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 fre- quency 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- 595 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 focused on 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). tance of 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 1.64 the 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 . 596 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 tennas with 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 con- ducting 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 geo- metrical 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 spher- ical 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 electri- cally close.