Dual frequency circularly polarised microstrip antenna R. Shavit, Y. Israeli, L. Pazin and Y. Leviatan Abstract: A dual frequency and dual circular polarisation multilayer microstrip antenna element for satellite communication is presented. The element is fed by a gap-coupled probe pin. The microstrip element exhibits a dual frequency band of operation for two orthogonal circular polarisations. A parametric study to optimise the element performance has been conducted. A prototype with dimensions that are based on the simulations has been built and tested. A good agreement between the measured and numerical results was found. 1 Introduction Microstrip patch antenna elements are very popular in wireless communication system applications. They offer an attractive way to integrate the RF front end of the system with its antenna and achieve a low profile, low weight, easy to fabricate and low cost solution. The increased demand for higher transmission capacity is driving the research to investigate new ways to increase the microstrip antenna bandwidth or, alternatively, operate in multiple frequencies while using two orthogonal polarisations. This requirement motivated our research to look for a dual frequency and dual circular polarisation antenna element for Ku band satellite communications. There are numerous ways to obtain a dual-band circularly polarised printed antenna as described in [1–4] . However, in all the above cases the same sense circular polarisation, right-hand circular polarisation (RHCP) or left-hand circular polarisation (LHCP), was investigated. In this paper, a dual frequency and dual circular polarisation microstrip element is presented. The linear polarised, dual-frequency stacked circular antenna de- scribed in [5] inspired to some degree the proposed element. The element is composed of two stacked circular patches fed in tandem by a single gap-coupled probe pin [6] . The proposed element design is unique in the sense that it enables one to adjust independently the phase of the radiated electric field of each of the patches by turning each patch around its common feeding point. This is an important feature for the design of a microstrip non- resonant antenna array fed by a radial waveguide [7] . It enables one to adjust the phase so as to offset the phase errors generated in a radial waveguide microstrip array. The proposed element has been studied numerically and later built and tested. The computation of the antenna parameters and currents has been conducted using the Microwave Studio (MWS) commercial software from Computer Simulation Technology (CST), which is based on the method of finite integral time domain (FITD) algorithm. A prototype of the element in Ku band has been built and tested. The agreement between the computed and the experimental results was good. 2 The element design The basic structure of the proposed element operating in two Ku frequency bands and two orthogonal circular polarisations is shown in Fig. 1. Two stacked circular patches are fed in tandem by a single pin. Each patch is coupled to the feeding pin through an annular gap. This unique type of feeding is necessary in order to introduce a capacitive effect to counterbalance the inductive effect of the feeding pin. The feeding pin is top loaded with a little circular pad as an additional matching element. The upper patch ‘ground plane’ is the lower patch. To ensure a large enough ‘ground plane’ for the upper patch in all of its turning angular positions, the diameter of the upper patch (operating in the upper frequency band) must be consider- ably less than that of the lower patch (operating in the lower frequency band). This may be achieved by choosing the relative permittivity e r of the substrate between the lower patch and the ground plane to be close to unity, and on contrary by choice of considerably greater relative permit- tivity of the substrate between the two patches. In the design of the proposed element the lowest layer is Rohacell foam with electrical properties e r ¼ 1.067, tand ¼ 0.0041 and thickness 0.761 mm. The substrate between the two patches is Rogers RO4003 with electrical properties e r ¼ 3.38, tand ¼ 0.002 and thickness 0.5 mm. The top pad is printed on the Rogers substrate RO4350 with electrical properties e r ¼ 3.48, tand ¼ 0.001 and thickness 0.1 mm. A parametric study to optimise the element performances was conducted using MWS software from CST. For the operating frequencies 11.95 GHz (RHCP, lower patch) and 14.25 GHz (LHCP, upper patch), the patch diameters found are 11 mm and 5.85 mm, respec- tively, and the diameters of the annular gaps are 0.9 mm (inner) and 1.5 mm (outer). In a microstrip non-resonant antenna array fed by a radial waveguide configuration the top patch rotation angle varies from ring to ring and correspondingly its dimensions may change. The eccentri- city of the annular gaps is chosen as 1.8 mm for the lower R. Shavit and Y. Israeli are with the Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel L. Pazin and Y. Leviatan are with the Department of Electrical Engineering, Technion, Haifa 32000, Israel E-mail: rshavit@ee.bgu.ac.il r IEE, 2005 IEE Proceedings online no. 20045137 doi:10.1049/ip-map:20045137 Paper first received 24th October 2004 and in revised form 6th February 2005 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 4, August 2005 267