) is given as a function of co-latitude angle. In the simulation, the phase center of the antenna was taken to be at the center of the dipole which was resonant at the simulation frequency, thus the phase center was taken to be at the center of the so-called active region of the LPDA. This is in keeping with the assumptionthat the phasecenter tracks the center of the active region in an WDA. As can be seen,over 90 degreesof co-latitude angle, the gain varies about 4 dB and thus the magnitude of the electric field varies by about a factor of 1.5. However, the phase Figure 2: Comparison of Electric Field Intensity Com- also varies significantly (more than 20 degrees)over this puted With Four Different Techniques range of co-latitude angle. As can be seenthe assumption that the phasecenter of an LPDA is the center of the active In summary, either of the two expressionsfor electric region only partially compensatesfor the phasevariation field can be used with electrically-short dipoles to give with co-latitude angle. This variation arisesfrom the fact more accuratepredictions of ~~~~ than the expressions that more than one dipole contributes to the radiation at given by reference [2, 31 or the ANSI standard. This is any frequency within the operating frequency range of the true for any separaliondistance or height although the im- antenna. Thus the radiation at any given frequency is aris- provement in accuracy is primarily for closely separated ing from severalspatially distinct antennas.

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active region of the LPDA. if this were not done {as is generahy the casewhen a single phase center is arbitrar- ily chosen for the entire LPDA), the agreementbetween results obtained from the different approacheswould have been quite poor.

7e-06

2 6c-o.5 s- gI 5c-o6

Co-latitude Angle (Degrees) 0 1 4.506

2 Figure 3: Gain variation of LPDA with co-latitude angle .g! 3%06

B 2c-06 Ei

MODIFIEDAPPROACH k-06 0 0.5 1 15 2 2.5 3 35 4 A more rigorous approach to computing EF is to begin with the ray model (using the appropriate reflection coef- Figure 4: Comparison of Electric Field Intensity Com- ficient for horizontal or vertical polarization) and derive puted With Four Different Techniques (instead of Equation 4)

E=&i APPLICATIONTOPXMANTENNAS . &qiqp@ i The P x M antenna,a detailed description of which is pre- sentedin another paper at this conference [l 11,is a par- + djqqejLqeZ) , ticular antenna for which the radiation pattern gain char- acteristics must be included in the expressionfor q if an accurateassessment of the antenna factor is to be ob- where F(8) is the complex directivity pattern of the trans- tained using a standardsite method. This is true in spite mit antenna and 81 and 92 are the co-latitude angles of of the fact that the P x M antennais a relatively low gain the direct (LOS) and ground bounce rays. This approach, antenna (4.77 dBi maximum gain) and the fact that the while not including the nearfield nonlinear retardation of P x M antenna has a well defined phase center and thus phase with distance, should, in principle yield very ac- no variation of phaseover the far field radiation sphere. curate results. That is, the method is exact in the in the farfield limit. In Figure 4, the electric field as a function of An analytical expression for ET of the P x M antenna height above the ground phtne is plotted for the LPDA in can be derived easily by noting that the farfield electric Figure 3. The LPDA is horizontally polarized and situated field in free spacevaries as: 1.5 meters above a perfectly conducting ground plane. The horizontal distance from the center of the active re- E = G [l +cos(0)] gion to the observationpoint is 3 metersand the frequency is 453 MHz (The LPDA is designed to cover from 150 m to 2 GHz so this frequency is not near either the up- where t3is the latitude angle measuredfrom the boresight axis as shown in Figure 1. In the presenceof a conduct- per or the lower operating frequency limits). The electric ing ground plane, the imagesof the electric and magnetic field is computed first using the NIX-4 “NE” card which dipole can he used to obtain the following expression for can be assumedto be accurate,second with GO assuming far-held electric field generatedby a P x M antennaradi- a dipolar field (Equation 4, but using the boresight gain ating 1 pW of power: of the LPDA), third with GO but adjusting the gain to account for changesin gain as a function of elevation angle, and fourth using Equation 14. The agreementbetween all four approachesis reasonably good but the inclusion of phase information does provide enhanced accuracy. Fi- nally, it is important to note that the phasecenter implic- itly used in Equation 4, is taken to be at the center of the

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where {p+l L&,” is the complex reflection coefficient for [4J ANSI C63.5-1988: American National Standard horizontal or vertical polarization impinging on an imper- for electromagnetic compatibility-radiated emis- fect ground with relative permittivity ERand conductivity sion measurementsin electromagnetic interference u: The factor of 2a reflects the gain of 3.0 for the P x M WI) control-calibration of antennas,IEEE, New antenna (as opposedto 1.64 for the half-wave dipole an- York 1988. tenna). [S] CISPR/mGl (Heirman) 4, August, 1990: Deter- mination of ant&a factors: CentraiOffice of the IEC, rue de VarembB,Geneva, Switzerland. 161Z. Chen and M. D. Foegelle, “A Numerical Inves- tigation of Ground Plane Effects on Biconical An- tenna Factor,” 1998 IEEE Symposium Record, Au- gust 1998,pp. 802-806

171 H. F. Garn, W. Miillener, and H. Kremser, ‘A Crit- ical Evaluation of Uncertanties Associated with the ANSI C63.5 Antenna Calibration Method and a Pro- posal for Improvements,” 1992 DEEEEMC Sympo- sium Digest. I81 J. DeMarinis, ‘Antenna Calibration as a Function of Height,” 1987IEEE EMC Symposium,pp. 107-l 14.

Figure 5: Predicted electric field for P x M antenna over 191 H. Iida, S. Ishigami, I. Yokoshima, and T. Iwasaki, ground plane ‘Measurement of Antenna Factor of Dipole Anten- nas on a Ground Plane by 3-Antenna Method,” IE- ICE Trans. Commum., Vol. E78-B, No. 2, pp. 260- The predicted electric field for a canonical P x M antenna is plotted in Figure 5 as a function of height above 267 Feb. 1995. the ground plane. flO1 A. Sugiura, T. Morikawa, T. Tejima, and H. Ma- suzawa, “‘EMI Dipole Antenna Factors,” IEKE CONCLUSION Tms. Commum., Vol. E78-33,No. 2, pp. 134-139 Feb. 1995. The limitations of the standard site technique for non- J. S. McLean and G. E. Crook, “P x M Antennas for dipole antennas have been demonstrated. A modifica- r.111 tion to the technique in which the maximum receivedfield Immunity Testing and other Field GenerationAppli- is calculated using the complex dire&v&y pattern of the cations,” presentedat IEEE 1999EMC Symposium. transmit antenna is proposed. Finally, an analytical ex- cm Robert S. Elliot, Antenna Theory and Design, pression for this maximum field for a P x M antenna is Prentice-Hall, Englewood Cliffs, NJ., 1981, pp. given. 329-332.

1131 S. A. Schelkunoff, Antennas Theory and Practice, References John Wiley & Sons,New York 1952,pp 368-370. I141 R. W. P. King, meory of Linear AntennaT, Harvard I31 C. R. Paul, Introduction lo Electromagnetic Cornpar- University Press,Cambridge, Massachusett, 1956. ibility, Wiley Interscience, New York, 1992. “IEEE Standard Definitions For Antennas,” IEEE PI A. A. Smith, Jr., R. F. German,and 3.3. Pate, “Cal- Tms. Antemw and Propagation, Vol. AP-31, No. culation of Site Attenuation From Antenna Factors,” 6, Nov. 1983. IEEE Trans. Ekcrromagnetic Comp., Vol. EMC-24, J. S. McLean, “A Re-examination of the Fundamen- No. 3, pp 301-315, August 1982. tal Limits of the Radiation Q of Electrically-small Antennas”, IEEE Trans.Ant. Prop., April 1994. [31 A. A. Smith, Jr., “Standard Site Method for Deter- mining Antenna Factors,” IEEE Trans. Electromag- R. F. Harrington, Time Harmonic Electromagnetic netic Comp., Vol. EMC-24, No. 3, pp 3 16-322 Au- Fields, McGraw-Hill 1961. gust 1982.

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