This article is from the Low Power Broadcasting FAQ, by Rick Harrison firstname.lastname@example.org with numerous contributions by others.
No transmitter puts out a pure signal. In addition to the main
signal, there will be harmonics and spurs. These impurities really
can interfere with other signals. So please, do yourself and your
colleagues a favor and make your signal as clean as possible!
Harmonics are found at multiples of the main frequency. For example,
an FM transmitter tuned to 100.1 MHz will also be emitting weaker
signals at 200.2 MHz, 300.3 MHz, and so forth.
Spurs (called "sprogs" in Britain) occur at unpredictable frequencies.
For example, if your transmitter has PLL tuning and uses a 4 MHz
crystal reference oscillator in the circuit, there might be some
mixing of signals in the circuit and you might find a spur at 4 MHz
above and/or below your main frequency. Spurs are especially
dangerous because it is hard to predict what frequency they will be on
or how powerful they will be. The only way to "see" them is with an
expensive piece of test equipment called a spectrum analyzer. (In
major cities, you may be able to rent a spectrum analyzer, but it
might be better to spend the money on a filter; see below. You can
see spectrum analyzer displays of some low power FM transmitters'
spurs and harmonics on the DSchmidt Technologies web pages.)
There is no way to completely eliminate spurs and harmonics; they are
a fact of life in radio-frequency circuits. Broadcasters have an
obligation to reduce the intensity of these "spectral impurities" so
that they do not have any effect on the rest of the world.
One step you can take is to install an external lowpass filter or
bandpass filter between your transmitter and antenna. If you're
transmitting with more than half a watt of power, you should do this.
A lowpass filter allows signals below a certain frequency to pass
through, but frequencies above that cutoff point are reduced in
intensity; the amount of reduction increases as the frequency gets
farther from the cutoff point. A bandpass filter reduces the
intensity of signals above and below its frequency range.
Splatter is another form of interference. When you try to tune in a
station and you hear some hissing and harsh sputtery noises from
another station on a near-by frequency, that's splatter. Splatter has
a variety of causes including excessively high level of audio fed to
the transmitter (over-modulation) and poor choice of operating
Splatter is also more of a problem in areas close to one of the
transmitters, where that rig's signal is much stronger than the one
being splattered on (the splatter-ee). This is why it's not a good
idea to operate a pirate station with more than 1 or 2 watts of power
in the middle of a densely populated neighborhood. Many of the
complaints that caused the authorities to attack free radio stations
in 1997 were inspired by splatter onto second adjacent channels. For
example, an unlicensed station on 99.1 in Tampa, Florida splattered
onto a licensed station on 99.5 MHz. Several people in the
neighborhood of the unlicensed station's transmitter complained when
they were unable to hear coverage of a football game on 99.5 one
[The web version of this FAQ also discusses images, co-channel
interference, and RF feedback in audio gear.]