Design and performance of an active radar calibration target for ultra-wideband radar cross section measurements

The University of Nebraska has developed an ultra-wideband coherent random noise radar that accomplishes phase-coherent processing of the received data. The system operates over the 1 - 2 GHz frequency range. In order to make calibrated radar cross section measurements of targets and terrain, a radar calibration target was fabricated and tested. The unique requirements for the ultra-wideband calibration target include (1) high radar cross section value to minimize effects of background reflections, (2) constant radar cross section over the frequency range to ensure calibration accuracy, and (3) wide beamwidth to minimize effects of antenna pointing errors. The design consisted of a receive and a re-transmit antenna between which a high-pass filter and a microwave amplifier were inserted. Log-periodic antennas were used as calibration target antennas owing to their broadband and wide beamwidth characteristics. The high-pass filter possessed a 12 dB per octave roll off to appropriately reduce the signal level at lower frequencies to compensate for the correspondingly lower propagation loss as predicted by Friss transmission formula. The high-gain broadband amplifier was used to provide a high- retransmitted power level back to the radar. The design and performance characteristics of the active ultra-wideband radar calibration target are discussed in this paper.