A TM triple-mode microwave filter

A novel realization of triple-mode microwave filters using only transverse magnetic (TM) mode degeneracies is presented. The filter is constructed using two triple-mode cavities cascaded back-to-back. The cavities contain one coaxial coupling probe for input or output coupling, six coupling screws for intercavity couplings, and three tuning screws for independent tuning of the three orthogonal modes. Tuning of the microwave filter was accomplished as for any six-pole quasi-elliptic function filter. The in-band and out-of-band responses are shown. Experimental results obtained with a prototype triple-mode six-pole, 8.5246 GHz, quasi-elliptic function, 80 MHz bandpass filter design at the degeneracy of the dual-TM₁₁₀ and single-TM₀₁₂ mode are shown to agree well with theory