No matter how carefully designed for purely electrical functions, an aerial may be, the performance, both initially and continuing throughout the life of the equipment, is almost entirely dependant upon the mechanical structure. Just how nearly the predicted performance is achieved is determined by the precision of manufacture and by the electrical properties of the constructional materials used. Consistency, accuracy and reliability of performance depend on the stability of the materials used under conditions of stress, such as those due to adverse weather conditions and in the ability to withstand ageing and weathering without either physical deformation or changes in electrical properties. In addition, power losses due to absorption in the portions of the supporting structure in the field of the aerials must be held to a minimum. This precludes the use of a metallic structure, which would not only absorb some energy but would also alter the radiation resistance, rendering impedance matching difficult and still further reducing the usefully radiated power.
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