As was discussed in the May Technical Corner article, it is
sometimes desirable to include couplings between non-adjacent resonators
to obtain more optimum filter transfer functions. An example
of a fourth order dielectric resonator design incorporating a single capacitive
cross coupling is shown in Fig. 1. 
This filter employs a short, single-section evanescent mode filter to couple between the first resonator and the last resonator. The couplings between the other resonators are inductive irises, tuned but not resonated. The evanescent filter coupling between resonators (1) and (4) uses a re-entrant section of coaxial line to provide the capacitance required to resonate the equivalent shunt inductance of the below cutoff (evanescent mode) section of waveguide which is what the iris really represents. The equivalent circuit of such a coupling was presented in the May article. The results are quite striking, with the simulated and actual response characteristics shown in Figs. 2 and 3, below.
Figure 2 - Simulated Response
Figure 3 - Actual Response
The single cross-coupling in this even order circuit (4th order) provides a pair of transmission zeros which emphasize the attenuation close to the passband. As has been previously discussed [1], [2], [3], inclusion of both capacitive and inductive couplings are possible, with respective effects on stopband attenuation or on passband group delay and amplitude flatness. The technique can also be applied to dual mode filters, bandstop filters or most kinds of TEM, quasi-TEM or evanescent mode filters.
1. RS Microwave Technical Corner, May 1997, http:// www.rsmicro.com.
2. R. V. Snyder, “Generalized Cross-Coupled Filters Using Evanescent Mode Coupling Elements”, IEEE MTT-S Symposium Proceedings , June 11, 1997, Denver, CO.
3. R. V. Snyder, “Inductive or Capacitive Couplings Using Resonated Evanescent Mode Filter Sections”, Filter Workshop, 1997 IMS, June 9, 1997, Denver, CO.