A Varactor-Tuned Helix-Based Chiral Layer

Tuning performance of a chiral layer using varactor diodes is presented here. The chiral structure consists of an array of metallic long helices fabricated in parallel on a PCB. The helices have the same handedness, sub-wavelength diameter, pitch, and spacing in the array. Varactor diodes are incorporated into the array to tune array cross-polarized transmission response, a qualitative measure of the effective tuned chirality. Each helix is connected to one of its neighboring helices through a set of varactors placed between them. Array has one varactor per one helix pitch. The tuning performance of the chiral array transmissivity is measured over frequency band of 5.5-12 GHz with the bias voltage sweeping from 1 to 15 V. Voltage tuning changes each varactor capacitance from 0.9 to 0.13 pF. This results in ~ 50% increase in the cross-polarization peak frequency, changing it from 6 to 9 GHz, for a linear polarization incidence perpendicular to the helices. Tuning rotates the transmitted wave polarization ellipse by ~15°.