FM35 19
In all cases, the field strength of a signal decreases in direct proportion to the
distance away from the antenna. Power decreases by the square of distance:
for every doubling in distance, the signal power is quartered, but the field
strength voltage is only halved. Using this theory, we can construct a simple
chart to show the maximum permitted performance of a non-licensed FM band
transmitter. The theoretical figures assume a simple 1 meter receiving an-
tenna in all cases and do not take into consideration that reception can be
greatly enhanced with larger, multi-element antennas and preamplifiers. In the
following chart, the field strength (theoretical minimum) gets even stronger as
you move from the edge of these circular boundaries toward the antenna:
This "exercise in meters and microvolts" demonstrates that the FCC clearly
intends to limit the theoretical range of non-licensed devices operating in this
band. It also shows the potential for causing interference at a home down the
street from you. But it also shows that you can legally put out quite a good
signal over wider areas than you might have imagined.
For other kinds of radio services, the FCC restricts such factors as transmitter
power or antenna height, which cannot really limit the possible "range" of a
transmission under good conditions. By restricting the maximum field strength
METERS FEET FIELD
STRENGTH
(µV)
TOTAL
RECEPTION
AREA
3
10 250 314 FT
6
20 125 1256 FT
12
39 63 4800 FT
24
78 31 19113 FT
48
157 15 1.8 ACRES
96
315 7.5 7.2 ACRES
192
630 3.8 28.6 ACRES
384
1260 1.9 114 ACRES
768
2520 .95 458 ACRES
1536
5036 .5 1830 ACRES
DISTANCE FROM TRANSMITTER ANTENNA
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