For accurate power measurements you need a 50 ohm output impedance, a short cable to a good through line meter and a high quality 50 ohm load right on the meter output (or a terminator type meter)?
if you have the equipment replace the above with a T adapter in the Tx output, the 50 ohm load on one port and a 100MHz or better digital scopes¡¯ x10 probe in the third port so you can use the scopes RMS output figure for your calculation ( fir extra points add a filter to be sure you¡¯re only seeing the fundamental frequency power)
if available see if if a 3dB attenuator on the TX output half¡¯s the power etc ?(if not somethings not 50 ohm?
with non 50 ohm outputs, different loads, different meters and different cables/connectors/lengths I can get any number of different power ¡®readings¡¯ you like?
Bruce Akhurst