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the antenna to the source (DUT).Whereas for calibration of commercialtesting, it is reckoned at a distance of3 and 10 metres from the centre of theantenna to the source (FCC testing).

Antenna manufacturers supplyLPAs duly calibrated as describedabove for EMI measurement applica-tions. The standard dealing with an-tenna calibration for 1 metre is SAEAPR 958. For 3 metres and 10 metres,antenna calibration is as per ANSIC63.5.Therefore any new development of an-tenna should conform to these stan-dards.

Presently, commercially availableLPAs conforming to these standards areusable only up to 5 GHz. Only a fewmanufacturers (including Antenna Re-

Printed-circuit antennae come inhandy because of their lightweight, ease of manufacture and

quality production in bulk. At high fre-quencies, especially from 1 to 18 GHz,horn antennae are invariably used forelectromagnetic interference (EMI)measurements. As a possible alterna-tive, log-periodic directional antennaeare also used for EMI measurements.In this article, two log-periodic anten-nae (LPAs) using two different dielec-tric materials (glass epoxy and teflon)are described.

The basics

The basic requirements of a log-peri-odic antenna for making EMI measure-ments are:

1. It should be able to perform bothemission and immunity measurementsaccurately or comparable to broadbandhorn antennae.

2. It should handle adequate RFpower for generating high fieldstrength (20V/m or above).

3. It should possess dualpolarisation and exhibit high gain andlow voltage standing-wave ratio(VSWR) besides good beam-width pat-tern.

While using LPAs for EMI mea-surements, calibration factor is an im-portant aspect. This differs for MIL-STD testing and commercial testing.The initial calibration for military test-ing is performed at a distance of 1metre as measured from the apex of

� S. SATHYAMURTHY

search, USA) deliver wire-element LPAsfor 18GHz applications and that too atreduced power levels (5 watts). AH Sys-tems, USA, supplies printed-circuitLPAs operating in the 1-18GHz range.

This design has been indigenisedhere as a possible replacement. Twoantennae experimented for the sameband with two different dielectric sub-strates (Er=4 and Er=2.55) have beendiscussed here.

Design parameters/specifications

The basic design requirements for thedevelopment of an LPA are:

1. Frequency coverage requirementis typically 100-1000 MHz and 1-10GHz for the two different designs.

Design of Printed-CircuitLog-Periodic Antennaefor EMI MeasurementsLearn to construct printed-circuit log-periodic antennae, compare theirmeasurement results and understand the effect of dielectric substrate ontheir performance

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Both the ranges suit SAE and militarystandard testing. The 100-1000MHzrange also suits commercial testing.

2. Power input is the ability of anLPA to handle directly from the RFamplifier with minimal mismatch.While 5GHz LPAs handle 100 watts,1GHz LPAs can handle as high as2 kW, both of them being wire-elementantennae. For printed-circuit antennae,it is less than 5W.

3. The power gain should be atleast 5 dB in the 200-1000MHz region.Some manufacturers produce LPAswith gain as high as 7.5 dB. For1-18GHz range, printed-circuit LPAsoffer 5 to 6 dB of gain.

4. Gain flatness required is ±1 dBover the entire range of frequencies.While this is a desirable figure, practi-cally the antennae exhibit higher val-ues.

5. Average and maximum VSWRshould not exceed 1.5 and 2, respec-tively, for the complete range of fre-quencies.

6. The average beam-width differsin ‘E’ and ‘H’ planes. The desirable val-ues are 127 to 152 cm (50 to 60 inches)in the E plane and 229 to 254 cm (90 to100 inches) in the H plane.

7. Front-to-back ratio of 12 to 15dB is an ideal requirement for LPAs.Unless asked for, these figures are notspecified by the manufacturers.

8. Impedance is invariably 50 ohms(nominal), while polarisation is linearin all LPA requirements.

9. The size of LPAs in the 100-1000MHz range comes to 6.3×74.9×74.9cm (h×w×l). And for 1-18GHz printed-element LPAs, it is 26×16×1 cm.

10. The average weight of a 1GHzantenna is 3.5 to 7 kg and for 18GHz itis less than a kilogram. All antennaeare mounted, in tripod for height ad-justments and for rotation in azimuthplane.

Construction

The antennae are constructed as perFig. 1. An LPA employs a number ofdipoles. The dipole elements increasealong the array by a current-scale fac-tor such that:

(li+1)/li=(di+1)/di=k ............. (1)From the principle of scaling, it is

evident that for this structure, extend-ing from ‘0’ to infinity and energisedat the apex, the properties repeat at ‘n’frequencies (kpi×f), where ‘n’ takes in-

teger values.When plotted on log scale,

these frequencies are equallyspaced at intervals of ‘log k.’Therefore the antenna is termedas ‘log-periodic.’ In this PCB,trace width of 0.5 cm was chosento get an impedance of 50 to 60ohms for the substrate height of2 mm. For a value of w/h=5/2=2.5,

Rin=60 ohms, for Er=2.5(teflon) ........Eq. (2)

Similarly, for glass epoxy,Er=4 and Rin=50 ohms. Accord-ingly, the transmission line wasetched in both the antennae (Fig.2(a) and (b), respectively).

Performance of glass-epoxy-based LPA

The EMI-frequency-receiving ca-pability of an indigenously de-veloped glass-epoxy-based LPAwas compared with that of animported LPA from AH Sys-tems, USA, and found to be ingood agreement for practicalmeasurements. The test resultsindicate that this antenna can beused for near-field diagnosticmeasurements in the 1-13GHzfrequency range. A distance of 1metre between the radiating andreceiving antennae was main-tained throughout the experi-ment at a constant height of 1.5metres from the levelled floor of

the shielded room. The resultsare shown in Table I.

Performance of teflon-based LPA

The teflon-based antenna wasfound to be much superior to theprinted-circuit-version (glass-ep-oxy) antenna. Also, unlike theprinted-circuit version, its perfor-mance is comparable to that ofthe test antenna throughout thefrequency range. Repeatable mea-surement results are possible withboth versions of the antennae.

Effects of dielectrics

The relative dielectric constant ofthe substrate plays an important role

Fig. 1: Log periodic array

Fig. 2: Design of LPAs

Fig. 3: Impedance characteristics of various substrates

(a) Glass-epoxy based (PCB) LPA

(b) Teflon-based LPA

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TABLE II

Performance of Commercially Available LPAsParameter AT1000 RF Amp 3145 DP EMCO 96005 EATON 3147 EMCO LPD118 Ant.

Resch.

Frequency 150-1000 80-1000 200-1000 200-5GHz 1-18GHz

Power Input 750-200W 1300W 1000W 100W 5W

Power gain 7.5 dB (average) NA 5 dB (min.) NA Excellent

Gain flatness ±1 dB NA NA NA NA

VSWR 1.5-1.8 <1.2 2:1 1.2 Excellent

Beam-width E-152 cm NA NA NA NA(60 inches),H-267 cm(105 inches)

Front-to- 15 dB NA NA NA Highback ratio

Ant. factor 7.5-22.5 dB 9-24.8 dB NA 11.7-42 NA

Polarisation Linear Dual Linear Linear Linear

TABLE I

EMI-frequency Received by theIndigenously Developed Glass-Epoxy-

based LPA and the Reference AntennaFrequency Frequencies received Frequencies receivedin GHz by the reference by the test

antenna (dBm) antenna (dBm)

1 –46.4 –46.8

2 –57 –58

3 –54.5 –55.5

4 –55 –52.7

5 –64.7 –66

6 –64 –62.8

7 –68 –71.5

8 –67 –65

9 –81 –79.1

10 –73.8 –71.6

11 –74.4 –75.5

12 –79 –79

13 –80 –80

in the impedance of the transmissionline. Therefore to achieve afeed-point impedance of 50 ohms,the width-to-height (w/h) ratio ofthe micro-strip patch has to be prop-

erly adjusted. To meet the requiredimpedance, ‘h’ is invariably keptconstant. Therefore width is theonly parameter that the designercan alter or choose a differentthickness of the substrate. Theresulting matching completelydepends on the material and accuracyof the artwork.

The impedance curve w.r.t. ‘w/h’ratio of the substrate is shown in Fig.3. Therefore dielectric constant has adefinite role in the design of theprinted-circuit LPA.

Besides, antennaeshould be adequatelytransportable andmountable at the testlocation (open site,shielded room, etc).The performance ofcommercially avail-able LPAs is shown inTable II.

Suggestionsfor making EMImeasurements

For diagnostic mea-surements, teflon-based printed LPA isthe best option and itcan directly replacethe test antenna. Forless accurate measure-ments, or just probingthe field strength, the

printed-circuit version is recom-mended. In both the cases, accuratecalibration of the antenna factor is amust prior to EMI measurements.Also, accurate positioning and mount-ing of the antenna are necessary. �

The author is Scientist G and additional director

(L) at Combat VEHS Research & Development

Estt, Ministry of Defence, Avadi, Chennai