a must be taken into account in the design. Mobile antennas on vehicles, ships, and aircraft, or near humans present challenging problems to the antenna designer. In most cases the physical, mechanical, and environmen- tal aspects take precedence over the electromagnetic design. In consequence the ingenuity of the antenna designer is required to produce an antenna that works well in adverse condi- tions. For example, antennas on aircraft must not disturb the aerodynamic profile, SO they cannot protrude from the aircraft body. The effects of corrosion, temperature, pressure, vibration, and weather are other factors to be taken into account. Antennas for personal radios are constrained by the role of the operator and by the need for very compact designs commensurate with satisfying radiation safety levels. The human body acts partly as a director and partly as a reflector depending on the frequency of use and the relative position of the antenna to the body. Portable radio equipment has to be considered a part of the antenna system, including the radio circuits, batteries, and case. In general, improved per- formance results when the antenna is held as far from the body as possible and as high as possible. HF Communications HF antennas are used in the range of frequencies from 2 to 30 MHz for mobile communica- tions and some fixed communications. Space precludes more than a brief mention of HF antenna types. Surveys can be found in Rudge (1986) and Johnson and Jasik (1984). HF antenna design is constrained by ionospheric propagation characteristics, which change daily, seasonally, and with the sunspot cycle. Antennas can receive either the sky wave reflected from the ionosphere or the ground wave if the 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; this in turn means that the local environment around the antenna will have a major impact on performance, which is particularly true for antennas mounted on vehicles, 