Any assessment of overall radar performance must include the performance of the radar antenna. When the antenna is installed on a moving vehicle, for example, an airborne platform, the installed rather than the free space antenna characteristics must be employed in the systems analysis. The nature and extent of the performance degradation after installation will depend both upon the type of vehicle, in particular its size and surface structure, and the siting of the antenna. Modelling the installed performance of radar antennas has proved to be very difficult owing to the difficulties in computing the complex interactions between the antenna and the structure.
Introduction There are various problems in using radars on a helicopter.
The sites which are suitable for installation are very limited and full coverage over 360.0 degrees is not achievable with one antenna unless the antenna can be accommodated under the helicopter. However, this is impractical for the antenna dimensions required for AEW or surveillance radar applications and the more usual sites are a side installation where 180.0 degree coverage is the maximum feasible and, as will be shown, this is unlikely to be achieved without antenna performance degradation due to scattering from the helicopter structure. In addition to the siting problems and the interactions between the antenna and the helicopter fuselage, the presence of the moving rotor blades will further perturb the installed antenna performance as compared to free space. In general, the bulk of the energy incident on the main rotor blades will be scattered downwards, thereby degrading the antenna performance in the direction of the earth's surface. The installed performance of the antenna, therefore has implications both for the levels and the distribution of clutter present in the radar receiver.