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April, 1951
layton: business radio
121
imagine what would happen if the long range frequency bands were used. The allocation of different spot frequencies for each user would be quite out of the question, since the most generous allocation of " space " in the frequency spectrum would only accommodate a fraction of the numbers involved. Excessive mutual interference between services in widely separated areas would therefore be inevitable, and as a result the usefulness of this form of communication would be severely restricted.
countered en route, it is possible to assess its chances of reaching any particular distant receiver.
Assuming that the fixed station aerial has been erected on the roof of a block of offices, or, for that matter, on a film studio roof, all that can be seen on a perfectly clear day from this position, the signal may be expected to reach. Further, since the earth's curvature tends to curve the path of the signal too, the visible horizon may usually be extended by about 20% when interpreting V.H.F.
DECREASING REFRACTIVE INDEX IN IONISED RETGICN CAUSES BENDING AWAV FROM NOSfMAL. AND HENCE COMPLETE CURVE ROUNO OF WAVE".
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EQUIVALENT REFLECTION POINT
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REFLECTION FROM EARTH OF M/p WAVE. — ' ~~
HoRizoM^ normal, limit of range
FOR V.M.FS
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ILLUSTRATING REFRACTION OP M/P WAVE BY THE IONOSPHERE AND HOW GREAT RANGES ARE THEREBY" ACHfEVED
The use of very high frequencies automatically suggests itself, for with these, interference problems are much more localised, and by judicious allocation of spot frequencies and intelligent aerial siting, most such problems become soluble. The need for V.H.F. 's will now be clear.
Results in Built-up Areas
Having established that for business radio purposes the transmitted signal must follow a substantially direct path, and that its fate will therefore depend on earthly objects en
range. As the height of this roof-top aerial is increased, the horizon begins to recede, and range is increased accordingly until a limit is reached at which the wave is finally dissipated in the myriad of nature's everabsorbing elements.
In general, buildings in the immediate vicinity of the mobile receiving aerial need not be regarded as serious obstacles because they reflect and scatter incident waves in much the same way as wet surfaces reflect and scatter light. Thus, the mobile counterpart of the studio installation, moving be