FM-100 Page 59
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 non-licensed 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).
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
11.4 ACRES
768
2520
.95
458 ACRES
1536
5036
.5
1830 ACRES
DISTANCE FROM TRANSMITTER ANTENNA
