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if someone is generating frequency, can receiver detect from where exactly it's emitting? If someone is emitting on illegal frequency band, how it can be caught?

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Every Government does things slightly different, but the way most organizations look for illegal wireless use is to use directional antennas. This is usually an array of antennas to cover a decently large frequency range and direction. They will then stick this array on top of a van and drive around. If an antenna shows a strong signal then they can know to travel in the direction that the "hot" antenna is pointed. They can then just play a game of Hot and Cold until they find where it is coming from.

Most likely if you are using a frequency that is allowed, but at slightly higher radiated powers, they probably wont notice. There is enough variations that can be caused by various objects in its path that it would be hard to tell if you were radiating too much or not.

If you are using a frequency that is not allowed, but keep the power low enough that it would just look like noise at the street then you will also most likely be fine.

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Essentially, Rule of thumb is, if you keep your wireless signals mostly on your private property then there is not much to worry about. As soon as you get strong enough that it could interfere with others then you can start worrying about getting caught. – Kellenjb Apr 26 '11 at 20:51
Thanks for your answer – Kulin Choksi Apr 27 '11 at 16:37
Seems like they could do a better job than this. Even consumer phones can detect their location based on making maps of Wi-Fi signal levels. – endolith May 3 '11 at 13:55
@endolith True, but the phone wouldn't be able to do that if there wasn't someone who came along before with a GPS and recorded wifi data vs gps location. They could drive around the radiator once and then use the signal strength at different locations to get a pretty good guess at where it is coming from, but even then reflections of RF can do weird things resulting in you possible thinking its next door or something. So they still have to go back to the hot and cold game in the end. – Kellenjb May 3 '11 at 14:19
"They could drive around the radiator once and then use the signal strength at different locations to get a pretty good guess at where it is coming from" Yes, that's what I mean. It seems like it would be much more accurate to computerize it than play hot and cold. – endolith May 3 '11 at 14:52
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The standard approach is 'triangulation' - you use a directional receiving antenna, and determine the direction to the source from two different locations. Draw the lines on a map and you have a pretty good idea of where the signal originates from.

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