Microstrip antennas and arrays. Part 1: Fundamental action and limitations

Consideration of the radiation mechanism of an open-circuit microstrip termination indicates that the terminal-plane region is the dominant radiating aperture. On this basis, the radiation patterns of several types of microstrip radiating elements in rectangular and cylindrical geometry are calculated and substantiated by experiment. In general, the patterns exhibit the characteristics of slot radiators, thus endorsing the action of an end-radiating aperture; the radiation admittance of the latter is obtained by novel measurements of the standing waves in a truncated microstrip line but, when the substrate extends beyond the radiating aperture, substrate surface waves are generated. An analysis of the substrate waves indicates that the side-lobe level of an array of microstrip radiating elements could be limited to around ¿¿20 dB, and severe unwanted coupling effects between microstrip circuits are also noted. The use of substrate troughs to reduce the surface-wave generation is discussed, and a strip width-modulation technique proposed to control the array aperture distribution and simplify the feeding configuration. The implications of these present findings on the potential performance of microstrip antenna arrays is discussed and design recommendations made; the latter are investigated in a companion paper.