An equivalent circuit model of a radial line planar antenna with coupling probes

Many communication systems require a microwave planar antenna with a gain in the range of 30 dB or more. Achieving this value of gain using a planar microstrip patch array with a conventional circuit-type feeding network faces difficulties. The paper concentrates on the planar radial antenna consisting of one central feed probe and a number of coupling probes arranged in rings feeding antenna elements. The usual design goal for this antenna is a loss-free, uniform (amplitude and phase) aperture illumination to obtain maximum gain. Realization of this illumination requires selecting proper positioning and length of the coupling probes. To achieve this task in a predictable manner, an approximate model for the feed network was developed by H. Miyashita and T. Katagi (see IEEE Trans. Antennas and Propag., vol.44, no.8, p.1158-65, 1996) assuming only interactions between the central probe and individual coupling probes. We describe a circuit model which takes into account mutual coupling between all of the coupling probes. The model is tested on the example of a 5-ring planar radial array as described by Miyashita and Katagi. The results show agreement with theoretical and experimental results presented by other researchers. The radiation pattern is sensitive to small changes in the length of the coupling probes.