Does the frequency vary in transmission?

this depends on if you are using amplitude modulated (am) or frequency modulated (FM) waves. am waves have a fixed frequency, and so can be sent a lot further, but do not give good signal quality because the amplitude carries the data and therefore it is affected badly by interference. FM waves have a constant amplitude, meaning they have better quality, but the frequency contains the data, meaning it changes all the time and therefore cannot be used for very long distances