British Kinematography (1951)

April, 1951 layton: business radio 123 afforded by such a lofty site can be enjoyed with remote control at very little capital outlay. Installation in London Area The Imhof installation at Hampstead Heath is an excellent example of the remote control technique. The fixed station is located in a small room at an inn on the top of the Heath and its aerial (100 ft. pole) stands behind the building. Telephone lines are used to link the equipment with the controlling office at New Oxford Street, and at this latter address a Radio and Television Service Manager uses the system to control his fleet of service vehicles. The average radius of coverage achieved is of the order of 35 to 40 miles, and in many directions is well in excess of this figure. Contrast this with the average range of 15 miles afforded by the previous local installation at New Oxford Street itself. This latter made use of a 60 ft. mast erected on the company's building, which is about 100 ft. high. Ground level at New Oxford Street is about 100 ft. above sea level, so that the local aerial was 250 ft. above sea level. Ground level at the inn is 440 ft. above sea level, so that the existing 100 ft. mast provides an aerial height of 540 ft. above sea level. The wide discrepancy in the results obtained with the two aerials is therefore not surprising. Single or Dual Frequency Single frequency working, that is to say, a system in which transmitter and receiver use the same frequency, does not in general appear to be popular with the licensing authorities. This method of operation permits of direct communication between one mobile unit and another and thus robs the master station operator of the power to discipline the use of the channel. If an organ'sation operating a fleet of, say, 20 vehicles used this system, and the drivers of half the vehicles attempted to hold conversation with the other half at the same time, the resulting mutual interference would be likely to render the received signals quite unintelligible. In such circumstances, the " master " station would be powerless to justify its title. Dual frequency working, which is orderly in its nature, is almost universally employed. All mobile units transmit at a frequency X and receive at a frequency Y, whilst the fixed station uses these frequencies in reverse. Thus a transmission from any one mobile will be received only by the master receiver, and if a number of mobiles call at the same time, all can be instructed to stand by for permission to pass their messages in turn. Communication between vehicles is then achieved only by relaying messages through the master station. It follows, therefore, that in a dual frequency system only one transmission can take place at a time. Each such transmission is originated or received by the master operator, who is aware of the nature of all communications and is in a position to control and record them. A further important fact in favour of dual frequency operation is that with this system it is possible to repeat frequencies in areas which lie much closer together than would be possible in single frequency schemes. In consequence, more licences can be issued for any given area. Simplex and Duplex Working Duplex operation, which does away with the need for a transmit-receive key, enables the equipment to be used exactly as if it were an ordinary wired telephone. Used in conjunction with vehicles, however, duplex working presents certain problems. The transmitter and receiver require separate aerials, and as it is difficult to provide adequate spacing for these, a need for specially designed R.F. systems arises and introduces complications where they are most unwelcome. Furthermore, as both transmitter a~d receiver are operating together, demands on the battery supply tend to be heavy. For the above reasons duplex operation, where required, is normally confined to the fixed station, and for mobile work the simplex technique is more popular. A sendreceive key mutes the receiver whilst the transmitter is in use and also operates a relay which switches a single aerial as required.