Analysis and Design of a High Directivity Compact Waveguide Coupler Using 3-Hole Aperture Array

The present paper describes a design of a high directivity compact multi-hole directional coupler using three rows of apertures in the broad wall of the waveguide. Shelton reported the design of a compact multi-hole coupler based on the classical theory. His design gives low directivity using two rows of holes. In this method holes diameter required to obtain tighter coupling becomes quite large. As a result the predicted performance of the coupler deviates too much from the experimental results. The large diameter also poses some mechanical problems in fabrication of the coupler. In view of this a three hole aperture instead of two hole one for multi-hole coupler was thought to be better possibility for tighter coupling. It is quite obvious that by using three holes instead of two holes the diameter of the aperture are reduced considerably, to achieve the same performance. As the Bethe´s theory of small aperture is the basis of the coupler design hence the predicted results of the coupler of smaller aperture diameter would be more closer to the experimental results than that of a larger aperture coupler. The performance of 3-hole aperture coupler has been analysed, by Levy´s Technique of equivalent circuit synthesis and incorporated it in designing a practical high directivity compact coupler. This design technique is different than that used by Shelton. This design of coupler is based on the distributed low pass filter design method. The design of compact coupler using 3-hole aperture has not been so far reported.