Extending the low-frequency limits of the compact-range reflector

Traditionally, the compact-range is not considered a viable method for conducting low-frequency (VHF/UHF) antenna or RCS measurements, because of the limited electrical size of the collimating-reflector system. Normally, compact-range measurements are conducted in the extreme near field of the collimating system, where the main reflector size is on the order of 25 to 30 wavelengths minimum, with edge treatments of at least four wavelengths at the operating frequency. This mode of operation limits measurements to the high UHF band (500-800 MHz) and above, for even the largest sized reflector systems. L or S bands are more commonly the low-frequency limit. Recent research with compact ranges indicates that acceptable VHF/UHF measurements can be conducted in the quasi-far-field region of the collimating system, with reflectors as small as five wavelengths, and with electrically short edge treatments. A good user knowledge of this mode of operation is required to maximize its utility. A qualitative measure of acceptable quiet-zone performance must also be established. This paper addresses the theory of operation, practical implementation, and inherent limitations of the non-conventional use of the indoor compact range for conducting low-frequency measurements.