Ramsey Electronics TV6 Manual page 19

For other kinds of radio services, the FCC restricts such factors as transmitter
power or antenna height, which cannot really limit the possible "range" of a
transmission under good conditions. By restricting the maximum field strength
at a specific distance from your antenna, the FCC clearly plans for your signal
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 in a school
room or home - as long as you do not cause interference 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
discussed your possible signal coverage in terms of square feet or square
meters.
(2) It's easy to see that your signal can easily serve a school or home.
(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 lots of homes, before adding apartments!
In fact, the most significant distance in the above chart is the 3 µV signal
strength permissible at 315 feet, covering a circular area of about 7.2 acres. A
quick check at a TV set's specifications shows maximum sensitivity of about 5
µV before considering high-gain antennas or preamplifiers. Your non-licensed
signal can provide serious competition to a public broadcast 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
Amateur'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 microvolt "delivered" to the receiver input by antenna.
If the antenna is not good, the receiver cannot respond to the presence of
fractions of a microvolt of RF energy.
TV6 • 19