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I am curious to know why can not signals of different tv stations mix together while transmitting. I have seen many towers built near each other in mountain areas in my country.
If two different signals reach the towers in the mountain, how cannot one signal interfere the other? I think this applies to mobile transmission as well. May be this is a stupid question, but I couldn't find an answer by googling.
The answer is frequency or channel separation.
The electromagnetic spectrum, specifically the broadcast frequency ranges, are divided up and managed by different regulatory agencies depending on where you are in the world. The job of these agencies is to provide rules that govern what types of transmissions are allowed on different bands, and who is authorized to do so. Within each band, the rules also specify the separation between different transmitters.
The short version: Two stations within geographical proximity can operate if they are on different frequencies, sufficiently separated that they don't interfere with one another. This doesn't mean it never happens. Equipment getting out of calibration, lack of maintenance or tuning, and other factors can all cause unwanted interference.
As Theran mentioned you need a non-linear process to mix frequencies together. Without a non-linear process the frequencies will remain the same no matter what they pass through, air, windows, walls. They superimpose so frequency A and frequency B can not produce frequency C without being transformed by something non-linear.
For example, say you have two processes. One multiplies signals together (non-linear) and the other just adds them (like putting some transmitting antennas on the same tower). The first order response looks like this:
The top process converts frequencies N and W to other frequencies (W+N) and (W-N) as well as N and W being present, however the process of just adding some antennas to a tower just adds frequencies N and W.
Non-linear frequency mixing can occur when receiving signals or amplifying them. These are referred to as distortion products and mixer products (because they are multiplied).
