Television Aerials and Systems

Abstract

As we have seen, the bandspread of one TV channel can be 8 MHz or more, depending on national standards. In order to accommodate a number of channels of such large bandwidth, it is necessary to use high carrier frequencies, in both the v.h.f. and u.h.f. regions; microwave transmissions may be adopted in the future. The propagation of electromagnetic waves is very severely affected by the order of their frequency. The higher the frequency the more pronounced is the ‘line of sight’ distribution of the signal. That is, at the higher frequencies the receiver installation ought to ‘see’ the transmitting aerial. The process of reflecting the signals at frequencies above 40 MHz from the upper layers of the atmosphere is no longer reliable for normal communication purposes. It will also be appreciated that the signal strength will be considerably reduced as the reception area becomes more distant. In countries where the population density is low it is necessary for the transmitter to cover a large land area; in these cases v.h.f. transmission is particularly useful, because it provides increased coverage. In the UK, with its large population centres, often only 40 miles apart, u.h.f. transmissions offer better facilities in allocating the available channels without adversely affecting the reception in the neighbouring areas. Additionally, since very little man-made interference is generated at u.h.f. frequencies, transmissions in this frequency band result in better quality reception.