Design and characterisation of a 200 GHz tunable lens-coupled annular-slot antenna with a 50 GHz tuning range

A frequency-tunable lens-coupled annular-slot antenna (ASA) with a nearly 50 GHz tuning range has been designed, fabricated and characterised at the G-band (140-220 GHz). Initial numerical electromagnetic simulation results show that the resonant frequency of the antenna can be tuned effectively, with a tunability of ~2.5 GHz/fF associated with the capacitive loading from the varactor. In addition, simulated far-field antenna patterns have demonstrated that the addition of the varactor used to implement the capacitive loading does not strongly affect the antenna´s radiation properties. This design has been demonstrated experimentally, using a Schottky varactor diode with a zero-bias junction capacitance of ~16 fF mounted on a 200 GHz ASA fabricated on a semi-insulating silicon wafer. Measurement at G-band shows a frequency tuning range of nearly 50 GHz by varying the voltage bias of the Schottky varactor diode from -5 to 1 V. The measurement results agree well with simulations based on an equivalent circuit model for the Schottky diode and the antenna. Measured far-field radiation patterns of the lens-coupled tunable ASA at 203 GHz show good agreement with calculations based on the ray-tracing technique, verifying that the diode mounting does not affect the antenna radiation properties significantly. This type of tunable ASA is very promising for realising reconfigurable detectors and focal-plane arrays at terahertz (THz) frequencies, and may find applications in future adaptive submillimetre-wave/THz communication systems.