Ramsey Electronics FM25A Instruction Manual download pdf (Page 24)

FM25A • 24

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.

• b. FCC Rule 15.215(a) says: "Unless otherwise stated, there are no restric-

tions as to the types of operations permitted under these sections." This

general provision appears to leave you free to use your FM stereo transmit-

ter in a manner similar to operations of an FM broadcasting station, or to

use it for any other non-interfering, practical application.

• c. FCC Rule 15.5: General conditions of operation: "(b) Operation...is sub-

ject to the conditions that no harmful interference is caused and that inter-

ference must be accepted that may be caused by the operation of an au-

thorized radio station, by another intentional or unintentional radiator, by

industrial, scientific and medical equipment, or by an incidental radiator. (c)

The operator of a radio frequency device shall be required to cease operat-

ing the device upon notification by a Commission representative that the

device is causing harmful interference."

• 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 equipment.

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 trans-

mitted signal. Since it is unlikely that you have the equipment to carry out accu-

rate 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 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 prac-

tice. "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 con-

trast, 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

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Ramsey Electronics FM25A Instruction Manual Page 24