Ramsey Electronics FM10A - Manual (Page 22)

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.
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
meters. 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