RF Power measurements with Spectrum Analyzer
The spectrum analyzer is a 50 Ohm load.  If the transmitter / matching network is specified to deliver +15dBm to a 50 Ohm load, you will measure +15dBm.

If you don't burn it out first!  Please make sure that the analyzer can handle a +15dbM input.  Some max out at +10dBm, for example.  You may need to use an attenuator between the transmitter and the analyzer.