A HIGH PERFORMANCE CIRCULARLY POLARISED STACKED PATCH ANTENNA WITH LOW MUTUAL COUPLING Kwok L. Chung and Ananda S. Mohan Microwave and Wireless Technology Research Group, ICT Group Faculty of Engineering, University of Technology Sydney 1 Broadway, NSW 2007, Australia E-mail: kchung@uts.edu.au, ananda@eng.uts.edu.au A broadband circularly polarised stacked patch antenna element and a 4-element planar array, which have high efficiencies at 10 GHz, are presented in this paper. In addition to the low axial ratio given by the array, all possible bandwidths are obtained to be greater than 20% with an overall antenna efficiency greater than 75%. The mutual coupling between proposed elements at a centre-spacing of 0.667λ o is also observed to be less than -25 dB. 1. Introduction A conventional two-layer electromagnetically coupled patch (EMCP) antenna is a specific type of stacked patch antenna. EMCP uses dielectric materials with low dielectric constants of around 2 to 2.5 for both the driven and parasitic layers, and uses airgap or foam as the layer separation [1-3]. However, when low dielectric constant materials are used as the driven layer the reinforcement of surface-wave power occurs. Surface-wave excitation has long been a major problem in microstrip patch antennas. It reduces antenna efficiency and increases mutual coupling when electrically thick substrates are used to increase the bandwidth for linear polarisation (LP). For circularly polarised (CP) patch antennas and arrays, surface-wave also causes severe unwanted coupling, radiation and high cross-polarisation discriminations (XPD), which in turn governs the quality of the axial ratio and its bandwidth. To overcome these problems, a radiating patch and feeding network printed on different layers using materials with different dielectric constants were suggested and realised in [7- 9]. The idea presented in [7] uses a high-dielectric-constant material such as Alumina for the feed substrate allowing simple integration with the microwave circuit and bandwidth improvement for LP patch antenna. The stacked patches printed on hi-lo dielectric-layer combinations are also developed for CP applications in [8] to enhance surface-wave efficiency. We presented an optimal design and tuning strategy for CP-EMCP antennas [9]. A superstrate with a low dielectric constant has been included for the parasitic top-patch, which does not just improve antenna efficiency [4] but reduces mutual coupling. Thus, the EMCP element presented in [9] can also be termed as a stacked patch antenna with hi-lo-lo dielectric-layer combinations. We have also addressed in [9] the effect of superstrate for the determination of the top-patch size and the CP performance such as the enhancement in XPD and bandwidths, and more importantly, the trade-off between the boresight axial ratio and the axial-ratio bandwidth as a function of the perturbations on the stacked patches. In this paper, we present another novel broadband stacked patch element in a CP-EMCP structure and show the performance of a 4-element planar array. The single element is first designed by using the design and tuning techniques introduced in [9], and a sequential feeding technique is then applied for the array to increase the gain as well as bandwidths. In addition to the impedance matching and CP performance for both the single element and the array, the overall antenna efficiencies as well as the mutual coupling between elements with different element orientations are examined.