Conical-Scan Antennas for W-Band Radar Systems

Conical-scam implementations for three antenna types are discussed in terms of the beam nuiatioa method, reference signal generation and angle error demodulation Various applications for these antennas including their use in teaching are discussed. Inder Terms-..eoaical-sean, millimetre wave, antenna I. IN'RODUCiTON Tracking radars require a mechanism to determine the offset of a target from the antenna boresight. At millimetre wavelengths, implementation of the monopulse technique is both difficult and expensive in contrast to the elegant simplicity of the conical-scan technique. This simplicity and ease of implementation makes the latter an ideal tool to teach a wide range of sensing concepts. A . Conical-scan Principles To determine the target offset fiom the antenna horesight, conical-scan radars locate the centre of the target image by nutation of a single feed that is displaced fiom the axis as shown in Fig. 1. When the target image is centred on the axis, the feed receives equal amounts of power from all nutation angles [l]. When the image is offset, more power is received hy the feed on the one side which results in the amplitude modulation of the received beam. This modulation is compared to the in phase and quadrature (I and Q ) reference signals generated hy the nutation mechanism to determine the sense of the error, while the amplitude determines its magnitude. It should be made clear that the received radar signal that is demodulated by the reference signals will have been gated in range to include only the target being tracked, and normalised using a automatic gain control (AGC) circuit to remove the effects of both target cross-section variations and range. B. Squint angle tradeoff A major consideration when designing conical-scan antennas is the beam squint angle as this determines the modulation depth and hence the error slope k, as well as the power loss on boresight Lt. The effective slope [Z] which combines the actual error slope and the on boresight loss G. M. Brooker i s now a researcher at the Australian C e n a for Field Robotics (ACFR), University of Sydney, AusudiaThe applications discussed here were developed while he war with Analysis Management and Systems in South Africa (e-mil gbraokeriiiiacfr.u~d.~d".~"). !&Lk is a maximum at a normalised squint angle (with respect to the 3dB heamwidth of the antenna) of 0.45. This optimum is seldom used and a smaller squint angle with a value of 0.33 is suggested hy [2], while an even lower value of 0.28 is suggested by [l]. The latter results in an on boresight power loss of only l a . For the beacon tracking case, [2] proposes an optimum normalised squint angle of0.5.