Calibration of a large receiving array for HF radar

This paper addresses the calibration of a receiving array for a large sky wave HF radar currently under construction for the Commonwealth of Australia. The array incorporates a receiver per antenna element followed by digital beamforming. Although this clearly eases many of the problems associated with analogue beamforming methods, calibration of the analogue components, including receiver local oscillator phases, is still necessary, and to a high degree of accuracy. In order to accommodate frequency and waveform agility a two-stage calibration process is adopted. Offline calibration is used for the antenna elements and feeders with real-time calibration for the receivers and associated local oscillator signal distribution system. The real-time calibration system uses a set of equal length cables to the receivers which is itself calibrated off-line by a coaxial cable loop to offer an optimum combination of calibration speed and accuracy. Simulation results indicating the effect of cable impedance imperfections for the loop are included together with practical means of verifying the performance