Antenna gain measurement
During a seminar on RF measurement, a relator told about a method involving three antennas; first you measure the signal coming from couple antennas A+B, then B+C and at the end A+C, with the corrections and what all the other people already said ( height of the antennas, distance between the antennas, ecc. )
Then you make three equations with three unknow data ( the gain of the three antennas, because you do not have any reference isothropic radiator ).
You solve the three equations and you will have the absolute gain of the three antennas.
P.S. I have never tested it because we have calibrated antennas and field, but anyway I think is worth the test.
Mandi