Electrical filters of a recently invented type are designed and constructed as transmission lines into which multistage photonic-band-gap (PBG) structures are incorporated. As used here, "transmission lines" refers to any or all of the standard electrical media, including coaxial cables, microstrips, and the like, used for propagation of electromagnetic waves along enclosed or well defined paths. The term "electrical filters" is used here in a conventional sense, denoting any of a variety of systems or components that block propagation of signals in certain frequency bands (stop bands) and/or support propagation in other frequency bands (pass bands). The PBG structures used in this invention can be any of a variety of dielectric or electrically conductive structures, containing precise periodic or quasiperiodic arrays of holes or other features that can be tailored to obtain desired stop bands.

Three or More PBG Structures are incorporated into a transmission line. The number of PBG structures and distances between them are chosen to obtain a pass band wider than could be obtained by use of only two PBG structures.
Prior to this invention, PBG structures had been used to create mirrors or Fabry-Perot resonators. Such a mirror or resonator consists of two PBG structures separated by a gap and typically provides a very narrow pass band within the stop band(s) of the PBG structures. It is necessary to maintain tight manufacturing tolerances in order to place the narrow pass band accurately in the desired frequency range. However, in some applications — especially those involving microwaves — there is a need to provide a wider pass band and to relax manufacturing tolerances. The invention helps to satisfy this need.

A filter according to the invention (see figure) includes at least three PBG structures at different positions along a transmission line. Each successive pair of PBG structures constitutes a successive PBG resonator. The distances between the resonators determine the narrow pass bands of the PBG-resonator pairs. Through appropriate choice of the number (N) of resonators and of the distances (S1,2, S2,3, ... SN-1,N ) one can obtain a single wider pass band. Depending on the requirements of a specific application, these distances could be equal or unequal.

This work was done by Derke R. Hughes of the Naval Undersea Warfare Center.