Ramsey Electronics FM100BWT User Manual page 13

METERS
3
6
12
24
48
96
192
384
768
1536
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).
DISTANCE FROM TRANSMITTER ANTENNA
FEET
10
20
39
78
157
315
630
1260
2520
5036
FIELD STRENGTH
(µV)
250
125
63
31
15
7.5
3.8
1.9
.95
.5
FM100B • 13
TOTAL
RECEPTION AREA
314 FT
1256 FT
4800 FT
19113 FT
1.8 ACRES
7.2 ACRES
28.6 ACRES
11.4 ACRES
458 ACRES
1830 ACRES