Harmonics and spurious emissions.

What is it?
These are unwanted frequencies that may be present on the output of a transmitter.
Harmonics are multiples of the main frequency.

What can suffer from interference from these signals?
Only other receivers, such as radio and television, can get interference due to these signals.

Tip. Modifications to CB radios, such as increasing the power or channels, can cause them to put out harmonics, which will cause interference. To spot such problems requires expensive specialist equipment.

Who's fault is it?
This is usually caused by a problem with the transmitter, so is usually a fault with the CB equipment.
Legal UK radios are tested for harmonics and spurious emissions, so an unmodified legal UK CB radio should not have this problem.

What causes it?
Because of the way the frequencies are generated in a CB radio (frequencies are mixed and multiplied), plenty of different frequencies are generated within the radio. Some of these signals can find their way to the output of the transmitter.
Also, for efficiency, the output devices of FM CBs are biased at a level where the signal will be distorted - creating harmonics.
As well as the radio itself, other parts of the CB system may introduce harmonics.

What should I do about it?

Check all your antenna connections:
Poor connections can act as diodes, so can create harmonics. This includes plugs, sockets, and connections between any sections of your antenna.

Tip. Interference to other equipment is the usual reason for a visit from enforcement agencies. Even if you are not to blame for interference, you would get busted for any illegal equipment found.

Do not leave your SWR meter in line:
As the components in the SWR meter can produce harmonics, it should only be connected while testing the antenna.

Modifications:
If your radio has been modified, particularly for extra channels or extra output, have it checked out to see if it is the problem. An unmodified, legal CB radio should not give out harmonics at a high enough level to cause problems.

Low Pass Filter:
If all else fails, a filter in the CB coax could be tried. Personally, I am not in favour of these. They are often cheaply designed and made, and may offer little filtering, while possibly allowing some signal to escape - with the risk of more problems.
At best, they just mask the problem - an unmodified, correctly working system should not have harmonic problems, so the cause should be found.

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