Jamming repeaters is illegal and if you get caught large fines and confiscation of equipment are usually the norm. However, if you have decided to go down this route be made aware of the risks and how not to take unnecessary chances. Here are a few guidelines :-
1.) Always go mobile
This is by far the best way to jam, with minimal risk of being caught. You cannot be found because when they take a bearing on you it is already history!
The risk becomes greater if you use the same route over and over again, like to and from work. If fellow hams at work notice a new 2-metre whip on the car and it is co-incident with the start of a new squeaky, they may put two and two together and become suspicious. They would then have to trap you and would probably do this with the help of the DTI. If you change your route regularly and stop jamming one mile from work you should be ok, but dont brag about it. The grapevine is very small and news travels fast, even if you have friendly amateurs who are anti-repeater, they may talk. Never jam from a cul-de-sac.
Even if you find a favourite spot make sure you dont spend more time than is really necessary. Between two and four hours is normally ok. Anymore than this and over a period of time they may get lucky!
2.) Background Noise
From ARG publication " How to jam it without tears".
Dont park on the perimeter road of Heathrow as Concord takes off. Its a bit of a give-away, or Big Ben as it starts to chime etc.
3.) Dubious Locations
If you do park up, make sure your vehicle does not look suspicious. With the activities of the IRA, people are being increasingly vigilant. Make sure your vehicle does not attract attention by standing out like a sore thumb. Try and keep antennas small.
4.) Disguise your identity and views
If you are already a licensed amateur, you may have a very characteristic rig. If your views are already known, it does not take a genius to put two and two together.
If you must talk, use a squeaky voice or play music or even a blank carrier will often suffice. If a blank carrier is used, make sure the microphone is disabled. This allows you to carry on a conversation if your not alone, without anyone over-hearing it.
5.) Have you been rumbled?
Amateurs often may make you think that they know whos jamming. They do this in various ways like saying, " Its that mobile again." or " Its that inverter whine again".
Dont be put off. If they knew who was doing it, they would give your name, callsign and address in the next breath. If this hasnt happened, your pretty safe!
6.) Never jam from home
This is self explanatory and the reasons are obvious.
I have been called by friends who have the DTI and police already on the premises. They will take material they believe is being used to jam the repeater, tapes etc., they will search the premises, and if they find a transmitter on the premises, weather or not it is installed, they will take it and use it in evidence. You will not see it again! Plus the fact, the police are no longer independent in this kind of situation. They are backing up the DTI officials and not just keeping the peace. The police are often violent in this situation!
Of course they need a search warrant and you must let them in, otherwise they will force entry. But you can insist that only the people named on the search warrant enter the premises. They must show you the warrant and you may read it.
A raid of this nature can take over 12 hours. If the police are in number a confiscation order may not be produced, but you are in a minority, buddy, and they will take anything they want. You have been warned!