As I posted in another thread, I've been researching the sensitivity of various FM receivers. Any statement with regard to the range of a Part 15 FM transmitter that doesn't take into account the specifications of the receiving equipment at the other end - and the most important spec is sensivitity - is just meaningless.
Although I've just started this task, I have some reasonably accurate numbers on high end home receivers, and good car receivers can approach, at least in FM sensitivty, the specs of those home receivers.
So let's do some analysis on the current Part 15 FM rules, which allow a field strength of 250uv/m at 3 meters.
The best home receivers (along with the best car receivers), have a useable mono sensitivity of around 1uv, a mono sensitivity with 30-50 db quieting of 1.5uv, and stereo sensitivity of 15uv with 30-50 db quieting.
Field strength is linearly proportional to the distance between the transmitting antenna and the receiving antenna. So, the field strength of a legal Part 15 signal at 200 feet (about 60 meters) would be 12.5uv/m, line of sight. However, the signal level at the input to the receiver would be around 5uv (math for this factor done elsewhere on the Part15.us site). That's easily within the ability of these receivers for a mono signal - a stereo signal would likely be too noisy, and the receiver would either switch to mono automatically (most car receivers do this) or you would have to switch to mono mode manually. In fact, you would need a field strength of around 40uv/m to receive a quiet, stereo signal with separation - that would translate to a range of 18.75 meters or roughly 60 feet.
Moving to 400 feet, the field strength of a legal Part 15 signal would be 6.25uv/m. That would translate to an input signal of around 2.5uv. You would still hear a mono signal with full quieting on the best home and car receivers.
Moving outwards to 800 feet, the field strength would be 3.125uv/m, and an input signal of 1.25uv. You would be able to hear a mono signal, but it would be weak and could be noisy. You would still have full quieting for a mono signal at 600 feet (with a field strength of 4.69uv/m and an input signal of 1.875uv).
It is likely that any discussion with the FCC and the NAB and others about increased field strength for Part 15 FM will focus on the potential for interference. So it is imperative that we look at the maximum ranges that any field strength can produce. But that's not going to be the typical range that most people will get - most will not have McIntosh home receivers, although it is likely that they will have a reasonable car receiver. I would like to propose that we define a new term - useable range - which also defines a typical home receiver and its sensitivity, and takes into account obstructions (i.e., if your antenna is indoors, a signal has to travel through walls to get to it, reducing its field strength, not to mention other buildings and even topographic features that might get in the way). It's the useable range that we, as Part 15 broadcasters, are most interested in. And we likely have to define both the useable mono range and useable stereo range.
Moving along a bit further, the Canadian maximum legal field strength for FM is 1000uv/m at 3 meters. You merely multiply the above ranges by 4 to get the Canadian equivalents. For example, at 600 feet, with the U.S. maximum, you can easily receive a mono signal with full quieting on a high end home or car receiver - in Canada, that translates to 2400 feet. You would be able to receive a very weak and potentially noisy mono signal at 3200 feet. And a stereo signal with quieting and separation at about 240 feet. But just a word of caution for Canadian readers - those numbers are for line of sight. In the real word, the further you get from the transmitting antenna, the more likely there will be obstructions between it and your antenna (trees, buildings, hills, etc.). And those obstructions will attenuate the signal field strength and reduce the range.
Still not earth shattering numbers, but significantly better than in the U.S. And the sky hasn't fallen in here yet. So I do think that there is potential to increase the U.S. field strength maximums if the requests are reasonable, and backed up by solid technical information re potential interference, as well as maximum and useable ranges.