Design of a planar, closely spaced yagi antenna

The Yagi antenna is a standard antenna to achieve high gain in a very simple structure. The antenna is composed of a driver and parasitic elements including a reflector and one or more directors. Designs of the Yagi typically have element-to-element spacing on the order of 0.15lambda to 0.4lambda. For some applications, however, it is desirable to reduce this spacing to achieve a physically more compact size. Previously, we reported on a three- element Yagi antenna with very close element spacing (0.02lambda). The folding concept was used to overcome the low radiation resistance due to the close spacing. The resulting antenna design was evaluated for potential HF skywave applications. In this paper, we extend our work to make the previously reported planar, which may be desirable for such applications as RFID. For the driver, multi-folding is used to step up the radiation resistance to 50Omega . The antenna dimensions are optimized using a genetic algorithm (GA) in conjunction with the numerical electromagnetics code (NEC) based on a wire implementation of the antenna. Two structures are investigated: a driver plus a reflector and a driver plus a director. Prototype structures are constructed and measured at 1 GHz.