ships, and aircraft. The analysis of the antenna must take account of the envi- ronment by techniques such as wire grid modeling (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 wires. 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 SO that the optimum propagation frequency can be used. The radiation patterns of the basic elements are wideband, but the input impedance or VSWR is narrowband. To overcome this limita- tion, a tuning unit has to be incorporated into the system. Wideband operation is achieved with automatic tuning units. References References Balanis, C. A. (1982) Antenna Theory: Analysis and Design, Harper & Row. Bennett, S. B., and Braverman, D. J. (1984) INTELSAT IV-A Continuing Evolution, Proceedings IEEE 72:1457. Bergman. J., Brown, R. C., Clarricoats, P. J. B., and Zhou, H. (1988) Synthesis of Shaped Beam Reflector Antenna Patterns, Proceeding, IEE 135(H)(1):48. Bjontagaard, G., and Pettersen, T. (1983) An Offset Dual Reflector Antenna Shaped from Near-Field Measurements of the Feed Horn; Theoretical Calculations and Measurements, IEEE Trans. 31:973. Cha, A. G. (1983) An Offset Dual Shaped Reflector with 84.5 Percent Efficiency, IEEE Trans. AP-31: Clarricoats, P. J. B., and Olver, A. D. (1984) Corrugated Horns for Microwave Antennas, Peter Peregrinus (IEE). Clarricoats, P. J. B., Tun, S. M., and Brown, R. C. (1984) Performance of Offset Reflector Antennas with Array Feeds, Proceedings IEE 131(H)(1):172. Clarricoats, P. J. B., Tun, S. M., and Parini, C. G. (1984) Effects of Mutual Coupling in Conical Horn Arrays, Proceedings IEE 131(H)(1):165. Collin, R. E., and Zucher, F. J. (1969) Antenna Theory, Parts I & II, McGraw-Hill. Elliot, R. S. (1981) Antenna Theory and Design, Prentice-Hall. Galindo, V. (1964) Design of Dual Refl