e impedance or from helixes. Antennas for mobile communications can be divided into those for base stations and those for the mobiles. Base station antennas are mounted on towers and usually require to have nearly uniform patterns in the horizontal plane with shaping in the vertical plane to conserve power. This can be achieved with a vertical array of vertical dipoles or other panelled dipoles. The influence of the tower on the antenna must be taken into account in the design. Mobile antennas on vehicles, ships, aircraft or near humans pres- ent challenging problems to the antenna designer. In most cases the Reflector Driven Directors physical, mechanical and environmental aspects take precedence over the electromagnetic design. In consequence the ingenuity of the antenna designer is required to produce an antenna which works well in adverse conditions. For instance antennas on aircraft must Figure 17.19 A Yagi-Uda array not disturb the aerodynamic profile SO cannot protrude from the body of the aircraft. The effects of corrosion, temperature, pressure, References 17/17 vibration and weather are other factors to be taken into account. Clarricoats, P.J.B., Tun, S.M. and Parini, C.G. (1984b) Effects of Antennas for personal radios are constrained by the role of the Mutual Coupling in Conical Horn Arrays, Proc. IEE, 131 (H), operator and by the need for very compact designs commensurate (1), 165. with satisfying radiation safety levels. The human body acts partly Clarricoats, P.J.B., Tun, S.M. and Brown, R.C. (1984c) Perfor- as a director and partly as a reflector depending on the frequency of mance of Offset Reflector Antennas with Array Feeds, Proc use and the relative position of the antenna to the body. The portable IEE, 131 (H), (1) 172. radio equipment has to be considered a part of the antenna system Collin, R.E. and Zucher F.J. (1969) Antenna Theory (Pts 1 & 11), including the radio circuits, batteries and case. In general, improved McGraw Hill, New York. performance will result when the antenna is held as 