If I have a network analyzer to test the antenna propagation. I used S21 parameter to do the testing. It shown a value in dB. Can anyone please tell me what is the value regarding? It's the ratio of the reciever power and the transmitter power (PR /PT )?
Using S21 you will see, as you correctly assumed, the ratio between transmitted and received power. I.e. loss of feeders, gain of both antennas and the pathloss.
If you do want to charactierize the matching of your antenna, use S11 to get this