British Kinematography (1951)

120 Vol. 18, No. 4 BUSINESS RADIO IN FILM PRODUCTION Howard M. Layton, Assoc Brit. I.R.E. IN recent years the film industry has made use of various types of radio-telephone apparatus to assist in the communication requirements of film production. The " Walkie-Talkie " type of equipment, famous for its war-time applications, has proved very useful on widely dispersed film location sites, and for co-ordinating " actions " in which large numbers of artistes have been involved. More recently, the rather longer range mobile equipment known as " Business Radio " has received some attention, and since communications with this latter may be quite reliable over distances of the order oi 20 or 30 miles, its scope within the industry is worthy of investigation. Indeed, one Business Radio installation has already been used as a means of passing instructions between a Studio Production Office and its Location Unit operating in various parts of London. It is intended, therefore, that the present paper should be regarded as a general introduction to Business Radio technique so far as it may concern the film industry. Reasons for V.H.F. Radio-telephony in business is licensed for operation on spot frequencies in the 70-100 Mc 's and 156-184 Mc 's bands. Frequencies of the order of 460 Mc rs and higher are also available, but as far as the writer is aware, mobile equipment suitable for these latter frequencies has not yet been marketed. All of these frequencies may be classified as V.H.F. , and in order to gain a full appreciation of the scope and limitations of available apparatus, a brief examination of some of the more important propagation characteristics of V.H.F. 's will perhaps be helpful. Long distance radio communication relies on the properties of those layers above the earth's atmosphere which form the ionosphere. When a wave-front meets this * [red Imhof, Limited ionised region, refraction or " bending takes place, and at medium carrier frequencies this bending is usually sufficient to turn the wave back towards the earth again. By further reflection from the earth's surface, a number of such " hops " is possible (Fig. 1), and in this way a signal may cover several thousands of miles before it is finally dissipated. The phenomenon known as ionospheric propagation is responsible for virtually all long range radio communication. At frequencies in excess of about 10 Mc/s, however, the angle at which a wave front is incident at the ionosphere determines the degree of refraction to which it is subjected, and for any particular frequency /, there is a critical value 9 of this angle, at which refraction becomes insufficient to turn the wave back to earth. At angles of incidence greater than 9, the wave, therefore, penetrates the ionospheric layers, and is lost in outer space. As / is increased, the critical angle 9 decreases, until at frequencies between 50 and 60 Mc/s, the effects of ionospheric propagation become small, and for the purposes of this discussion may be neglected altogether. Avoidance of Interference Neglecting tropospheric effects, radio communication above about 60 Mc/s is therefore restricted to the direct ray track, which seldom penetrates far beyond the transmitting aerial's horizon. This horizon, interpreted for ground-to-ground communication over flat countrv, would lead to an expectation of range of the order of 20 to 35 miles, according to aerial height. Thus, in contrast with the ranges of hundreds of miles which are obtainable at medium frequencies, it is customary to think of V.H.F. ranges in terms of only tens of miles. As the number of mobile radio svstems in operation is already large and is increasing almost daily, it would not be difficult to (Radio Telephone Diw).