Radio Transmission distance (FM) I am trying to find a formula (if one exists, and this is the right place) to try and calculate how far a radio station can transmit, under ideal conditions. I have searched Google, and I can not find a solid answer.


*

*Broadcast Frequency: 89.7 MHz 

*Effective Radiated Power: 910 Watts

*Transmission Power: 936 Watts

*Transmitter Height: ~39m

*Terrain: Flat


Thanks for the help.
 A: You can make some assumptions like a typical FM receiver needs -110 dBm to work.  Then assume you have an isotropic antenna in both cases because you didn't say anything about the antennas so we'll ignore the gain.
Next take a look at the path loss based on the 910 W (+59.6 dBm) power.
Your path loss can not exceed 59.6 + 110 = 169.6 dB  (with loss dB and dBm are the same, but dB and dBm to watts is different).
The free space loss model is 
$L_{fs}=32.45 + 20*log(d_{km}) + 20*log(f_{MHz})$
You'll need to solve this for $D_{km}$ and set $L_{fs}=169.6$ and $f_{MHz}=89.7$.
There are many other path loss models.  NIST suggests several


*

*Free Space Model (shown above...least accurate) 

*CCIR 

*Hata

*Walfisch-Ikegami (WIM)


Having the antenna height will reduce your path loss some.  In the popular WIM model in a line of sight (LOS) situation like yours where the base antenna is >30m high with no obstructions in the direct path then you can use a more realistic estimate:
$L_{wim-los}=42.64+26*log(d_{km})+20*Log(f_{MHz})$
Also the ITU has a specification ITU 1546 Method for point-to-area predictions for terrestrial propagation (pdf here).  You can also find MatLab solutions and a lot of literature on their technique and field test results for broadcast FM.  (FYI the models are all very similar in structure to NIST's models).
A: Would very well depend on the sensitivity of the receiver. Perhaps you want to know by how many dB the signal is attenuated over distance?
See: Determining a radio signal's range
