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Radio Interference (Deliberate)

01 January 1970

Radio Interference.
Radios by their nature can pick up other radios on the same frequency, radios not part of your group. They can also pick up environmental interference from any other of the many electrical devices that surround us.

There are a number of ways that we configure your radios to counter interference so that there is none of significance. However it will always be possible for deliberate interference to take place should someone wish to do so.

A brief understanding of how potential interference can take place will help you avoid most problems.

Channels
Radios are designed very carefully to only pick up signals on one channel and to reject even hugely powerful signals on adjacent or other channels. For interference to take place the interference must the on the exact frequency you are using.

Range
Interference can only potentially occur from radios that are within range of your radios, so if you know over how far your radios will work you will then know that interference can practically only come from within this area.

Channel Sharing
Because there are only a limited number of radio channels available all radio channel are shared worldwide. However since the sharing user will mostly likely be out of range then interference is not normally a problem.

Licence-Free and Licensed radios are all shared. With some Ofcom licences you can specify the area you will be using the radios and they will assign you a frequency which will not be issued to another radio user in you area. This of course means you cannot move your radios to another area without potentially causing interference (and contravening your licence).

With licence-free radios other techniques are used to prevent interference and these are outlined below.

Capture effect.
All land based and marine radios use Frequency Modulation (FM). One of the big advantages of FM is that a receiving radio will only pick up the strongest signal. The strongest signal will almost always come from your own radios because they will be nearer than the interfering radio. So in practical terms your radios will normally always be able to communicate over interference.

Capture effect does not prevent you hearing interference but it will allow you to transmit over distant interference.

CTCSS.
If your radios only had a simple FM receiver you would hear all and any interference in range, deliberate or otherwise regardless. CTCSS (explained here) is a system fitted to every radio we have ever sold and this keeps your radios silent to interference provided the interfering source is not using exactly the same CTCSS code.

In practical terms CTCSS will keep your radios silent to virtually all interference from other users and environmental.

Deliberate Interference.
The one type of interference that cannot be easily countered by the design of the radio is deliberate interference.

For deliberate interference the rogue user will need to do a number of things to cause a problem.

First they must be in range which if they are using a conventional hand held radio means within your normal radio coverage area and they must have a radio capable of transmitting on your channel and CTCSS code.

Next they must know your radio channel. This can be found fairly easily if you are using as licence-free radio as there are only 8 channels to choose from. With licensed radio a fairly low cost radio scanner will enable them to find your channel.

Next they must find your CTCSS code. This is much harder to do as there are over 100 to choose from. There are some radios and radio scanners that will decode the CTCSS tone for them but these are less commonly available.

In most cases Channels and CTCSS will be found by accident by someone playing with the settings of their radio rather than a methodical approach outlined above.

What the interfering party does next is when your problems can start.

Handling Interference.
If you hear accidental or deliberate interference the best thing to do is to quietly change channel.

Most deliberate interference is casual if very annoying. If the interference is deliberate don't announce a channel change over the radio (unless you have some pre-arranged code for doing this) as this will probably encourage the interferer to search for you on another channel.

It is important to avoid all dialogue with the interferer as well because usually a reaction is exactly what the interferer is trying to prompt. If you show no sign you have heard them invariably they will get bored and give up creating interference.

Reporting Interference
Getting help from the authorities will be difficult as tracking down radio interference needs a lot of resources. If a crime has not been committed it is unlikely much effort will be made. If interference is of an abusive or persistent in nature try to make a recording of as much of it as you can and a log of times that the problems occur.

If you do have an OFCOM licence contact your radio supplier and OFCOM for help.

Further prevention measurer.
Changing the radio system to one with encryption is the only practical option to prevent this type of problem. Most digital radios even low cost ones will have very high levels of encryption which make it impossible for your radio to hear interferers.

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