IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 1325

A Broadband Planar Monopulse Antenna Arrayof C-Band

Zhong-Wu Yu, Guang-Ming Wang, and Chen-Xin Zhang

Abstract—In this letter, a broadband planar monopulse antennaarray of C-band for the application of monopulse radars is pre-sented. A novel wideband monopulse comparator as the sum–dif-ference feed network has been designed, and a multilayer 4 4 mi-crostrip antenna array as the radiation part has been fabricated.The measurement results demonstrate that the impedance band-width ����� �� of the proposed monopulse antenna array isup to 28.2% and the null depth can reach �� dB, which rangesfrom 5.05 to 6.675 GHz. At the same time, the peak sum beam gainis up to 18.4 dB. Compared to other monopulse antennas, the pro-posed monopulse antenna array exhibits excellent characteristics.

Index Terms—Monopulse antenna, monopulse comparator, nulldepth.

I. INTRODUCTION

M ONOPULSE antennas are commonly applied in radarand other communication systems. The traditional

monopulse antennas are usually in the form of Cassegrainparabolic structures, and waveguide monopulse comparatorsare often rather heavy and complicated [1]. However, planarmicrosrtrip structure monopulse antennas can overcome theseshortcomings, which exhibit numbers of attractive qualities,such as simple structure, small volume, low profile, and conve-nient manufacture. Due to these, significant research activitiesand interests have been aroused in the academic field recentlyto explore various monopulse antennas [2]–[8].

Laheurte [2] presented a uniplanar monopulse antenna basedon odd/even mode excitation of the coplanar line. In that system,two series-fed slot arrays were fed by a coplanar waveguidetransmission line excited in its coplanar mode or coupled slot-line mode, and that monopulse antenna just operated in a 2.8%band around 4.2 GHz. A low-cost and monopulse antenna usinga symmetrical bidirectionally fed microstrip patch array was de-scribed in [3]. However, only one-dimensional monopulse per-formances were obtained, and the impedance bandwidth wasless than 1.5%. In [4], a low-cost monopulse radial line slot an-tenna with the slots placed on the upper plate in concentric ringswas introduced. The proposed antenna worked in rather narrow-band, which ranged from 13.4 to 14 GHz. A compact single-layer monopulse microstrip antenna array was devised in [5], inwhich the effects of spurious radiation and blockage caused bythe comparator on the sidelobe level were estimated and space

Manuscript received August 20, 2009; revised October 13, 2009. First pub-lished December 08, 2009; current version published December 22, 2009.

The authors are with Air Force Engineering University, Shaanxi713800, China (e-mail: dabaoyzw1258@sina.com; wgming01@sina.com;zcx_1969@163.com).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2009.2038077

Fig. 1. Configuration of the proposed monopulse antenna array.

mapping (SM) technique was applied to design microstrip sub-array. The bandwidth of the antenna was only 5.6%. As the mostrecently reported in [6] and [7], the substrate integrated wave-guide (SIW) feeding technology was applied in the monopulseantenna design. V-type liner tapered slot antenna was adopted asthe radiation part. The monopulse antennas based on SIW feedtechnology worked in millimeter-wave band and could only ob-tain one-dimensional monopulse performances. The operatedbandwidths were less than 15%, respectively.

In this letter, to broaden the impedance bandwidth ofmonopulse antennas, a broadband planar monopulse antennaof C-band has been proposed. A novel broadband monopulsecomparator that acts as the sum–difference feed network is pre-sented. Meanwhile, a multilayer 4 4 microstrip patch antennaarray as the radiation part has been designed. The monopulseantenna considered here can obtain two-dimensional perfor-mances. The momopulse comparator and the radiation antennaarray are not in the same layer. The measurement results showthat impedance bandwidth can be up to 28.2%, and the nulldepth is below dB among operated band. Furthermore,the peak sum beam gain of the proposed monopulse antennaarray can achieve 18.4 dB, and excellent radiation patterncharacteristics are also observed.

II. DESCRIPTION OF THE MONOPULSE ANTENNA ARRAY

The configuration of the monopulse antenna array is shown inFig. 1. As can be seen in Fig. 1, the proposed monopulse antennaarray is comprised of two parts, which are the comparator as thesum–difference feed network and the radiation patch antennaas the radiation part. The comparator that adopts four magic-Tsis used to form sum and difference beam lobes. Compared toother comparators presented in other open literature, the com-parator considered here exhibits excellent characteristics. Now,Part A introduces the comparator configuration, whereas Part Bpresents the radiation patch antenna structure.

1536-1225/$26.00 © 2009 IEEE

1326 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 2. Photographs of the proposed monopulse comparator. (a) Front side.(b) Back side.

Fig. 3. Frequency responses of the SUM and DELTA channels based on S-pa-rameters simulations using the measured eight port S-parameter file.

A. Monopulse Comparator

In order to obtain two-dimensional performances of themonopulse antenna, the monopulse comparator or sum–dif-ference feed network requires four magic-Ts to construct.However, one 3-dB coupler added by 90 delay line can operatein rather narrow bandwidth constructed in [5]. As is known,the T-junction exhibits broad impedance bandwidth, and theoutputs keep the same amplitude and phase. Moreover, themicrostrip–slotline transition structure can also exhibit the

Fig. 4. Antenna element structure. (a) Front view. (b) Side view. � �

��� � � ���� � ��� � � ���� � ���� � � ���� � � ���� � � ���

(unit: mm).

same amplitude and reverse phase in rather broad bandwidth[9]. A novel broadband microstrip–slotline transition structureis introduced into the traditional hybrid ring. Compared tothe transition structure introduced in [9], the proposed mi-crostrip–slotline transition structure based on a semiannularsplit can effectively decrease the slotline radiation loss. Basedon this, a wideband magic-T is designed, and then the com-parator is fabricated using four magic-Ts. Photographs of themonopulse comparator are presented in Fig. 2.

As the key part of monopulse antenna design, the characteris-tics of the comparator require evaluation. The comparator holdseight ports. The measured S-parameters of the comparator arethrough an HP 8720ET Vector Network Analyzer. The S-param-eter file of the comparator is obtained through 28 two-port mea-surements. Then, a one-fourth divider is designed, whose out-puts associate Port 1–Port 4 of the comparator. Then, the evalu-ation diagram is formed, in which the input port of the divider isdefined as Port 1 and Port 5–Port 8 change into Port 2–Port 5 inthe evaluation diagram. Compared to the method of comparingamplitudes and phases, this method is easy and visual [8]. Thefrequency response diagram based on measured results of thecomparator is shown in Fig. 3. The sum and difference charac-teristics are rather excellent. As can be seen, the sum port in-sertion loss is less than 0.5 dB, whereas the difference port islower than dB. Though the operated bandwidth comparatorpresented in [8] is nearly 100%, the overall characteristics areinferior to the comparator considered here, especially the sumport insertion loss that has reached 3 dB in high frequency band.

YU et al.: A BROADBAND PLANAR MONOPULSE ANTENNA ARRAY OF C-BAND 1327

Fig. 5. Photographs of monopulse antenna array. (a) Excitation patch array.(b) Parasitic patch array.

The measurement results demonstrate that this comparator canbe used as the feed network of the monopulse antenna array.

B. Radiation Antenna Array

The planar antennas mainly include two style antennas, whichare the microstrip patch antenna and the slot waveguide antenna.Though the slot waveguide antenna exhibits compact structureand good directivity, it requires high product cost and fabrica-tion difficulty. Therefore, the microstrip patch antenna is ap-plied as the radiation part. Because of the high Q, the singlemicrostrip patch antenna keeps relative bandwidth of less than10% [10]. Therefore, multilayer structure is adopted to broadenthe antenna bandwidth. The antenna element configuration isshown in Fig. 4. It contains three layers, which are dielectriclayer 1, dielectric layer 2, and air layer. The dielectric constantsof layers 1 and 2 are 2.65 and 4.1, respectively; the air layeris the middle part. In order to increase the range of action, thehigh-gain antenna is often required. Therefore, a one-fourth di-vider is employed, which consists of one T-style divider and

Fig. 6. Measured VSWR curves. (a) Sum channel. (b) Difference channel 1.(c) Difference channel 2.

two Wilkinson dividers to form one 2 2 patch antenna array.This patch antenna array is used as the subarray, and then themonopulse radiation antenna array is constructed. Photographsof the monopulse radiation antenna array are shown in Fig. 5.The distance between adjacent antenna elements is 45 mm. Theoverall dimension is 210 210 mm .

III. RESULTS AND DISCUSSIONS

Four cables are applied to associate the comparator and theradiation antenna array. Then, the impedance characteristics ofmonopulse antenna array are obtained through the HP 8720ET

1328 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 7. Measured gain of sum and difference beams.

Fig. 8. Measured sum radiation patterns. (a) E-plane (� � ��� GHz). (b)H-plane (� � ��� GHz). (c) E-plane (� � ��� GHz). (d) H-plane (� � ���

GHz).

Vector Network Analyzer. Because the cross difference is outof use, only two difference port impedance characteristic dia-grams are listed. The VSWR curves of one sum channel andtwo difference channels are shown in Fig. 6. As can be seen,from 5.05–6.675 GHz, the VSWR is less than 2, and there aresome ripples in the operated band, which are caused by themeasurement system self and interconnecting cables and is notled by the comparator or the radiation antenna array. The peaksum beam gain is up to 18.4 dB, which can be observed inFig. 7. Figs. 8 and 9 are the measured radiation patterns. Thetwo chosen frequency points are 5.1 and 6.5 GHz. The pro-posed monopulse antenna array exhibits good radiation char-acteristics, which has null depth of less than dB and lowsidelobes among the observed frequency bandwidth. Intercon-necting cables can affect feed phases; therefore, if the cablesare adjusted more properly, the null depth would be improved.The impedance bandwidth of the monopulse antenna dependson the monopulse comparator and radiation patch antenna array.In this design, if the impedance bandwidth of the radiation patch

Fig. 9. Measured difference radiation patterns. (a) E-plane (� � ��� GHz).(b) H-plane (� � ��� GHz). (c) E-plane (� � ��� GHz). (d) H-plane (� �

��� GHz).

antenna array can be broadened further, the monopulse antennaarray can operate in broader bandwidth.

IV. CONCLUSION

In this letter, a novel broadband planar monopulse antennaarray for C-band application has been designed. In order to con-struct it, a novel wideband monopulse comparator as the antennafeed network is presented, and a 4 4 multilayer high gain an-tenna array is realized. The measurement results show that theproposed monopulse antenna can be applied in radar and otherwireless communication systems successfully.

REFERENCES

[1] P. J. B. Clarricoats and R. D. Elliot, “Multimode corrugated waveguidefeed for monopulse radar,” IEE Proce. Microw. Opt. Antennas, vol.128, no. 2, pp. 102–110, Apr. 1981.

[2] J.-M. Laheurte, “Uniplanar monopulse antenna based on odd/evenmode excitation of coplanar line,” Electron. Lerr., vol. 37, no. 6, pp.338–340, Mar. 2001.

[3] S. G. Kim and K. Chang, “Low-cost monopulse antenna using bi-di-rectionally-fed microstrip patch array,” Electron. Lett., vol. 39, no. 20,2003.

[4] M. Sierra-Castañer, M. Sierra-Pérez, M. Vera-Isasa, and J. L. Fer-nández-Jambrina, “Low-cost monopulse radial line slot antenna,”IEEE Trans. Antennas Propag., vol. 51, no. 2, pp. 256–263, Feb. 2003.

[5] H. Wang, D.-G. Fang, and X. G. Chen, “A compact single layermonopulse microstrip antenna array,” IEEE Trans. Antennas Propag.,vol. 54, no. 2, pp. 503–509, Feb. 2006.

[6] Y. J. Cheng, W. Hong, and K. Wu, “Millimetre-wave monopulse an-tenna incorporating substrate integrated waveguide phase shifter,” IETMicrow. Antennas Propag., vol. 2, no. 1, pp. 48–52, Feb. 2008.

[7] Y. J. Cheng, W. Hong, and K. Wu, “Design of a monopulse antennausing a dual V-type linearly tapered slot antenna (DVLTSA),” IEEETrans. Antennas Propag., vol. 56, no. 9, pp. 2903–2909, Sep. 2008.

[8] K. S. Ang, Y. C. Leong, and C. H. Lee, “A wide-band monopulsecomparator with complete nulling in all delta channels throughout sumchannel bandwidth,” IEEE Trans. Microw. Theory Tech., vol. 51, no. 2,pp. 371–373, Feb. 2003.

[9] J. Chramiec and A. M. Glass, “Analysis of microstrip-slotline ring 3dB directional couplers,” IEE Proc., vol. 133, no. 3, pt. H, pp. 187–190,Jun. 1986.

[10] R. Garg, P. Bhartia, I. Bahal, and A. Ittipiboon, Microtrip Antenna De-sign Handbook. Norwood, MA: Artech House, 2001.