Theory and Demonstration of Narrowband Bent Hairpin Filters Integrated With AC-Coupled Plasma Limiter Elements

A novel narrowband filter with integral high-power microwave limiter behavior is proposed to protect cost-sensitive microwave systems. The filter-limiter consists of a third-order microstrip bent hairpin filter with encapsulated gas plasma limiters, designed for operation at 870 MHz. An equivalent circuit model is presented for the ac-coupled plasma-shell components used in this study, and parameter values were extracted from measured results and electromagnetic simulation. The theory of operation of the proposed filter-limiter was experimentally validated and key predictions were demonstrated including two modes of operation in the on-state: a constant output power mode and constant attenuation mode at relatively higher power. The filter-limiter operates passively from incident microwave energy, and can also use an external priming voltage source to reduce the limiter turn-on power threshold and reduce output power variation during limiting.