If you dig deeper, you end up with I/Q signals. If you take the input from the antenna and delay it by 90 degrees, you then have something which can have a positive or even negative frequency. Using multiplication of complex terms, you can add or subtract frequencies without generating unwanted images.

You could also use signals from 2 antennas 90 degrees apart to get I and Q. This gives you the added ability in that signals from one side has a negative frequency. It's some really useful stuff.