Total posts : 45366
Daniel gave a good overview of this topic.
The reality is that part 15 FM limits result in a typical range of 200 feet. This is not a rule. It is just a guideline for what to expect. It is also a guide, with no legal basis, with which a hobbyist who wants to approximate compliance can adjust their transmitter/antenna without the need for expensive and difficult field strength measurements.
If one checks the FCC field notices and citations one will see that the FCC is acting against many each week who exceed the part 15 limits on FM.
Be careful with FM broadcasting.