Design of a Wideband Ring Microstrip Antenna Fed by Two L-probes with a Rat-Race Coupler

This paper presents design of a wideband ring microstrip antenna (MSA) fed by two L-probes with a rat-race coupler. The proposed antenna consists of a square ring patch, two feeding L-shaped probes (L-probes) to excite the ring patch, and a rat-race coupler on the ground plane. The two L-probes are fed by the rat-race coupler differentially and unwanted radiations from the vertical portions of the two L-probes are cancelled out each other. Therefore, cross polarization in H-plane pattern of the proposed antenna, which is a typical phenomenon of the wideband MSA fed by an L-probe, can be reduced. The design frequency of the proposed MSA is around 0.6 GHz. In order to make the ring patch and the rat-race coupler equal in size, the rat-race coupler is miniaturized by bending the rat-race circuit. To achieve wideband characteristics at around the design frequency, dimensions of the proposed antenna are tuned by using electromagnetic simulator IE3D. It is confirmed by the simulation that fractional bandwidth less than -7.4 dB reflection is approximately 34%. Very low cross polarization in H-plane pattern below -30 dB is also confirmed by the simulation.