FM25B
•
29
to "die out" at a specific distance from your antenna, no matter what kind of
transmitter power or extra-gain antenna you are using. On the other hand, the
FCC standards do make it legal and possible for you to broadcast on a school
campus, campground or local neighborhood, as long as you do not cause in-
terference to broadcast reception.
“Why talk about acres"?
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
meters.
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 preampli-
fiers. Your non-licensed signal can provide serious competition to a public
broadcast station fifty miles away, a station which someone in your neighbor-
hood 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 an-
tenna is not good, the receiver cannot respond to the presence of fractions of
a microvolt of RF energy.
