Radar Jams H-F Communications

It has been well known for many years that UHF radar can jam cosite located 2-30 MHz communications systems. For the most part, the problem is due to unexpected spurious responses and audio rectification in the victim receiver. Not well known, however, is the fact that the primary mode of coupling may not be antenna-to-antenna interference but rather radar-antenna-to-victim-cable coupling. The paper presents case histories in support of the above together with analytical and hardware solutions. Generally this situation is not recognized because of the appearance of broadband interference across most of the tunable spectrum of the HF receiver. One solution is to use protective low-pass filters having a cut-off frequency sufficiently below that of the radar. Another solution is the selection of a coaxial cable from the victim antenna to its receiver having a considerably lower transfer impedance to eliminate the antenna-to-cable coupling mode. Also demonstrated is the inadequacy of present MIL-STD 461A/B, CS-04 limits in which a maximum out of band injection of 0 dBm is called for. It is shown that under certain conditions the available interference power may approach levels of + 30 dBm. This clearly demonstrates the need for increasing the MIL specifications limits where high-power radars and victim receivers are cosite located.