This paper describes the general realization of elliptic function narr
ow-bandpass filters constructed from two adjacent planes of microstrip
quasi-TEM line resonators. This dual-plane configuration allows nonad
jacent coupling to be realized through small slots between the planes.
Alternate sign coupling is realized by the appropriate positioning of
the slot along the resonator length. A model developed from Bethe's s
mall-hole coupling theory is used to predict these couplings. Adjacent
resonator couplings are modeled using a coupled-line analysis modifie
d to characterize the coupling of an inhomogeneous medium. The filter
synthesis technique previously developed for waveguide cavity filters
can be directly applied in designing a dual-plane filter.