A Circularly Polarized Balanced Radar Front-End With a Single Antenna for 24-GHz Radar Applications

In this paper, we propose a circularly polarized balanced radar front-end topology with a Tx leakage canceller. This radar is composed of a dual-orthogonal fed microstrip patch antenna, a 90deg delay line, a differential cross-coupled oscillator, two Lange couplers, a Wilkinson combiner, and a mixer. We implemented circularly polarized balanced Doppler radars by using a printed circuit board and InGaP/GaAs heterojunction bipolar transistor technologies. The front-end chip and module with antenna are 1 mm times 1 mm and 3 cm times 3 cm, respectively. The average Tx-to-Rx isolation is about 36 dB. This radar can measure speeds as low as 0.5 mm/s, which corresponds to a 0.076-Hz Doppler shift. The received signals of the circularly polarized balanced Doppler radar are about two times larger than those of a linearly polarized balanced Doppler radar due to the topology. This result occurs because the proposed radar has an intrinsic loss of only 3 dB in the receiving path.