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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.

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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.

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  • \$\begingroup\$ ok I understand the frequency allocation part, but if two frequencies mix together, a signal with a different frequency can be produced right? So why doesn't that happen? \$\endgroup\$ – DesirePRG Sep 14 '13 at 6:07
  • \$\begingroup\$ You can modulate one frequency with another, which is used for example in FM broadcast radio. Two waves at different frequencies traveling through the air don't "combine" like that. You might want to ask about how electromagnetic waves propagate on physics.stackexchange.com as to how they travel simultaneously and yet can be received independently as it's more a physics question at that point. \$\endgroup\$ – JYelton Sep 14 '13 at 6:43
  • \$\begingroup\$ @DesirePRG, the simple summation of two signals at different frequencies does not create any new frequencies. All a linear time invariant (LTI) system can do is amplify, attenuate, or phase shift frequencies that are already present. Mixing of frequencies to create sums and differences requires a nonlinear process, such as a diode mixer in a heterodyne radio, or the KTP frequency doubler crystal in a green laser pointer. If our hearing was linear, then tuning forks would be nearly useless because the beat frequency would be inaudible. \$\endgroup\$ – Theran Sep 14 '13 at 6:59
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    \$\begingroup\$ @DesirePRG Two people whistling a different note at the same time might sound like a third note or a song, but the original two still exist and can be picked out. Thats how radio signals work. \$\endgroup\$ – Passerby Sep 14 '13 at 12:50
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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:

mixing/adding

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).

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