The choice of operating frequency in HF surface wave radar design

Radars operating in the HF band (3-30 MHz) are primarily of interest because of their ability to provide an over-the-horizon (OTH) surveillance capability. There are two types of such OTH radars - skywave (typically capable of detecting. targets at ranges of thousands of kilometres) and surface wave (typical detection ranges of hundreds of kilometres). Skywave radars, which are generally large, complex and expensive, rely on the ionosphere to refract signals beyond the horizon. The dynamic nature of the ionosphere places considerable demands on the radar, one of them being a very wide operating bandwidth (e.g., several octaves). Surface wave radars, on the other hand, are generally less expensive and complex, and can operate robustly over a somewhat narrower band, easing the constraints on RF and antenna design. This leaves the radar designer with an interesting choice with regards to the system operating frequency and bandwidth. The paper discusses the various factors that influence that choice and highlights some of the difficulties of selection.