2.3 ECCM Design Considerations

One of the reasons for renewed interest in HF communications, particularly by the military services, is their use as a backup system for satellite communications. To provide a measure of privacy, secrecy, and antijam capability, several schemes for spectrum spreading the signal have been proposed. Two basic types of spread-spectrum signals have gained wide acceptance. One of these involves using a pseudonoise (PN) code operating at high speed to switch the carrier phase at rates typically in the megahertz range. This produces a white-noise-like signal occupying several megahertz of bandwidth. Because of the wide bandwidth, the power density of this direct-spread signal is quite low, creating a minimum amount of interference to conventional narrowband signals in the band.

The second type of spread-spectrum signal in wide use utilizes frequency hopping over the band to spread the spectrum. The frequency hopping rate may vary from a few per second to thousands per second. Where the RF band occupancy is high, such as in the HF band, this is often the preferred ECCM method, because by proper choice of frequencies, strong fixed-channel stations in the band and interference to friendly signals can be avoided. A combination of the two methods, using a limited amount of direct-sequence band spreading along with frequency hopping, may also be employed.

Since the subject of this book is SSB systems and circuits, a comprehensive discussion of ECCM communication is beyond its scope, but those aspects that touch upon the design of SSB radio...