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122
BRITISH KINEMATOGRAPHY
Vol. 18, No. 4
"tween tall buildings which shield it completely from the fixed station, will normally receive signals without difficulty.
Naturally, a signal reflected from the wall of a building must suffer some degree of attenuation, and for this reason very thickly built-up areas are not always negotiated without some difficulty.
This is especially true when such areas are situated near the natural horizon of the fixed station aerial, for in these circumstances the wave front arrives at a very low angle, and may have to force its way through a considerable " depth " of man-made obstructions before reaching the mobile unit. Sporadic fading and the occurrence of
Where the headquarters of a business radio installation is situated in low country or in a valley, the range restrictions which it would normally suffer can be overcome through the use of a very high aerial mast, or by what is known as " remote control." The latter, which is by far the more practical and cheaper expedient, may be explained as follows : —
A neighbouring hill (up to 10 or even 15 miles away), which from a business radio standpoint would normally be regarded as a liability, is turned to good use as a location for the fixed station aerial. It is usually possible to find some building on such a hill where an attic or cupboard may be rented
THE CONTROLLING OFFICE AT A IS BADLT SITUATED ro& V.H.F COMMUNICATION PJRPOSES THE ACTUAL STATION AND AERIAL M---.ST ARE THEREFORE SITED FlQ .2. AT *B AND CONTROLLED FROM 'a OVER CPO TELEPHONE
LINES
TWiS DRAWING, ALTHOUGH EXAGGERATED, WILL SERVE
To ILLUSTRATE THE VALUE OF REMOTE CONTROL TECHNIQUE
" blind spots " are effects which are thereby commonly experienced.
Remote Control
Large obstacles, such as hills, are by no means readily negotiated, and communication is likely to be completely curtailed when a hill interrupts the direct path between transmitting and receiving aerials. Indeed, if the fixed station is located in a valley, completely encircled by ground higher than the aerial mast, communication will in general be restricted to that valley. Clearly then, aerial height is one of the most important single factors in determining the range that a given installation will provide,
for the master set. The aerial, carried on a pole of moderate height, is then erected on the roof or chimney of these premises, and the complete station is linked with the distant control point over a pair of Post Office lines. Needless to say, the equipment is left untended and need be visited only at rare intervals for inspection and maintenance purposes. If the hill is 300 ft. higher than the surrounding country, and the aerial is 50 ft. above ground (20 ft. pole on a house chimney), a total height of 350 ft. is achieved. The prospect of erecting a mast of comparable effective height on low ground would be unthinkable for most ordinary installations, yet the considerable advantages