Experiments with the Forbidden Regions of Open Periodic Structures: Application to Absorptive Filters

Experiments with open periodic structures are presented which show a large change in transmission at the transition from allowed to forbidden regions. At the boundaries of the forbidden regions, the radial distribution of the fields of a wave impressed upon the structure changes from one corresponding to slow-wave propagation to a field distribution corresponding to radiation. At these boundaries, the local field shapes do not change very much. However, at the boundary a large decrease in transmission through the structure is found, but with little change in input VSWR; this implies a change from real characteristic impedance to about the same value of radiation resistance. This behavior suggests applications to sharp cutoff absorptive filters.