A Microstrip Based Active Antenna Doppler Transceiver Module

We present the design and test of a microstrip active transceiver element suitable for use as a CW Doppler module. The circuit consists of a microstrip annulus from which a small section has been removed, resulting in a two-port antenna. This antenna is then coupled to the drains of two FETs through a short section of microstrip line. By adjusting the length of this section of line, the oscillation condition is satisfied, the two FETs oscillate in the odd mode, and a microwave signal is radiated from the circuit. The Doppler-shifted return signal which is picked up by the antenna is applied back to the drain ports of the FETs, which also function as self-oscillating mixers. The IF signal is coupled out using audio transformers. By measuring the in-phase and out-of-phase IF signals, we can determine the co-polarized and cross-polarized components of the reflected signal.