Multibeam microstrip patch antennas excited by parallel-plate beam-forming network with shaped reflecting boundary and optimized slot feeding transition structures

This paper presents the design of a multibeam phased array of microstrip patch antennas at 38 GHz with a parallel-plate beam-forming network (BFN). This antenna set, including the antenna elements and BFN, is equivalently triple-folded into two stacked printed circuit board substrates for a compact design of nearly 60% size reduction in comparison to conventional implementation on a single planar dielectric substrate. In particular, the BFN employs a shaped metal reflecting boundary in the bottom substrate to transform the cylindrical waves of feed excitations into planar ones for a relatively linear phase variation to feed the column subarrays on the top substrate. A slot feeding transition structure is designed to couple the energy from the BFN to feed the microstrip patch antenna ports. These two mechanisms are then used to compensate the phase distortions due to BFN cavity disturbance and mutual coupling from subarrays of antennas at millimeter wave frequencies. Numerical techniques to optimize both the metal reflecting boundary and slot feeding transition structure are introduced to minimize phase distortions. Numerical and experimental results are presented to demonstrate the radiation characteristics of the integrated antenna set.