This article is from the Low Power Broadcasting FAQ, by Rick Harrison email@example.com with numerous contributions by others.
Most receivers with digital tuning will only lock onto signals that
are on standard broadcast frequencies. In the US, AM stations are at
10 kHz intervals, ranging from 540, 550, 560 ... to 1700, and FM
stations are spaced at 0.2 MHz intervals, ranging from 88.1, 88.3 ...
to 107.9 MHz. (In Europe, AM stations are spaced at 9 kHz intervals.)
Do not use an out-of-band frequency; they are reserved for other
services. (For example, the frequencies just below 88 MHz are used for
TV broadcasts, and the frequencies just above 108 MHz are used for
aircraft navigation and communications.)
Make a survey of the band you are planning to use. Get some graph
paper or notebook paper and make a list of all the channels. Listen
during the day and at night, making a note of what station(s) you can
hear on each channel. Use a good receiver with digital tuning and a
decent antenna, not some cheap piece of junk clock-radio or dime-store
pocket radio. Repeat this band-scanning process several times during
the course of a couple of weeks. (If you really want to be thorough,
get a list of all the licensed stations in a 100-mile radius. You can
get this data from trade publications, or on the web from the
Station Location Page: http://www5.jagunet.com/~kodis/station.html )
Now, sit down with your data and search for an appropriate channel.
An appropriate channel for low power broadcasting is one that is not
occupied by a local station, or by an often-audible distant station.
The "first adjacent" channels -- the next channel above and
the next channel below the one you're considering -- also must not be
occupied by local stations, because they will "splatter" onto
your signal, and your signal will splatter onto theirs. (An
explanation of splatter is coming up later in this document.)
If there is a TV station broadcasting on channel 6 in your area, it is
unwise to operate on 88.1 or 88.3 MHz. TV receivers have broadband
tuning circuits (a TV channel is 6 MHz wide, enough spectrum to hold
30 FM stations), so broadcasts at the low edge of the FM band can
easily interfere with reception of channel 6.