ISSN 1064�2269, Journal of Communications Technology and Electronics, 2012, Vol. 57, No. 9, pp. 1031–1038. © Pleiades Publishing, Inc., 2012.Original Russian Text © N.A. Efimova, V.A. Kaloshin, E.A. Skorodumova, 2012, published in Radiotekhnika i Elektronika, 2012, Vol. 57, No. 9, pp. 1020–1027.

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INTRODUCTION

Antennas on the basis of irregular lines with theTEM mode, in particular, horn and biconical anten�nas are widely used as ultra�wideband antennas ofultrashort and microwave bands (see, e.g., [1]). Bicon�ical [2] and polyconic [3] antennas ensure very largematching bandwidth; however, they have small gain.Moreover, the gain of these antennas varies nonmono�tonically with frequency. TEM horns (Fig. 1) havehigher directivity than biconical horns; however, theiraperture efficiency is insufficiently high, especially inthe upper part of the frequency band. In this paper, weanalyze a TEM horn whose output surface is filledwith a dielectric and is shaped as a lens in order toimprove matching and increase the gain and the aper�ture efficiency of the antenna. The analysis is per�formed in an ultra�wide frequency band. We shouldnote that application of lenses as phase correctors inpyramidal and circular horns is well known and suchantennas are referred to as horn–lens antennas [4].Therefore, we will call the antenna under study ametal–dielectric horn–lens TEM antenna or, briefly,an MDHLA (Fig. 2).

In this study, we consider three variants of excita�tion of this antenna. The first variant is excitation withthe help of a coaxial line parallel to the horn axis(Fig. 3a), the second variant is excitation with the helpof a coaxial line orthogonal to the horn axis (Fig. 3b),and the third variant is excitation by a microstrip line(Fig. 3c) with the use of a graded transition from thecoaxial line to the microstrip line (Fig. 3d). The tran�sition geometry is selected so as to ensure an imped�ance of 50 Ω in any its section, in accordance with theformulas presented in [5].

Basic geometrical parameters of a horn are itslength and two angles: angle α, which determines theangular dimension of the horn plates; and angle βbetween the plates (see Fig. 1). There are several theo�retical results that can be used for description of thedependence of the characteristic impedance on theseangles. In order to select the relation between theangles and ensure the best matching, we use the resultsobtained in [6] as the initial approximation. Subse�quent optimization is performed on the basis ofnumerical modeling.

A disadvantage of TEM horns matched to a 50�Ωcoaxial line is large difference between angles α and β.In order to lower this difference, we analyze horns filledwith a dielectric with the permittivity ε = 2.25. In orderto focus the spherical front of the TEM mode into aplane front at the horn output, the surface of the dielec�tric is shaped as a lens.

Analysis of a Horn–Lens TEM AntennaN. A. Efimovaa, b, V. A. Kaloshina, and E. A. Skorodumovaa, b

a Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Mokhovaya ul. 11 str. 7, Moscow, 125009 Russia.

b Moscow Technical University of Communications and Informatics, Aviamotornaya ul. 8a, Moscow, 111024 RussiaReceived December 10, 2011

Abstract—Matching and radiation characteristics of a horn�lens TEM antenna are studied theoretically andexperimentally in an ultra�wide frequency band. Theoretical results are obtained by means of electromag�netic modeling on the basis of the finite�element method. Antennas with different geometries of the lens anddifferent methods of excitation of the TEM horn are considered. Direct excitation by a coaxial line and exci�tation with the use of a graded transition from a coaxial line to a stripline are studied. The results of calcula�tions are compared with experimental results.

DOI: 10.1134/S1064226912090045

ON THE 100th BIRTHDAY OF Ya.N. FEL’D

α

β

Fig. 1. Geometry of the TEM horn.

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Longitudinal sections of two analyzed MDHLAs,one with focusing symmetric relative to the horn platesand the other with asymmetric focusing, are shown inFigs. 4a and 4b, respectively. In the latter case, the lensensures both focusing and rotation of radiationthrough the angle equal to one�half of angle β. Thegeneratrix of the first lens was calculated from the con�dition of obtaining a plane wave front perpendicular tothe plane of symmetry of the lens and the generatrix ofthe second lens was calculated from the condition of

obtaining a wave front perpendicular to one of theplates forming the horn.

1. THEORETICAL ANALYSIS

The theoretical analysis of MDHLAs was per�formed on the basis of electromagnetic modeling withthe use of the finite�element method. The results ofthe analysis of the frequency characteristic of theMDHLA reflection coefficient are shown in Fig. 5.The total length of the horn and lens is 400 mm, thewidth is 360 mm, and the height is 170 mm (for theantenna with asymmetric lens) or 230 mm (for theantenna with symmetric lens). Curve 1 in Fig. 5 corre�sponds to excitation of the antenna with symmetriclens by a coaxial line parallel to the horn axis; curve 3,excitation of the antenna with symmetric lens by amicrostrip line; and curve 3, excitation of the antennawith asymmetric lens by a coaxial line orthogonal tothe horn axis. For comparison, the frequency charac�teristic of a classical TEM horn (without dielectric)with the angles whose values correspond to the lowermatching frequency are also presented (curve 4).

As seen from the figure, the antenna with asym�metric lens excited by the coaxial line orthogonal tothe horn axis ensures the best value of the reflectioncoefficient in the frequency band. The antenna withsymmetric lens has worse matching, irrespective of theexcitation method (with the help of the coaxial or themicrostrip line), though it has larger aperture.The worst matching takes place in the case of the hornwithout dielectric.

Fig. 2. Geometry of the MDHLA.

(a) (b) (b)

(d)

Fig. 3. Excitation of the TEM horn by (a, b) the coaxial line, (c) microstrip line, and (d) with the use of the coax�to�microstriptransition.

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ANALYSIS OF A HORN–LENS TEM ANTENNA 1033

Calculated frequency characteristics of theantenna gain, aperture efficiency, and sidelobe levelare presented in Figs. 6, 7, and 8, respectively. Curve 1in Figs. 6–8 corresponds to excitation of the antennawith symmetric lens by the coaxial line parallel to thehorn axis and curve 2, excitation of the antenna withasymmetric lens by the coaxial line orthogonal to thehorn axis. For comparison, the gain of the horn with�out dielectric is shown in Figs. 6 and 7 (curve 4). Curve3 corresponds to the frequency characteristic of ametal–dielectric horn with spherical output surface.The gain and the aperture efficiency of the antennawith symmetric lens excited by the microstrip line arenot shown, because these curves are indistinguishable

from the corresponding characteristics for the antennawith symmetric lens.

As seen from Fig. 6, the gain of the MDHLAmonotonically increases with frequency. For antennaswith symmetric lens, it reaches 29.5 dB at 10 GHz. Forthe antenna with asymmetric lens, it is approximately27 dB at the same point. The gain of the metal–dielec�tric horn without focusing is maximal (approximately17 dB) at 2 GHz and remains almost the same athigher frequencies. The gain of the TEM horn withoutdielectric reaches its ultimate value (about 12 dB) at3 GHz.

Figure 6 demonstrates that the antenna with sym�metric lens ensures the maximum gain. This increased

(а) (b)

Fig. 4. Longitudinal sections of the MDHLAs with (a) symmetric and (b) asymmetric focusing.

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Ref

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coe

ffic

ien

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Fig. 5. Frequency characteristics of the reflection coefficient for (1, 2) the symmetric lens, (3) the asymmetric lens, and (4) theclassical antenna.

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(as compared to the antenna with asymmetric lens)gain is explained by larger dimensions of the antennaaperture. This fact is confirmed by Fig. 8, where onecan see that the antenna with asymmetric lens has themaximum aperture efficiency. At low frequencies, theaperture efficiency of this antenna is slightly higherthan unity and, at frequencies above 3 GHz, itbecomes 0.8 and remains almost constant at higher

frequencies. Antennas with symmetric lens, irrespec�

tive of the excitation method (with the help of the

coaxial or microstrip line), have an aperture effi�ciency of approximately 0.7. As seen in Fig. 8, aper�ture efficiencies of the TEM horn without dielectricand a metal–dielectric horn with flat output surfacerapidly decrease with increasing frequency.

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Fig. 6. Frequency characteristic of the antenna gain.

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Fig. 7. Frequency characteristic of the antenna aperture efficiency.

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ANALYSIS OF A HORN–LENS TEM ANTENNA 1035

The lowest sidelobe level is attained for the antennawith symmetric lens excited by the microstrip line (seeFig. 8). As before, the TEM horn without dielectricdemonstrates the worst result.

The E�plane patterns of the MDHLAs with sym�metric and asymmetric lenses excited by the coaxialline at frequencies f = 1.0 and 0.6 GHz are shown inFigs. 9a and 9b, respectively. The H�plane patterns arepresented in Fig. 10. As seen from the figures, for the

analyzed models, the width of the pattern main lobemonotonically decreases with increasing frequency inboth planes. Note that, due to larger difference in theaperture dimensions in the E plane, the difference inthe E�plane widths of the pattern beam is also larger.

2. EXPERIMENTAL STUDY

In order to confirm the numerical results, anexperimental prototype of the MDHLA with asym�metric focusing and excitation by the coaxial line per�pendicular to the horn axis (Fig. 11) was manufacturedand tested. The dielectric filling of the horn was madein the form of a layered structure composed ofpolypropylene sheets. Each polypropylene layer wasshaped as an angular sector with curvilinear cylindri�cal surface at the output. As a result, the lens at theMDHLA output had a stepwise surface. The dimen�sions of the antenna coincided with the correspondingdimensions of the computational model (see above).

In the experiment, the E� and H�plane patternswere measured at two frequencies (2 and 4 GHz) alongwith the reflection coefficient in a frequency band of100 MHz to 10 GHz. All measurements were per�formed with the use of a HP�8720C vector networkanalyzer.

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Sid

elob

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vel,

dB 1

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2

Fig. 8. Frequency characteristic of the antenna sidelobelevel.

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(а) (b)

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Fig. 9. E�plane patterns of the MDHLAs with (a) symmetric and (b) asymmetric lenses at frequencies f = (1) 2.0, (2) 1.0, and (3)0.6 GHz.

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Experimental E� and H�plane patterns at 2 and4 GHz are presented in Figs. 12 and 13, respectively.For comparison, theoretical patterns are also shown.

As seen from Figs. 12 and 13, the widths of themain lobes of experimental and theoretical patterns

agree rather well, except for the case of the E�planepattern at 2 GHz (Fig. 13a), where the width of themain lobe of the experimental pattern is noticeablyless than the width of the main lobe of the theoreticalpattern. Theoretical and experimental sidelobe levels

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(а) (b)

Fig. 10. H�plane patterns of the MDHLAs with (a) symmetric and (b) asymmetric lenses at frequencies f = (1) 2.0, (2) 1.0, and(3) 0.6 GHz.

Fig. 11. Experimental prototype of the MDHLA.

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ANALYSIS OF A HORN–LENS TEM ANTENNA 1037

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Fig. 12. E�plane patterns for f = (a) 2 and (b) 4 GHz. Solid and dashed lines correspond to theoretical and experimental results.

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Fig. 13. H�plane patterns for f = (a) 2 and (b) 4 GHz. Solid and dashed lines correspond to theoretical and experimental results.

are close to each other, while the fine structures ofsidelobe radiation agree only qualitatively.

Experimental and theoretical frequency character�istics of the reflection coefficient are shown in Fig. 14.The horizontal straight line marks a level of –10 dB.As seen in Fig. 14, the lower matching frequency cal�culated at this level (439 MHz) agrees rather accu�

rately with the experimental frequency (422 MHz). Asfrequency further increases, theoretical and experi�mental frequency characteristics of the reflectioncoefficient agree qualitatively; however, the amplitudeof oscillations of the theoretical characteristic is lowerthan the amplitude of oscillations of the experimentalcurve.

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CONCLUSIONS

Performed theoretical and experimental studies ofthe MDHLA have shown that this antenna is a prom�ising candidate for application as an ultra�widebanddirectional antenna with stably high value of the aper�ture efficiency.

For the same length of the antenna, the MDHLAwith symmetric lens ensures higher values of theantenna gain and the MDHLA with asymmetric lens,higher aperture efficiency and lower value of the low�frequency boundary of the matching band.

The antenna excitation method only slightly affectsthe radiation and matching characteristics of theMDHLA.

ACKNOWLEDGMENTS

We are grateful to V.L. Biryukov and L.I. Pangonisfor their help in preparation and conduction of theexperiment.

This study was supported by the Russian Founda�tion for Basic Research, project no. 10�07�00705�a.

REFERENCES

1. E. G. Farr, C. E. Baum, W. D. Prather, and L. H. Bowen,Ultra�Wideband Short�Pulse Electromagnetics 4 (Ple�num, New York, 1999), p. 131.

2. V. A. Kaloshin, E. S. Martynov, and E. A. Skorodu�mova, in Radars & Communications 3 (Proc. 3rd All�Russia Sci.�Eng. Conf., Moscow, Oct. 26–30, 2009)(IRE RAN, Moscow, 2009), Vol. 1, p. 97.

3. V. A. Kaloshin, E. S. Martynov, and E. A. Skorodu�mova, J. Commun. Technol. Electron. 56, 1093 (2011).

4. G. Z. Aizenberg, V. G. Yampol’skii, and O. N. Ter�eshin, Antennas of Ultrashort Waves (Svyaz’, Moscow,1977), Part 1, p. 179 [in Russian].

5. M. A. R. Gunston, Microwave Transmission�LineImpedance Data (Van Nostrand Reinhold, New York,1972; Svyaz’, Moscow, 1976).

6. F. C. Yang and K. S. H. Lee, Sensor and SimulationNotes, No. 221, 20 (1976); http:/www.ece.unm.edu/summa/notes/SSN/note221.pdf

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Fig. 14. Comparison of the (solid line) experimental and (dotted line) theoretical frequency characteristics of the reflection coef�ficient.

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