Getting the Best out of Biconical Antennas for Emission
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Getting the best out of biccmicall antennas for emission measurements and test site evahatiom Martin J Alexander Marth H Lopez Martin J Salter NationalPhysical Laboratory CENAM National PhysicalLaboratory Teddington km 4.5 can-. 8 Los cuts Teddington TWll OLW,UK. El Marqu&, Cl?76900 QuerCtraro,M&&o TWll OLW, UK. Abstract: Thebiconical antemlahas been adopted for EMC testingbecause of its broadband,frequency coverage, 20 MI-J?to 300 MH?z,and compactsize. Potentiallythe antennaworks very well, but some models have poor reproducibility resulting in perceptionsof excessivelylarge uncertaintiesfor emissionmeasurements and predictingpessimistic performances of test sites.The antelmacan Side View End View be accurately modelled, which enables normalised site attenuation @ISA) values to be based on more rigorous Fig. lb Biconical antennaelements with cross-bars theoreticalprinciples, and challengesthe validity of the currently acceptedpractice of calibrating antennasat 3 m and 1 m The tuned dipole antelmais the industry standardand its use is distances.Antemla factor can be measuredwith uncertaintiesof enshrinedin CISPR Publication 16-l’. The problem with using f 0.2 dl3, which togetherwith calculability,make the biconical this antennais that it has to be tuned at every measurement antenna a worthy reference antenna. The use of properly frequency.These days modern receivers and spectrumanalysers constructedcollapsible elements is recommendedfor portability. are able to make the equivalent of swept frequency measurements,which is highly desirable in ensuring that all 1 Introduction unwantedEMC responsesare identit?ed.It is customaryto use broadbandantennas, the biconical being the acceptedtype LIPto The tuned“ half-wave” dipole antennahas the statusof being the 300MHz, andthe log-periodicantenna taking over to 1 GI-Tzand acceptedreference antenna for measurementof E-field strength. beyond. CISPR 16-1 requires that the antenna factor of This is becausethe theory of the radiation propertiesof such broadbandantennas be referredto the tuneddipole, including the antennasis well establishedand the antemlafactor is accurately effect of radiationpatterns. It is advocatedin this paperthat it is calculable, and the antenna is relatively easy to construct. possible to be weanedoff dependenceon the hwd dipole and Around the 1950sthere was a requirementfor a more compact that broadbandantennas should be establishedas reference antennafor making emissionmeasurements in screenedrooms. antennasin their own right. This is feasible on two accounts. The size of a typical room ruled out the use of a dipole tunedto Firstly there is a handful of laboratoriesin the world that offer 30 MHz since it was 4.8 m long. The biconical antennawith cost effective and accuratecalibration of broadbandantennas. “wire cage” eleme& was introduced,whose overall length is Secondly it is possible to model these antelmasso that they approximately1.4 m in the dimensionsspecified in MIL-STD- becomecalculable reference antennas. 461, still usedto this day, seeFig. la. Over tile yearsthe baluns have been improved,enabling a frequencyrange of 20 MHz to The performanceof broadbandantennas can be modelledwith 300 MI-h to be covered,with an improvedreflection coeficient. great precision using NEC2*3 . This paves the way to perfonn Schwazbeclcintroduced the singlecross-bar in eachelement, see experimentswith antennason the computer,which hasbeen done Pig.lb, which suppresseda high-Q resonanceat 287 MHz. for this paper to investigatethe methodsof biconical antemia calibration for 1 m distanceaccording to ARP958”, and NSA Balun , Metal Supports measurementsat a distanceof 3 m accordingto ANSI CG3.4$. The NEC calculatedproperties of antennashave beenvalidated by measurementto uncertaintiesas low as& 0.1 dB on a National Standardground planeG. The applicationof the tuneddipole antemlato openarea test site validation and EMC testing has its own problems, For many x years only free-spaceantenna factors were considered.More Side View End View recent technology has enabled overall EMC measurement uncertainties to be reduced, which has shown up antelma Fig. 1a Biconical antenna“ wire cage”elements propertiesthat were originally ignored,see clause4.3 of ANSI 0-7803-4140-6/97/$10.00 84 C63.57.Antenna factor, AF, is affectedby mutualcoupling ofthe 10 antennawith its image in the ground plane, such that AF can changeby 6 dB when a dipole tuned to 30 MHz is scannedin Sleightfrom 1 m to 4 m. Variation of Al! with height for a 16 biconical antennais dealt with in Section 2. When it bec<ame acceptableto make measurementsand perform calibrationsat a 14 distanceof 3 m the mutual coupling betweenthe antennashad alsoto be considered’. So far in this introduction intrinsic electromagneticproperties of IO antennas have been considered. Ways of measuring these propertieswill be given in the following sections,together with appropriate ways of using the measureddata. These will be _.-.-.- 1.5 m - 1.0m comparedwith cunent published n@hods, with shortcomings ----- Frees/~ace 6 highlighted.But now anotheraspect of the use of antennasmust be introduced. Some antenna models have exhibited poor 4 reproducibility of results. With hindsight one can say that these 30 50 70 90 110 130 150 170 190 210 230 250 models had design deficiencies, but this has wrongly led to exaggeratedmeasurement uncertainties for EMC activities.It has Frequcncy(MHz) led to speculationabout OATS defectsand aboutthe criticality Fig. 3 Antennafactor calculatedat 7 heightsand in free-space of equipment layout. Two common problems are bahm imbalance in biconical antennas and breakdown of RF Themeasured AFs are about 1 dB higherwhich is partly because connection on log-periodic array elements. Both of these the balun is not in the model, thereforeno balun loss, and also problems are eliminated by the choice of well designedmodels the model may not be complete,for examplethe solid conical of antenna. sectionswhich hold the wires togetherwere not in the model. The measurementswere of a 1:1 or 50 ohm balun. 2 Antenna factor clmwteristics of biconical antennas Note that for frequenciesbelow 55 MHz the antennafactor changesvery little with height. This is becausethe antemlais The antennafactor of a horizontally polarisedbiconical antemla behaving like a short dipole with a large input reactance.If the varies by up to 1.8 dB when the antennais scannedin height higher antennafactor can be tolerated,an accuratelycalibrated from 1 m to 4 m over a conductingground plane. This has been biconical antennawill generallymake it easierto obtain lower accurately measured”Or height increments of 0.5 m against measurementuncertainties than will a tuneddipole antennaused calculablestandard dipoles on a national standardGO m gound below 55 MHz. Becauseof the antenna’shigh self-impedanceat plane, Fig.2. In contrastantenna factor changesmuch less for a these lower frequencies,the effect of mutual coupling to its vertically polarised antenna. The change can be ignored for sunoundingsis negligible. heights above 1.5 m, where the antennafactor is effectivelythe free-spacevalue. This large variation of antenna factor with height causes a - problem with the accuratecalibration of biconical antennasby the ANSI method7.One of the three <antennashas to be usedat a - fixed height and as the height scannedantenna. Therefore one is trying to solve for four mknownsfrom threemeasurements. The calculation of antennafactors assumesonly three unknowns, so - the antennafactor of all three antennasis in error. The solution is to do the measurementwith all three antemlasat fixed heights above the ground plane, or to adopt free-spaceantenna factor, AI;,, as the primary pammeterfor antennas.The latter solution - is being proposedby an ad-hocworking groupof CISPRIA. ‘\,- The insertion loss between two bicones was measured and ,\, compredwith prediclion. The agrm~nent wt.~s better 111~1 1 dI3 asshown in Fig. 4. The biconeelements were on detachableNPL \ balls whoseS-parameters could be measured.The biconeswere -- horizontally polarisedat a height of 2 m and separatedby 10 m. 30 50 70 90 110 130 150 170 190 210 230 250 The shapesof the curvesare very similar, andthe model is being refined to give better agreementwith measurement,by altering Frequency(MHz) the segmentationand including the solid metal endsof the cages Fig. 2 Antennafactor measuredat 7 heightsand in free-space in the model. 85 Biconical antennaswere originally intendedfor use over the frequencyrange 20 MHz to 200 MHz. Thefirst designshad a poor return loss, typically less than 4 dB over most of the frequencyrange. This figure was improvedto 6 dl3 by a new balun designwith a 1:1 transformer,which also gavea good performanceup to 300 MHz. Howeverthe AF was typically 19 dB/m at 30 MHz. Thenext ma jorimprovement was to usea 4:l balun,otherwise known as 200 ohm,which gavean AF of 13dB/m at 30MHz andimproved return loss. The poor return loss below 40 MI-Iz is causedby the high reactancetypical of a shortdipole antenna. In orderto makethe antennaa more eficient transmitterfor immunity testing, the antennaelements can be replaced by longerelements. The propertiesof thebilog antennaare equally calculable using 20 40 60 60 100 120 140 160 100 200 220 240 260 260 300 320 NEX, which can be used to explore radiationpattems and Frequency (MHz) uncertaintiescaused by the variation of phasecentre position Fig. 4 comparisonbetween NEC andmeasured SA with tiequency.This is the subjectfor <anotherpaper. L?esignimprovements Collapsiblebiconical