Wideband Cavity Backed Patch Antenna for Air Surveilance Radar Application

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Wideband Cavity Backed Patch Antenna for Air Surveilance Radar Application

Anand Prakasp, Divyansh Kumar Singh, Prashanth. S, Vidyanand. P

D&E-RS-NSII,BEL Bangalore, India

Abstract This paper describes a compact rectangular patch antenna design for achieving the required bandwidth. The antenna is a probe fed micro strip antenna printed on a RT Duroid 5880 layer (r=2.2,thickness=1.6 mm) enclosed in a cavity. In the design process, the effect of cavity on the impedance bandwidth of the antenna is first studied and then an additional loading to cavity structure is added such that it resonates at the required frequency band of interest. The antenna is working for 400 MHz bandwidth (>12%) with return loss <-10 dB in S-band and with an antenna element gain of more than 7.2 dB. This design is simulated in FEM based 3D-EM simulator HFSS.

KeywordsCavity patch antenna, probe feed , patches, Loading.




Fig. 1. Cavity patch antenna configuration (top).

L Lp


    Micro strip antennas are used for numerous applications because of advantages like low profile, ease of fabrication, low weight etc., But as these antennas are resonant they are limited in bandwidth. Various techniques are available to improve the bandwidth both impedance bandwidth and pattern bandwidth. These techniques include different feed techniques, stacked patches, adding parasitic ,etc.,

    It is shown that bandwidth can be increased to 60% by using special techniques. Another basic advantage of micro strip patch antenna is that dual frequency operation is easily achieved and if the resonances are brought closer to each other, these resonance merge to give a wider bandwidth.

    In this paper, design is carried out by using a cavity backed probe fed rectangular patch antenna. Cavity backed patch antennas are widely used in today's applications as they possess inherent advantages like reduced sizes, suppressed surface waves and hence low mutual coupling in array configurations. But adding cavity to a patch antenna limits the bandwidth as cavities are considered to be resonant structures. Micro strip patch with loaded cavity are used to characterize and improve the bandwidth for the required frequency of operation.


    The antenna uses three different configurations namely probe feeding and cavity backing and loading. Probe feeding

    Fig.2.Cavity patch antenna configuration (sideview)

    h D

    Fig. 3. Cavity patch antenna configuration (Top view)

    W=68.4mm; Wp=41.1mm; L=39.3mm; Lp=22.6mm;

    D=9.8mm ; Depth of cavity Dc=7.6mm ; Position of probe pin from edge of patch is xo=1.7mm;

    The coupling of energy to probe fed antenna is given by

    ( )………….(1)

    Effective dielectric constant for different layers is calculated using [1]:

    = + ……………………………..(2)


    h- denotes the height of patch.

    is used in the patch geometry as feeding in presence of cavity is relative easy and also coupling to the patch is better because of direct transfer of energy. The antenna configuration is shown below

    = ( + )


    Design steps are as follows:

    1. Calculation of resonant frequency assuming dominant TM10 mode :

      = (

      ) (( + )

      ) ( ) ………(4)

      () ()

      where L, empirical correction factor are as per [1].(Ch.4,page:267).

    2. The effect of cavity on patch resonance is given by [3]. It is observed that effective dielectric constant changes due to the proximity of cavity walls on the patch which in turn changes the patch resonance. Initial design is carried without a cavity and resonant length () is determined as per resonant frequency () . Now cavity is introduced and a shift in the patch resonance to lower frequency is observed. Hence the patch resonant frequency(fr) is re- optimized with new length (). Since effective dielectric constant is increase s by an amount equal to Eq.5 the length () less than ().

      () = …………………………(5) K is determined by using [3].

    3. Coupling between the patch and loaded cavity is best understood from the electric field distributions on different surface planes as shown in (fig.4, fig.5 and fig. 6).

    Fig. 4. Electric Field pattern in(XZ- Plane)

    Fig. 5. Electric Field pattern in(YZ- Plane)

    Fig. 6. Electric Field pattern in(XY- Plane)

    Energy coupling between the patch and cavity is strongly influenced by the loading structures in cavity. The structure is simulated and optimized in 3D EM simulator ANSYS HFSS.


    The antenna is optimized in the frequency range of 3.1 to

    3.5 GHz (400 MHz). The fig.7. represents the simulated and measured results of Return loss. The return loss is observed to better than 10 dB over the full bandwidth.

    Fig. 7. Simulated & Measured Return loss in dB.

    Fig. 8. Simulated & Measured Azimuth Radiation pattern

    Measured and simulated Azimuth radiation patterns is shown in fig. 8.The simulated gain is > 7.1 dB and beam width is varying from (60.7 deg. to 70.4 deg.). The measured gain is > 7.2 dB and beam width is varying from (58.4 deg. to

    63.8 deg.).

    Fig. 9. Simulated & Measured Elevation Radiation patterns

    In the above fig. 9 Measured and simulated Elevation radiation patterns is depicted. The simulated gain is > 7.3 dB and beam width is varying from (78.5 deg. to 88.5 deg.). The measured gain is > 7.2 dB and beam width is varying from (95.2 deg. to 98.1 deg.).


    1. Ramesh Garg " Microstrip Antenna Design Handbook", Arctech House Inc,.

    2. " Dual layer cavity backed patch antenna for wider bandwidth", R.K. Samminga, V.S.Gangwar, Sangeeta Singh, Dr.Anil Kumar Singh.

    3. "Effect of cylindrical cavity on the resonance of a circular microstrip patch antenna with variable air gap", Manotosh Biswas, Jawad Y. Siddiqui, Member, IEEE, Debatosh Guha, Senior Member, IEEE, and

      Yahia M. M. Antar, Fellow, IEEE

    4. Investigations Into a Cavity -BackedCircular-Patch Antenna" , Nemai Chandra Karmakar, Senior Member, IEEE

    5. Effect of a Cavity Enclosure on the Resonant Frequency ofInverted Microstrip Circular Patch Antenna", Debatosh Guha and Jawad Y. Siddiqui.

    6. A Scheme to Lower the Resonant Frequency of the Microstrip Patch Antenna", J. L. Volakis, Senior Member, IEEE, and J. M. Jin, Member, IEEE.

    7. Theoretical modelling of cavity backed microstrip patch antennas using a hybrid technique".Jui-Ching Cheng,Nihad Dib and Linda Kathehi

    8. I. Bahl and P. Bhartia, Microwave Solid-State Circuit Design.NewYork: Wiley, 1988.

    9. C.A.Balanis,"Antenna Theory Analysis and Design",John Wiley & Sons,Inc.,Second Edition,1982.


    Anand Prakash received his M.E. degree in Microwave Engineering from Birla Institute of Technology Mesra Ranchi. In 2013 joined BEL Bangalore as Deputy Engineer and started career in design, development and evaluation of Antenna, Array Antenna systems and RADAR system design. Authored 2 papers in national and international journal and conferences. Presently working in the field of design and evaluation of pased array antennas, Beam forming and

    Fig. 10. Simulated 3D radiation pattern

    Fig.10 depicts 3d-radiation pattern of compact rectangular patch antenna with loaded cavity .

    EMI/EMC .

    Divyansh kumar singh received his B.Tech. degree in in Electronics and Communications. In 2015 joined BEL Bangalore as Probationary Engineer and started career in development and evaluation of Array Antenna systems,RF sub – module testing and RADAR system design. Presently working in the field of development and evaluation of RF sub modules for E/F Band and SNR .





    7.2 – 8.5

    7.1 – 8.6


    7.2 – 8.5

    7.3 – 8.6

    Az-Beam width(deg.)

    58.4 – 63.8

    60.7 – 70.4

    El-Beam width(deg.)

    95.2 – 98.1

    78.5 – 88.5





    7.2 – 8.5

    7.1 – 8.6


    7.2 – 8.5

    7.3 – 8.6

    Az-Beam width(deg.)

    58.4 – 63.8

    60.7 – 70.4

    El-Beam width(deg.)

    95.2 – 98.1

    78.5 – 88.5

    TABLE I Simulated/Measured Radiation Pattern Comparison

    The measured and simulated antenna parameters are summarized in the table. 1 above.


A compact rectangular patch with loaded cavity backed antenna is designed for required bandwidth and detailed study of the effect of cavity on patch resonances is observed. The antenna has a very useful application as a phased array since cavities prevent the surface wave propagation thus eliminating scan blindness. The scan loss can be made lesser by varying the available degrees of freedom. There is close coherence between simulated and measured results.


The author would like to thank G.M. NSII Shri Umesh K S and A.G.M Smt. Dharani B for their continuous support and permission to publish this work.

Prashanth.S received his B.E. degree in Telecommunication from Dr. Ambedkar Institute of Technology, Bangalore. He joined BEL Bangalore in June 2001.He has contributed in development of various Navigation and Surveillance Radars. His field of interest includes Active phased array radars, RADAR system design, and system integration. He is presently involved in design and development of Active

phased surveillance radars in C/D band, E/F band & SNR.

Vidyanand.P, is currently working in Bharat Electronics Limited, Bangalore, as Senior Dy.General Manager. He obtained his M.Tech degree in Electronics and Communications Osmania University College, in 1992. He has contributed in development of various Navigation and Surveillance Radars. His fields of interests are developing Missile Gathering & Guidance and

Monopulse Receivers, Radar System Design, Development and Engineering.

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