This paper presents for the first time a fully electronically reconfigurable waveguide filter tunable in bandwidth and center frequency based on liquid crystal (LC) technology. A continuously reconfigurable two pole bandpass filter is designed and characterized in the Ka-band at 30 GHz. To be able to tune both center frequency and bandwidth independently, the resonators and coupling structures are filled with LC as tunable material. Hence, the filter's center frequency and coupling strengths can be tuned and, furthermore, tuning with constant filter characteristic is possible. To tune the LC, a novel two-layer electrode design for waveguide structures is presented, which is simple to integrate and provides a high tuning efficiency with low insertion loss. By applying different bias configurations, the LC's effective permittivity can be varied, and therefore, also the resonators' electrical lengths. The presented two pole filter can adapt its center frequency from 29.8 GHz to 30.7 GHz with a maximum 3 dB bandwidth variation from 660 MHz to 870 MHz. The measurements are carried out with bias voltages up to ±250 V.INDEX TERMS Microwave filter, liquid crystals, millimeter wave communication, tunable circuits and devices, K-band.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

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In this paper, different techniques for the generation of additional transmission zeros (TZs) in planar waveguide filters are investigated. In the classical theory, TZs are generated only by destructive interference of non-adjacent cavities, limiting the available number of TZs to the filter order itself. However, more approaches for the generation of TZs are known, including bypass-coupling in oversized cavities, frequency-dependent coupling apertures as well as dispersive/resonant TZs which can be realized by the direct source to load cross-coupling. The aim of this paper is to combine several of the strategies in one physical filter set-up to increase the maximal number of TZs beyond the filter order. Different Ku-band fourth-order filter set-ups are presented, showing in total between six and eight real as well as complex TZs. Three filters are manufactured as a proof of concept and compared with the simulation, showing very good agreement.