Research into stepped frequency continuous wave ground penetrating radar (SFCW GPR) at UCT has been carried out since 1990. A first generation system comprising of Hewlett Packard test equipment controlled by a PC was assembled. Cavity-backed log spiral antennas were designed and built by the University of Stellenbosch for the specific use of GPR. Measurements were the first generation system proved the concept of SFCW GPR and thus a dedicated second generation system was planned.
A SFCW GPR system was designed to replace the first generation system. Various designs for transmitter and receiver configuration were investigated and those found most suitable were used for the implementation.
The SFCW radar consists of a wideband CW transmitter and a coherent receiver. A 300-1000 MHz transmitter was constructed using varactor-tuned oscillators as frequency sources. A double-sideband, low-IF receiver was constructed for the 300-1000 MHz signal, to mix it to an IF of 10.7 MHz and I-Q demodulate it. The transmitter was found to operate according to specifications. The receiver was found to operate satisfactorily, but the dynamic range was less than expected.
A limiting problem encountered in the first generation GPR was a large direct coupling signal from the transmitter into the receiver. This large signal reduced the effective receiver dynamic range. A method of cancelling this large direct coupling signal was implemented, using a bi-phase modulator to generate the cancelling signal in antiphase to the coupling signal. A 20 dB reduction in coupling was shown.
The system was used to measure cable lengths to within the inherent accuracy of the system. A metal plate target was detected by the system feeding two antenna and a concrete floor was detected under 1m of sand. It was thus shown that the SFCW system could be used as a second generation GPR.