Design of Composite Microstrip-Monopole Antenna With 180$^\circ$ 1 dB Beamwidth Based on Complementary Sources Concept

Abstract

A new perspective based on complementary sources radiation is presented for analyzing a widebeam composite microstrip-monopole antenna. The typical cavity model theory shows that the radiation patterns of the microstrip antenna could be calculated by using two parallel equivalent magnetic currents, and the E-plane beamwidth of the traditional microstrip antenna is mainly determined by the array factor. Loading the monopoles through the metal patch of the original microstrip antenna can construct complementary sources. Two monopoles are symmetrically loaded and two pairs of complementary sources are excited. Interestingly, the two pairs of complementary sources are aligned with in-phase magnetic current radiation sources and antiphase electronic current radiation sources, which indicates two heart-shaped radiation patterns pointing in opposite directions. The total far-zone radiated fields synthesized by the two heart-shaped radiation patterns can present wide E-plane beamwidth performance. An example antenna working at 4.4 GHz is designed for verification. With the introduction of a differential feeding scheme for a symmetrical radiation pattern, the E-plane radiation pattern can cover the upper half plane, i.e., 180°, with a fluctuation of less than 1 dB and an antenna gain of over 3 dBi.