Chapter 8: Special Radar Issues

OVERVIEW

This chapter addresses effects of aircraft structure on antenna patterns, aperture efficiency of low sidelobe antennas, and the determination of target altitude. Each of these subjects is of considerable importance to the design of future airborne early warning (AEW) radars.

Very low two-way (transmit and receive) sidelobes are needed to control side-lobe clutter that often sets the minimum detectable target radar cross section (RCS) for Doppler radars on moving platforms (Section 6.5.2). Thus, future AEW antennas will need to be designed for very low sidelobes, and these antennas must be positioned to minimize the effects of aircraft interference on the antenna patterns. Improved radar target height measurements are needed for targets over land and sea (Section 2.3.1). The traditional height finder measures elevation angle with a narrow beam. However, to obtain such a beam at AEW frequencies requires an antenna of large height, making it difficult to mount on aircraft. Although generally unsuited for small aircraft, a dedicated tracking radar of relatively high frequency may be the best option for obtaining accurate target height measurements.

Section 8.1 includes a discussion on the general effects of antenna/aircraft interference, and uses sample computer-calculated patterns to illustrate the antenna pattern degradation that can be caused by major aircraft structure such as the wings, engine nacelles and supports, and the empennage. Section 8.2 discusses aperture design requirements for obtaining low sidelobes in an idealized, interference-free environment, and graphs are included to...