Optimizing the HUSIR Antenna Surface

For a given target at a given range, the sensitivity of a radar is proportional to the transmit power and the gains of the transmitting and receiving antennas, which are the same for a monostatic radar such as the Haystack Ultrawideband Satellite Imaging Radar (HUSIR). As the operating frequency increases, it becomes more difficult to generate high transmit power, so it becomes vitally important that the gain of the antenna is maximized. The gain of a reflector antenna is inversely proportional to its surface errors in an exponential manner. Highly efficient reflector antennas require surface tolerances on the order of one-thirtieth of a wavelength or less. At HUSIR's operating wavelength of 3 mm (100 GHz), for example, the surface tolerance must be on the order of 100 µm root mean square (rms). Achieving and maintaining such a tolerance on the 120-foot-diameter HUSIR antenna is very challenging; careful design of the antenna structure, meticulous implementation of the design, and an accurate metrology system to measure the surface are required. The efficiency of a reflector antenna is inversely proportional to the rms surface errors as shown by Ruze [1]. A simplified version of the Ruze equation that is appropriate for HUSIR is plotted in Figure 1. The original 120-foot-diameter Haystack antenna was completed in 1964, representing what was then considered the state of the art in the construction of large moving structures, with a specified surface (half-path-length error) tolerance of 0.025 inch (0.64 mm) rms [2, 3]. Several improvements were made over the years, reducing the tolerance to 0.017 inch (0.43 mm) rms in 1967 [4]. With the addition of thermal control of the back-The primary objective of the HUSIR upgrade was to significantly improve the imaging resolution achievable by the Haystack radar. To meet this image resolution objective, a W-band (96 GHz center frequency, 3 mm wavelength) capability was added to the radar. W band provides the necessary bandwidth for high-resolution imaging, but its much smaller (than the original X band) operating wavelength makes the requirements on the antenna much more stringent and challenging to achieve.