Hardware cancellation of the direct coupling in a stepped CW ground penetrating radar

A stepped CW ground penetrating radar using UHF frequencies is being developed by the University of Cape Town. One of the major problems encountered is a large direct coupling between the transmit and receive antennas. This coupling limits the dynamic range of the system by inserting a large signal into the receiver. The hardware implementation of a digitally controlled RF cancellation circuit is discussed. This RF system comprises of an analogue bi-phase modulator used as a wideband phase-shifter and a PIN diode attenuator. This system, controlled by a PC, was attached to the front end of the SFCW radar. Results of the cancellation circuit used to cancel a cable over a 200 MHz bandwidth and a coupling vector are presented