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Aerial
There is very little possibility of amplification in the horizontal plane with
the frequency used. Aerials with amplification arrangements are bulky and
expensive.
Height of Aerial
The wave propagation at 29 Mc/s within short ranges is practically linear,
i.e. in principle one can attain the maximal visual range provided that the receiver
sensitivity and transmitter efficiency are sufficient. The visual range is
y' 2 • rh, where h = height of aerial and r = radius of the earth. Doubling of
the height of the aerial increases the visual range 1.4 times (40 % ). The aerial
should be placed as high as possible. On economic grounds one should make
use of existing buildings, masts or the like.
Power Output
One means of obtaining a larger range, better coverage within a desired
area, and stronger signal in shadow areas, is an increase of the aerial power.
The normal power output is 5 W. On an increase to 50 W the field strength
will be yTO, i.e. rather more than three times as great. A further increase of
the power involves unduly large and expensive transmitters. A power level of
50 W has therefore been found to be suitable for coverage of large areas.
Receiver Sensitivity
The ERIC ALL receiver needs a field strength of at least 50 jxVJm in order
to function. By way of comparison a car radio receiver needs a field strength
of about 1 |uV/m. The sensitivity of the receiver was selected on the basis of
price and dimensions.
Terrain
The terrain affects the propagation of radio waves to a high degree and the
range therefore varies from case to case. Over wet ground or water the range
is greater than over dry ground. Elevations in the terrain cause radio shadows.
Summary
As appears from the foregoing account it is difficult to give general figures
of range, but in practice it has been found that the following approximate
ranges can be reckoned on (out-of-doors at street level):
In area of In area of
Power Aerial multi-storey low
buildings buildings
5 W 30 m 1.5 km 3.5 km
50 W 30 m 6.0 km 12.0 km
Concluding Remarks
Among the advantages of radio alarm compared with the other systems
mentioned, the foremost is the lower cost for municipal fire brigades. Then
comes the fact that the individual member of a fire brigade can move freely
within the duty area, whereas earlier he was forced to remain within hearing
distance of a telephone or alarm bell. Experience from Oxelosund and the
ten other places which today use the ERICALL system has shown, too, that
the personnel are very much in favour of the method.