Receiving aerials for British television

摘要

The subject is treated under two headings, dealing separately with electrical and mechanical requirements.The electrical treatment includes references to aerial analyses made during the last thirty years. When an experimental investigation is undertaken it is apparent that the effect of the proximity of feeder and earth and the difficulties in obtaining reliable measurements make it impossible to check any of these theories to an accuracy sufficient to prove which is most correct. Measurements suggest that a dipole constructed from fine-gauge wire possesses a loaded bandwidth in excess of that required for full video-frequency resolution from the present British system. The properties of two- and four-element parasitic arrays are discussed, and it is found that bandwidth measurements on these do not confirm the narrowing predicted by theory. This may be due to the compromise design necessitated by the dual requirements of vision and sound. Simple apparatus to assist the development of small scale models is described. Travelling-wave directional aerials are discussed, and some space is devoted to loft and indoor aerials. Siting problems are given prominence, particularly when delayed images due to multipath reflections are considered, and the difficult problem of reception in a valley, where the reflected image may be stronger than the original, is discussed. It is shown that delayed images (“ghosts”) due to feeder mismatching are extremely rare.The mechanical treatment emphasizes important requirements such as strength, resistance to atmospheric corrosion, and the precautions to be taken in the selection of materials. The phenomenon of acoustic “humming” generated under certain conditions of wind velocity is treated, and an effective cure is described.The amount of apparatus, and the extensive experimental facilities required for systematic aerial development, may seem to be excessive having regard to the final simplicity of the aerials. The reason for this is stated and minimum requirements are tabulated.The main conclusions' suggest the need for a more certain appreciation of the technique of aerial measurements, and for some means of overcoming the serious multipath-reflection problems peculiar to hilly and mountainous terrain, which are serious although such localities may lie within a short distance of the transmitter.