Reduction of interference by high power HF radar transmitters

GEC-Marconi is involved in the development of a network of long-range skywave over-the-horizon radars for Australia, to provide wide area surveillance in the maritime approaches to the north and north west of Australia. High transmitter power must be provided for these radars to meet operational requirements. A basic transmitter subsystem consists of an array of antennas, with each being driven by a solid-state high power amplifier (HPA). An important requirement is for these radars to achieve new standards of performance in terms of noninterference with other users of the HF band by conforming to criteria taken from the US Department of Commerce NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management. This paper is concerned with the purity of the transmitted signal. The NTIA Manual defines a spectral template within which the radar emissions must be confined. The conventional approach of simply weighting the transmitted radar waveform in order to control the out-of-band emission is inadequate and inefficient with the waveform parameters typically used for HF radar. Additionally, because of the high transmitter power to be used, high amplifier efficiency is required to reduce running costs. This requirement conflicts with the requirement for high amplifier linearity which is necessary to keep distortion products to a low level.