Tunable bandpass frequency selective surface with wideband tuning and low insertion loss based on tunneling waveguide array

In this paper, we present a tunable bandpass frequency-selective surface (FSS) with wide tuning range. The FSS consists of a metallic rectangular waveguide array integrated into the double-sided printed circuit board (DSPCB) and an array of embedded varactors with a bias network on the backside of the DSPCB. The waveguide array exhibits a frequency-selective transmission peak due to the tunneling effect, and the transmission frequency can be adjusted by modifying the capacitance of varactors using different bias voltages. One benefit of this setup is that the varactors, along with the bias network, are situated on the same layer, resulting in a significant reduction in overall thickness to just 0.006λ_c (where λ_c represents the free-space wavelength at the highest transmission frequency). Additionally, the insertion loss related to the parasitic resistance of varactors is notably reduced due to the innovative integrated design of biasing lines with varactors and the nearly full-metal waveguide array. A prototype has been manufactured and tested, demonstrating that by varying the varactor capacitance from 0.12 to 1 pF, the passband showcases a frequency tuning range of 2.32:1, spanning from 5.8 to 2.5 GHz with an insertion loss ranging from 0.2 to 2.6 dB.