Novel microstrip low-pass/high-pass directional couplers/filters

The design of novel microstrip low-pass/high-pass couplers/filters is presented. The design is based on the equivalence principle of nonuniform transmission line directional couplers and the design procedure basically involves the determination of a realizable coupling coefficient function in the design bandwidth. The coupling coefficient distribution function is modified to obtain the necessary characteristics. This novel design offers the following features: if low-pass output is obtained from the coupled output, then the high-pass output is obtained from the direct output; and if high-pass output is obtained from the coupled output, then the low-pass output (from DC) is obtained from the direct output. The predicted performances with couplers that have dimensions within practical manufacturing limits are excellent. The unique feature of this novel design technique is the ability to obtain both low-pass and high-pass filter characteristics from a single component