Appendix B: Understanding Field-Strength; Appendix B: Field Strength - Ramsey Electronics FM10C Manual

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 under-
stand 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.
Previous limits on nonlicensed FM-broadcast band devices were defined as a
maximum field strength of 40µV per meter measured at a distance of 15 me-
ters. The June 1989 revised rule specifies a maximum of 250 µV per meter,
but measured at 3 meters from your antenna. Both limitations are the same in
practice. "250µV per meter" means that an accurate field-strength meter with
a 1-meter antenna may indicate a maximum signal field strength of 250µV (In
contrast, non-licensed operation from 26.96 to 27.28 MHz is limited to a field
strength of 10,000 µV per meter at 3 meters).
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.
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
at a specific distance from your antenna, the FCC clearly plans for your signal
to "die out" at a specific distance from your antenna, no matter what kind of
transmitter power or extra-gain antenna you are using. On the other hand, the
FCC standards do make it legal and possible for you to broadcast on a
school campus, campground or local neighborhood, as long as you do not
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