HF communications

HF antennas are used in the range of frequencies from 2 to 30MHz for mobile communications and some fixed communications. Space precludes more than a brief mention of the types of HF antennas. Surveys can be found in (Rudge, 1986; Johnson, 1984). HF antenna design is constrained by the ionospheric propagation characteristics which change both daily, seasonally and with the sun-spot cycle. Antennas can receive either the sky wave reflected from the ionos­phere or the ground wave if transmitter and receiver are close together. The wavelength in the HF band is such that antennas are usually only a fraction of a wavelength in size andthis in turn means that the local environment around the antenna will have a major impact upon the performance. This is particularly true for antennas mounted on vehicles, ships and aircraft. The analysis of the antenna must take account of the environment by techniques such as wire grid modelling (Mittra, 1975). This is computer intensive and ine­vitably approximate which means that much HF antenna design is empirical.

Most HF antennas are based on dipoles, monopoles or wire antennas. Complementary elements such as loops or slot antennas are also used. Directionality or gain is achieved by arrays of ele­ments. A prime requirement of most HF antennas is that they are broadband in order that the optimum propagation frequency can be used. The radiation patterns of the basic elements are wide band but the input impedance or VSWR is narrow band. To overcome this limitation a tuning unit has to be incorporated into the system. The wide band operation is achieved with automatic tuning units.