LF: Radio signals

[edgar <edgarjtwining@virginbroadband.com.au>]

Hi Group,

What is the mechanism for the way radio signals combine in the air?

For example

1.There are two transmitters each sending out the same fixed frequency 
sine wave signal.
2. There are no other signals in space, and no background noise.

3. There are no propagation factors modifying the signals.

e.g. buildings, ionosphere layers.

4. Both transmitters are the same distance from the receiver.

5. Both signals have the same polarisation.

How have the signals combined when they reach a receiving antenna at a 
reasonable distance from both transmitters?
It the process addition, multiplication or some other process?

Regards, Edgar
Moonah, Tasmania.