horn antenna

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Tom MilliganAssoc. Editor,Antenna Designer's Notebook8204 West Polk PlaceLittleton, CO 80123 USATel: +1 (303) 977 7268Fax: +1 (303) 977 8853E-mail: [email protected]

Design of Broadband Constant-BeamwidthConical Corrugated-Horn Antennas

Majid Abbas-Azimi, Farhad Mazlumi, and Fereidoon Behnia

Electrical and Computer Engineering School, Faculty of EngineeringNo. 49, North Kargar Ave, Tehran, Iran

Tel: +9821 61114940; Fax: +98 21 82094214; E-mail: [email protected]

Abstract

In this paper, a new design procedure is proposed for the design of wideband constant-beamwidth conical corrugated-hornantennas, with minimum design and construction complexity. The inputs to the procedure are the operating frequency band,the required minimum beamwidth in the entire frequency band, and the frequency in which the maximum gain is desired tooccur. Based on these values, the procedure gives a relatively good design with a relative bandwidth of up to 2.5:1. Based onthe proposed procedure, a corrugated-horn antenna with a constant beamwidth over the frequencies of 8 to 18 GHz wasdesigned and simulated using commercial software. The designed antenna was also constructed, and its electromagneticperformance was measured. The measured results of the constructed prototype antenna confirmed the simulation results andsatisfied the design requirements, validating the proposed design procedure.

Keywords: Corrugated horn antennas; shaped beam antennas; antenna radiation patterns

1. Introduction

Corrugated horns have wide application in communications,radio astronomy, satellite tracking, radar, and remote sensing.

They are used as improved feeds for reflector antennas [1-3], andalso as direct radiators for wideband measurements [4]. Corrugatedhorns can produce radiation patterns having extremely good axialsymmetry, relatively constant beamwidths as a function of frequency, high beam efficiency, low cross-polarization levels, verylow sidelobes and back lobes, good return loss, and relatively-thesame phase centers in all cut planes containing the antenna's boresight [1-6]. These outstanding features come from the fact that theysupport the so-called hybrid mode, HEl l' The corrugated surfaceinside the antenna produces equal boundary conditions for allpolarizations, and tapers the field distribution in the E and H planesin the aperture. This leads to a hybrid mixture of TEll and TMll

IEEEAntennas and Propagation Magazine, Vol. 51, No.5, October 2009

modes that behaves as a single mode, in which both componentspropagate with the same velocity [6].

Different design approaches for wideband corrugated hornshave been adopted in the literature [1-12]. In most of the designs,such as those proposed in [1-3, 9-12], a linear corrugation profilewith slot depths greater than a quarter of a wavelength at the lowend of the frequency band and less than a half wavelength at thehigh end was used to obtain a maximum relative bandwidth of2:1.Kerr designed a similar structure with a relatively constant beamwidth of about 30° to 45° within a relative bandwidth of2.1:1 [13].

To increase the frequency bandwidth of corrugated horns,more-complex designs have also been suggested in the literature.Examples of these designs are tapered slotted corrugations with aclaimed maximum bandwidth of3 :1 [14], a ring-loaded mode converter with a maximum bandwidth of 2.4:1 [1, 2, 5], and a vari-

109

able-pitch-to-width-slot mode converter with a maximum bandwidth of 2.05 :1 [5]. In [15], a procedure was presented for themode-launching region of corrugated horns, in order to obtainwideband low-return-Ioss single-mode operation over a fractionalbandwidth of 50%. Recently, great efforts have been carried out onprofiled corrugated horns [16-22]. All the wideband designs mentioned above were rather complicated, and had time-consumingdesign procedures. Furthermore, their mechanical complexities, therequired accuracy of their construction, and their high cost areother significant concerns.

This paper presents a design procedure for wideband corrugated-horn antennas with a maximum bandwidth of about 2.5:1.The proposed design procedure leads to antennas with lessmechanical complexities compared to their counterparts. Thisantenna can be even constructed by a two-axis milling machine.T~e proc~dure gi~es a good initial design, which then can be optimized with available powerful software for obtaining betterantenna performance.

The structure of the paper is organized as follows . First, theprinciples of corrugated-horn antennas are reviewed. A design procedure is then proposed for the design of corrugated horns . Tovalidate the design procedure, a corrugated-horn antenna isdesigned based on the proposed design procedure. The designedantenna is simulated using Ansoft HFSS [23] and CST MicrowaveStudio [24]. Finally, the measured results of the designed antennaare presented.

A very gradual change from a slot depth of AI2 to A/4from the throat toward the aperture can avoid the excitation of the EHl t mode.

Although the principle of operation is based on AI4deep slots, the surface impedance changes slowly withfrequency , so the outstanding features of this antennacan be maintained over a relatively wide frequencyband.

Corrugated-horn antennas generally are composed of fourmajor parts (Figure 1): the aperture, the corrugation profile, themode converter, and the input waveguide. The aperture diameterand the flare angle have a significant role in determining thebeamwidth and directivity of the antenna for wideband use. Thespecifications of corrugations have effects on the satisfaction of thebalanced hybrid-mode condition. The mode converter lies betweenthe body of the corrugated horn and the circular smooth-walledinput waveguide, the fundamental mode of which is the TEll

mode . A good design for the mode converter guarantees a smoothtransition from the TEll to the HEll mode supported by the

corrugated horn, and, hence , obtaining better impedance matchingat the antenna's input.

3. Design Procedure

Figure 1. The different parts of a typical corrugated-hornantenna.

In this section, a procedure is proposed for designing a wideband wide-flare-angle corrugated-horn antenna with constantbeamwidth over a maximum bandwidth of 2.5:1 (i.e.,fmaxl fmin =2.5 , where fmin and fmax are respectively the minimum and the maximum limits of the desired frequency band) .Since corrugations perpend icular to the wall of the horn are oftenproposed in the literature for wideband use [1, 2], the design procedure is based on the antenna structure shown in Figure 2. Withrespect to Figure 2 and assuming that all the angles are in degrees ,the optimum values of the antenna's parameters are calculatedaccording to the procedure described below.

2. Corrugated Horn Principles

. A nearly linear aperture electric field is the requirement forproducing a symmetrical main beam with a low cross-polarizationlevel in the horns [2]. The corrugations in a waveguide or anantenna produce a nearly linear aperture electric field of a hybridmode, which cannot be produced by waveguides support ing onlypure TE or only pure TM modes. The corrugated wall can generatea condition in which both the impedance and admittance at the surface of the corrugations become zero. This boundary condition cansupport the propagation of the so-called balanced hybrid mode,

HEll'

To obtain good performance from the corrugations, the following tips should be cons idered [I, 2, 5, 6, 11]:

There should be enough corrugations per wavelength( A) to have zero tangential transverse electric field atthe surface of the corrugations.

The corrugations ought to be narrow and AI4 deep

(more than AI4 deep for annular corrugations). Thecorrugations act as transmission lines that transform ashort circuit at the end of the slots into an open circuitat the corrugation's boundary, causing a relatively zerotangential transverse magnetic field at this boundary.

Slot depths less than A/4 generate positive (inductive)

reactance and result in the propagation of the EH((

mode, which is a surface-wave mode. This mode has aconcentration of power around the radius of the corrugations, and causes high levels of cross polarization.

FeedSection

InputWaveguide

ModeConverter

Aperture

110 IEEEAntennas and Propagation Magazine, Vol. 51, No.5, October 2009

mum directivity occurs. The corresponding maximum directivity,D, is approximately calculated (in dBi) as

The widths of the slots and teeth ( wi and ti , respectively) are

calculated according to the condition O.7 ~~ s 0.9 , and suchw +t

that there are at least four corrugations per Ae along the slant

radius. For simplicity, suppose that "i =w2 =...=wn =wand

tl =t2 =... =tn =t . The values of wand t are chosen according to

these two conditions . Considering these criteria, the widths of theslots and the teeth can be calculated as

-,\

\II '?\

II, I \I I \

......-__----,.0(/\7 i I I: 110 / • I J

..! ?t'!!!~~!2..~ R__!..-_-L _Axis I i i

f- - - - -I.- - - - . :.. -L\-jFigure 2. The parameters of a typical corrugated-hornantenna.

(250JD ""2010glO e; .

Ac"i =w2 =... =wn =W =- ,4.8

(5)

(6)

The procedure needs to define a center frequency the associated wavelength of which scales the antenna's dimensions. Thecenter frequency, Ie' should be closer to the low end of the fre

quency band, since the balanced-hybrid-mode condition fails morerapidly at the lower frequencies [II] . A proper value of Ie is

calculated in terms of Imin as follows:

To obtain a good return loss (especially at the beginning of the frequency band), the depths of the first slots (i.e., d. , i =1,2,3) are

calculated according to [5]

I e"" 1.2lmin · (I )

d . = K AcI 4 '

(7)

(8)

The radius of the input waveguide, «o- should be such that Imin is

spaced out far enough from the cutoff frequency of the TEll

mode. A suitable suggestion is [5]

where

[ ( J-1.134]

K =exp 0.473 2;:i . (9)

(2) The depths of the other slots are equal to the depth of the third slot,i.e., d3 = d4 = ...=dn .

where Ae is the wavelength associated with the center frequency

introduced in Equation (I).

In a wide-flare-angle corrugated horn, there are approximately linear relationships among the beamwidths for the -3 dB,-lOdB, and -20dB pattern levels and the semi-flare angle [6].The semi-flare angle, 80 , is hence calculated in terms of the

desired beamwidth as follows:

In order to have better control over the VSWR, especially atthe lower frequencies, a smooth-walled hom after the inputwaveguide and before the corrugations should be chosen. It is better if the length of this part, to, is at least equal to the width of the

corrugations , w.

The aperture radius, a, is calculated such that the horn's directivityis maximized at a desired frequency. According to references [1,2], the maximum directivity is obtained if

where Am is the wavelength of the frequency at which the maxi-

IS7Ama""-280 '

BW , if BW is - 3 dB beamwidth0.7

BW , ifBW is -10 dB beamwidth .1.5

BW , ifBW is - 20 dB beamwidth2.3

(3)

(4)

4. A Design Example

Suppose a corrugated horn is to be designed having a constant half-power beamwidth (HPBW) of25° in the 45° slant plane,and a VSWR less than two, over a frequency band of 8 to 18 GHz.Assuming that the maximum gain is to be reached at the highestfrequency, the optimum dimensions of the horn antenna areobtained by using the design procedure as follows: ao = 14.92mm,

80 =35°, a =37.5 mm, w =6.5 mm, and t =1.3mm. According to

Equation (5), D ""17dBi.

The corrugated-horn antenna with the dimensions calculatedabove was modeled in both CST Microwave Studio Version 5.1and HFSS Version 10. In the simulations, the antenna was fed by apure TEll mode at the input waveguide. To feed the designed

corrugated horn with a coaxial input, the hom antenna was com-

IEEE Antennas and Propagation Magazine, Vol. 51, No.5, October 2009 111

o...----- - --,-- - ---;;,............;;:-- - -,-- - ---,,.......,

~.g-1O.~0..E<: -20

"0Q)

-- -Simulation- Measurement

-50 0 50Angle off boresight (degrees)

Figure 8. The simulated and measured normalized 45° slantplane radiation patterns of the corrugated antenna at 8 GHz.

Figure 3. The constructed conical corrugated-horn antenna,fed by a coaxial-to-circular-waveguide adaptor.

O~-----,---~~------r----_,

-- -Simulation- Measurement

-50 0 50Angle off boresight (degrees)

<a /§ -40 ,/o 'Z

-~PO!-:O-----:;;--!====::==:=!-------;~---;-;I00

,.......,

~';;' -10"0B~-20

~Corrugated-hom antennas can be constructed with different

methods, such as classical milling, electroforming, and rapid prototyping [1, 10, 12]. Classical milling was used to construct thewhole antenna (Figure 3). The electromagnetic characteristics ofthis antenna were measured, and are presented in Figures 4-10. Inthese figures, the measured results are accompanied by the simulation results. A comparison between the measured results and thoseobtained from the simulations showed good agreement, validatingthe simulations of the designed antenna.

bined with a coaxial-to-circular waveguide adaptor [25]. The totalstructure, including the adaptor and the hom, was then simulatedagain.

The measured VSWR (Figure 4) showed a little rise at10 GHz in comparison to the simulation results, which might havebeen due to the coaxial-to-waveguide transition assembly. Theeffect of this matter can be seen as a decrease in gain (Figure 5) atthe same frequency. Figure 5 shows that the measured-gain curveincreased, and the maximum gain was close to 17 dBi, as wasexpected in the design.


Or----....----::::;;_-oo;;;;;:---~------,

40

---Simulation- Measurement

,.......,

~ -5C)

"'0

.~ -100..

~ -15 I

"0 /

. ~ -20~

§ -25oZ

-3_~0 -20 0 20Angle off borcsight (degrees)


The HPBW as a function of frequency of the antenna in the45° slant plane is depicted in Figure 6. As shown in this figure, theminimum of the measured HPBW was about 24.5°, which wasvery close to what was expected in the design. Also, the HPBWvariations were less than 12° over the entire bandwidth. To explorethe pattern's symmetry over the bandwidth, the antenna HPBWs inthe E and H planes are plotted in Figure 7. This figure showed thatthe HPBWs in the E and H planes were not equal at frequenciesgreater than 14 GHz, and had a maximum difference of 12°. Thisimplied that the radiation patterns were not completely symmetrical at frequencies greater than 14 GHz.

The normalized radiation patterns of the antenna in the 45°slant plane at 8, 12, and 18 GHz from the CST simulations and themeasurements are presented in Figures 8-10. At frequencies higherthan 15 GHz, the effect of the asymmetrical feed section with thecoaxial line could be seen as an asymmetry in the radiation pattern,such as in Figure 10. The measured sidelobe levels were betterthan nearly 25 dB.

112 IEEE Antennasand Propagation Magazine, Vol. 51, No.5, October 2009

181612 14Frequency (GHz)

10

"-HFSS

..·• ..·CST............ .....--Measurement

...............

~.... :

.............. ~:.-. ...... . ............. ~ ...::'f~':4-~ .-.. ~/F"'" .....

.................

.......................

8

10

60

50

16

---HFSS·..······CST

............... ............... - Measurement


2

1.8 .....

~1.6

r/.l:> 1.4

1.2

18

Figure 4. The simulated and measured VSWR of the corrugated-horn antenna accompanied by the coaxial-to-circularwaveguide adaptor.

Figure 6. The simulated and measured half-power beamwidthof the corrugated-horn antenna accompanied by the feed section, in the 45° slant plane.

20,--- - -,.-- - - ...--- - --.-- - - .,--- - -.,

1816

1 ,~ ..."He'" --.- - ...... . .i.


--

10

10

8

60 ,--- - .....-- - ----r- - ----;;== = = = = ===;t"-Simulated E-plane--Measured E-plane

50 , ·..··I -+-Simulated H-plane

'gl - Measured H- lane~40

oS'0. ~ 30S<':lQ)(!l 20 .

1816

.......! .

10

18

Figure 5. The simulated and measured gain of the corrugatedhorn antenna accompanied by the coaxial-to-circularwaveguide adaptor.

Figure 7. A comparison of the half-power beamwidths in the Eand H planes (obtained from HFSS and measurements).

IEEEAntennasand Propagation Magazine, Vol. 51, No.5, October 2009 113

5. Conclusions

A design procedure has been introduced for the design ofwideband wide-flare-angle corrugated-hom antennas with relatively constant beamwidth. The procedure is straightforward, anddetermines the physical dimensions of a corrugated hom with alinear profile and simple corrugations.

To examine the accuracy of this design procedure, an example antenna was designed over 8 to 18 GHz with specific electromagnetic features. The designed antenna was easily constructedusing a milling method. The electromagnetic performance of theantenna was then measured. The measurement results showed thatthe antenna's VSWR was less than 1.85 over the entire bandwidth.The antenna's gain increased from 12.5 dB to 16 dB. Theantenna's beamwidth also had a minimum beamwidth of 24.5° inthe 45° slant plane, and maximum variations of about 12°. Thesemeasurement results confirmed the results of the simulations andsatisfied the design requirements.

6. References

1. P. J. B. Clarricoats and A. D. Olver, Corrugated Horns forMicrowave Antennas, London, IEE, 1984.

2. A. D. Olver, P. 1. B. Clarricoats, A. A. Kishk, and L. Shafai,Microwave Horns and Feeds, London, lEE, 1994, Chapter 9.

3. A. W. Love, Electromagnetic Horn Antennas, New York, IEEEPress, 1976.

4. T. Chu and W. E. Legg, "Gain of Corrugated Conical Horns,"IEEE Transactions on Antennas and Propagation, AP-30, July1982, pp. 698-703.

5. C. Granet and G. L. James, "Design of Corrugated Horns: APrimer," IEEE Antennas and Propagation Magazine, 47, April2005, pp. 76-84.

6. T. A. Milligan, Modern Antenna Design, Second Edition, NewJersey, IEEE Press/John Wiley, 2005, Chapter 7.

7. C. A. Balanis, Antenna Theory: Analysis and Design, ThirdEdition, New York, John Wiley, 2005, Chapter 13.

8. R. E. Collin, Antennas and Radiowave Propagation, Singapore,McGraw Hill, 1985, Chapter 4.

9. S. K. Buchmeyer, "Corrugations Lock Horns with Poor Beamshapes," Microwaves, January 1973, pp. 44-49.

10. A. D. Olver, "Corrugated Horns," Journal of Electronics andCommunication Engineering, 4, February 1992, pp. 4-10.

11. B. M. Thomas, "Design of Corrugated Conical Horns," IEEETransactions on Antennas and Propagation, AP-26, March 1978,pp.367-372.

12. Y. Beniguel, A. Berthon, C. V. Klooster and L. Costes,"Design Realization and Measurement of a High PerformanceWide-Band Corrugated Hom," IEEE Transactions on Antennasand Propagation, AP-53, November 2005, pp. 3540-3546.

114

13. 1. L. Kerr and M. J. Timochko, Broadband Corrugated Hornwith Double-Ridged Circular Waveguide, US Patent 4,021,814,May 3,1977.

14.Z. Frank, "Very Wideband Corrugated Horns," ElectronicsLetters, 11, March 1975, pp. 131-133.

15. X. Zhang, "Design of Conical Corrugated Feed Horns forWide-Band High-Frequency Applications," IEEE Transactions onMicrowave Theory and Techniques, 41, August 1993, pp. 12631274.

16. G. L. James, "Design of Wide-Band Compact CorrugatedHom," IEEE Transactions on Antennas and Propagation, AP-32,October 1984, pp. 1134-1138.

17. A. D. Olver and J. Xiang, "Design of Profiled CorrugatedHorns," IEEE Transactions on Antennas and Propagation, AP-36,July 1988, pp. 936-940.

18. J. Teniente, R. Gonzalo, and C. Rio, "Ultra-Wide Band Corrugated Gaussian Profiled Hom Antenna Design," IEEE Microwaveand Wireless Components Letters, 12, January 2002, pp. 20-21.

19. R. Gonzalo, J. Teniente, and C. Rio, "Improved Radiation Pattern Performance of Gaussian Profiled Hom Antennas," IEEETransactions on Antennas and Propagation, AP-50, November2002,pp.1505-1513.

20. A. A. Kishk and C. S. Lim, "Comparative Analysis BetweenConical and Gaussian Profiled Hom Antennas," Progress in Electromagnetics Research PIER, 38, 2002, pp. 147-166.

21. P. J. B. Clarricoats, R. F. Dubrovka, and A. D. Olver, "HighPerformance Compact Corrugated Hom," lEE Proceedings onMicrowave, Antennas, and Propagation, 151, December 2004, pp.519-524.

22. J. Teniente, R. Gonzalo, and C. Rio, "Innovative High-GainCorrugated Hom Antenna Combining Horizontal and Vertical Corrugations," IEEE Antennas and Wireless Propagation Letters, 5,2006, pp. 380-383.

23. Ansoft Corporation, User's Guide - High Frequency StructureSimulator, HFSS Ver. 10, 2005.

24. Computer Simulation Technology Corporation, CST Microwave Studio Tutorial, CST MWS Ver. 5, 2003.

25. M. Abbas-Azimi and F. Mazlumi, "Design of WidebandCoaxial to Circular Waveguide Adaptors by the Use of CircularDouble Ridged Waveguide," in preparation.

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IEEE Antennasand Propagation Magazine, Vol. 51, No.5, October 2009