I still find it somewhat perplexing that people will pay money to subscribe to Part 15 Forums when you can get the same information (and more - it tends to be unbiased) elsewhere.
Anyway, this was a new topic at one of those sites. I'm never quite sure if these kinds of posts are just seeds to get discussion going, and we'll never know, as all replies are hidden at that site unless you're a member (and pay).
The answer to the post, of course, is not simple. The FCC does state that the effective range of a Part 15 AM signal is 200 feet, but in the rules, there is no restriction as to range - only the allowable input to the final stage of the transmitter (100 milliwatts) and the antenna system including ground (3 meters). But that does not take into account a huge number factors, not the least being the kind of receiver/receiving antenna you have. Other factors range from things you can't control (such as ground conductivity in your area, obstructions, background noise levels, even weather) to things you can control (your transmitter grounding, audio processing, transmitter efficiency, etc.). And how do you even define range - is it just barely hearing a signal? Or being able to listen to it, even if it's weak? Or a strong, city grade one?
Anyone who has ever used a Part 15 (or RSS210 in Canada) transmitter knows that your range can be anywhere between less than 200 feet up to 2 miles and more. The best I've been able to do is about 2 miles, hearing my weak signal clearly on a good car radio. I was using a Hamilton Rangemaster transmitter, tuned exactly to Part 15 levels and mounted at a height of about 3 feet outside, grounded to a metal mast that was pounded about 5 feet into the ground. Not even any radials.
My best range had a great deal to do with location (and ground conductivity). Similar installations elsewhere could do little more than 1/4 mile. And unfortunately, there's not much you can do about where you live.
When you think about it, blanket statements about range are really of little use, considering all the factors that are in play.
And don't even get me started about FM, which really is the target of so much misinformation. AM certainly has the potential to get far greater range than FM, but, given the right circumstances, and application, FM can still be useable. Parking lot radio comes to mind, as the area tends to be open, free of obstructions, and you have the best receivers at play (car radios). On Bowen Island, my target audience was the ferry lineup, which was similar to parking lot radio. It also helped that Canada allows more field strength on license-free FM than the U.S. (100uv/m at 30 meters, as opposed to 250uv/m at 3 meters) - that extra strength gave the signal the extra oomph so that local portable radios were able to listen in as well.
I don't know where the 200ft distance came into it. Maybe since no one has a $10,000 FIM field intensity meter that just got spread as a guideline to judge legality. In Canada it is thought that since the BETS measurement(talking FM here) is taken at 30 meters(100ft roughly) that your range is 30 meters(someone at the CRTC once told me that) but that is far from the truth. How can 250uV/M@3 meters get you 200ft but the Canadian BETS rule of 100uV/M@ 30 meters only gets you 100ft?, when BETS allows, in the real world, not in theory, 8 to 9 times more than the FCC?
Even the claim of 200ft(FCC rule) is only likely outside, line of site, and a good receiver, and I repeat GOOD receiver. Depending on the environment and receiver quality you could be getting 200 ft but be quite a lot over the limit....just because you are not more than 200ft doesn't mean you are legal!
And I've been talking about FM here, with AM I saw the 200ft guideline mentioned too!
With a Procaster operating legally with no power adjustment capabilities from inside with no ground except an in the cabinet self devised "ground" and a 104" length of wire tapped on the wall I got, in the daytime, a listenable signal at 1630 outside for about 1/2km around me with, again, a good receiver.
The biggest problem with statements like AM or FM or whatever will only go 200 feet is that it doesn't take into account those numerous other factors I mention in my post, particularly the receiving radio & antenna. It's a statement dummied down so much that it means nothing.
If there was any logic applied to that statement in the FCC rules (and, for FM, in the Industry Canada rules), it can only be that most people will use radios with horrible sensitivity and no antenna. I personally lean towards the opinion that there is no logic in the statement, and it's just thrown out there to scare people into limiting their range by what is being said, and not by the rules themselves.
And while I'm on a bit of a rant, I'll bring in Part 15/Industry Canada transmitter certification as well. The only way that most users of this equipment know that it is legal (since range is dubious at best) is to look for that certification sticker. Few have invested in a calibrated FSM (field strength meter) for either AM or FM. The labs that issue the certification are supposed to test the transmitter in its useable configuration, and determine that it meets the rules.
There's been plenty of talk around about how some transmitters have received this certification, and yet it's at best uncertain that they meet the rules. Those devices range from the Whole House 3 transmitter (that gets more than 200 feet range, and even has a 'hidden' high power switch to get even more) to the Retekess TR508, which has a claimed power output of 100/500 milliwatts. Yet both have indeed been certified in the U.S. and are in the FCC database.
The Whole House has been certified in Canada under RSS210, which is identical to Part 15 FM, but it's output is probably closer to BETS maximums than RSS210 (the Decade MS-100, BETS-certified, gets a bit more range under identical conditions, but not much more). The Retekess is also certified in Canada. When it first came out here there was confusion and some resellers stated it was RSS210 compliant (likely based on the Part 15 certification). However, if it ever was RSS210 certified, it's not now. It turns out that it's in the Industry Canada equipment database as being RSS123 certified, which means you need a license to use it, and even then only in a strictly bounded area (such as a large property, a public place, etc.).
But now to the problem. People are buying both of these transmitters, assuming that they are legal to use as they are certified. How can the FCC come back and say that they're not being operated legally, when they themselves (through outsourcing to the certification labs) have said they're OK.
Even in Canada there's an issue. The Retekess has an Industry Canada ID, yet there's nothing on the label to identify what kind of certification it has. You have to know enough to look up that ID in the Industry Canada database to determine what it's certified for. And most wouldn't do it, seeing the Part 15 certification (as mentioned previously).
This is an FCC problem, and an Industry Canada problem, not an end user problem. Both regulatory bodies have to fix their certification processes.