FM Transmitter Power and approx Transmission Distance Calculation
No


That is supply current,  efficiency will be &lt; 60% and is measured by ratio with Pout. 

Try different loads (50~300) such as 50 Ohm load, non-inductive, rated 5W with very short leads and measure Vpp then convert to Pd (rms) by \$\frac{V^2pp}{  8R_L}\$ 


10 log Pd(rms) = [dB] or + 30dB for dBm (0dBm=1mW)
There are many variables such as antenna impedance matching losses , directional gain ( e.g. Yagi)  for Tx, Rx and Rx noise level.  Look for a Friis Loss calculator and this applies to line of sight.