The TE111 cylindrical cavity mode provides good mode separation, small size, and an unloaded Q which is greater than that of comparable volume rectangular cavities. However, cavities which support higher order modes can also be considered. Physically larger cavities can provide even higher unloaded Q s and thus lower insertion loss, but a greater density of spurious modes will result from this choice.

The resonant modes that can be formed using the ten lowest cutoff frequency circular waveguide modes were systematically searched with a computer routine [4]. This routine calculates the cut-off frequency of several modes for a given waveguide dimension, either circular or rectangular.

Minimum mode separation was set at 5 percent. Resonances which did not provide a significant unloaded Q improvement over the TE111 case for the increased size were rejected. The candidate modes that survived the screening are shown in Figure 3.

Figure 3: Candidate modes for higher order mode filters.

The TE011 mode offers the greatest improvement in Qu for cylindrical shapes but is degenerate with the TM111 mode, which has a lower Qu. The TE1m1 sphere resonance yields an even higher unloaded Q but has a three fold degeneracy. Using these configurations increases the design complexity, since the extraneous modes must be moved out of the band. Degenerate modes can be used such that a dual or triple mode filter results [4].

Although it has been demonstrated that for higher order mode cavities, a Qu of two and...