Simultaneously fixed/tunable multi-band modulated-width transmission line with varactor loading

A novel type of reconfigurable microstrip circuit is presented. Because of the periodic modulation of the basic geometry, electromagnetic band-gaps are present in the frequency response of the circuit as verified by numerical and experimental data. Insertion of active elements (varactors) allows modification of the position of the first two band-gaps. Equivalent circuit modeling of the varactor diodes in conjunction with a mixed full-wave/circuit simulation is used to characterise the effects of the varactor bias voltage tuning. The dispersion characteristics are deduced from scattering parameters obtained from the circuit model. The relatively simple bias network in this design makes implementation simple and low cost. In the example presented here, a fixed low-pass response extends from DC to 4.3 GHz, with tunable band-pass responses from 5.7/1.3 to 10.0/2.9 (center frequency/bandwidth, GHz) for bias voltages from 1 to 10 V. Multiple higher pass-band responses are also observed.