Novel Microstrip Multifunction Directional Couplers and Filters for Microwave and Millimeter-Wave Applications

Abstruct-The design of a new class of microstrip couplers and filters is presented in this paper. The synthesis functions obtained from the solution of first-order nonlinear differential equation of nonuniform lines with a loose coupling assumption are modified and validated for higher coupling values. The design employs a nonuniform coupled line configuration along which a realizable continuous coupling coefficient is obtained by modifying the reflection coefficient distribution function. This modification results in a frequency selective coupling which minimizes the out-of-band coupling in the specified frequency range. As a result it is possible to realize -3 dB directional couplers using double-coupled lines without the need for tandem connections or extreme photolithographic techniques. Experimental results for microwave band-pass and periodic couplers are presented together with the computed results. Potential applications of these novel components are discussed and the work is extended to include millimeter-wave realization.