Design of microstrip antenna feeds. Part 1: Estimation of radiation loss and design implications

Feed arrangements for microstrip antennas commonly involve abrupt transitions between a coaxial or triplate feeder and the microstrip line; the latter may additionally embody stepped microstrip matching sections. A canonical transition model is analysed using an approximate variational method and estimations of the unwanted radiation loss associated with all the above mentioned transitions and matching elements are obtained. Computed results are given and discussed in relation to the very limited amount of independent experimental data available. The effect of these extraneous radiation losses on the antenna sidelobe level performance and ways of reducing the losses are considered. It is concluded that if the constructional simplicity of the transitions is to be retained there is little scope for drastically reducing the radiation losses but a few design recommendations are made. A triplate corporate feed having multiple transitions is treated in a companion paper.