Ramsey Electronics TV6 Manual page 18

APPENDIX B: UNDERSTANDING LEGAL "FIELD STRENGTH"
A "microvolt" is one-millionth of one volt and designated "µV" in the following
explanations.
The new FCC Part 15 Rules specify a maximum "Field Strength" of your
transmitted signal. Since it is unlikely that you have the equipment to carry out
accurate field strength measurements in microvolts, it is useful to understand at
least the theory of field strength so that you can understand both what you can
expect from such transmitters, and what limits the FCC intends.
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 antenna
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.
DISTANCE FROM TRANSMITTER ANTENNA
METERS
3
6
12
24
48
96
FEET
10
20
39
78
157
315
TV6 • 18
FIELD
STRENGTH
(µV)
100
50
25
12
6
3
TOTAL
RECEPTION
AREA
314 FT
1256 FT
4800 FT
19113 FT
1.8 ACRES
7.2 ACRES