Microstrip Resonator Doubling as a Filter and as an Antenna

This letter proposes a novel concept for a microstrip resonator that can function as a filter and as an antenna at the same time. The proposed structure consists of an outer ring, an open loop-type inner ring, a circular patch, and three ports. The frequencies where the proposed structure works as a filter and as an antenna, respectively, are determined primarily by the radius of the inner ring and the circular patch. At the resonant frequencies of the filter and the antenna, high isolation between the filter port and the antenna port can be obtained.