Ramsey Electronics FM25A Assembly And Instruction Manual - page 26
FM25A
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There are three reasons to translate our look at "field strength" into "acres".
1. The first one is easy: the numbers would get too cumbersome if we dis-
cussed your possible signal coverage in terms of square feet or square me-
ters.
2. It's very easy to see that your signal can easily and legally serve a school
campus or wilderness campground.
3. And, if we remember that typical urban single-family home sites run from
1/4 to 1/2 acre on the average, it should become extremely clear that your
obligation to avoid interfering with broadcast reception can easily involve
hundreds of homes, before adding apartments!
In fact, the most significant distance in the above chart is the 1.9 µV signal
strength permissible at 1260 feet (about 1/4 mile), covering a circular area of
about 114 acres. A quick glance at stereo FM receiver specifications shows
typical sensitivity of 1.7 µV before considering high-gain antennas or preamplifi-
ers. Your non-licensed signal can provide serious competition to a public broad-
cast station fifty miles away, a station which someone in your neighborhood
may have set up a special antenna to enjoy.
Calibrated "field strength meters" such as described in the ARRL Radio Ama-
teur's Handbook can detect signals down to about 100 microvolts. To measure
RF field strength below such a level, professional or laboratory equipment and
sensitive receivers are required. A "sensitive" receiver responds to a signal of 1
or even .5 microvolts "delivered" to the receiver input by antenna. If the antenna
is not good, the receiver cannot respond to the presence of fractions of a micro-
volt of RF energy.
SUMMARY
The present edition of Part 15 of the FCC rules does not provide detailed guid-
ance on ALL aspects of using a low-power transmitter such as the FM25A. The
main point is that you may not cause any interference whatsoever to licensed
broadcast services and that you must be willing to put up with any interference
that you may experience.
In addition to operations not requiring authorization, you also have the option of
writing a clear and polite letter to the FCC Engineer-in-Charge of your local dis-
trict, describing your intended operation. Mention the operating frequency and
planned hours of operation. This could be a good step to take if your project is
in behalf of a school, Scout or community group.
If you become further fascinated with the service rendered by low-power
broadcasting, other FCC regulations explain how to apply for a license or other
authorization which may permit you to upgrade your FM25A or other equipment