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FM30
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space for several different FM stations, this is a "generous" limitation de-
signed to accommodate cruder FM devices. Properly built and adjusted,
the FM30 kit operates well within this limit. In fact, its signal should sound
no "wider" than any other FM station when listening on an ordinary FM
radio.
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b. FCC Rule 15.215(a) says: "Unless otherwise stated, there are no re-
strictions as to the types of operations permitted under these sections."
This general provision appears to leave you free to use your FM stereo
transmitter in a manner similar to operations of an FM broadcasting sta-
tion, or to use it for any other non-interfering, practical application.
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c. FCC Rule 15.5: General conditions of operation: "(b) Operation...is
subject to the conditions that no harmful interference is caused and that
interference must be accepted that may be caused by the operation of an
authorized radio station, by another intentional or unintentional radiator,
by industrial, scientific and medical equipment, or by an incidental radia-
tor. (c) The operator of a radio frequency device shall be required to
cease operating the device upon notification by a Commission representa-
tive that the device is causing harmful interference."
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d. The most specific FCC regulation of 88-108 MHz FM Broadcast band
unlicensed operation is that the "field strength" of the signal must not ex-
ceed 250 microvolts/meter at a distance of 3 meters from the transmitter
(FCC rule 15.239). If you have any concern about this emission limit,
have your device checked by a technician with accurate measuring equip-
ment. Remember that the "field strength" of a signal is determined as
much by the antenna as by the RF output of the transmitter itself.
APPENDIX B: UNDERSTANDING LEGAL "FIELD STRENGTH"
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 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).
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