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).