I've been thinking about this a lot lately.
We all know that FCC accredited labs are supposed to test transmitters under controlled conditions to determine compliance (or non compliance) with the Part 15 rules. These conditions do not come close to mirroring those found in the real world.
It is further known that there are many factors that affect the field strength of FM transmitters. Antenna orientation is one. Cables (power, audio) are another. Even touching the transmitter, or placing it above a metal surface, can greatly increase field strength. Height is yet another, as it greatly reduces the potential nulling effect of ground reflections.
So, that transmitter that was measured at 250uv/m at 3 meters in the lab under those controlled conditions could have widely varying and much greater (it would be tough to have less) field strength in the real world. And unless you have expensive test equipment, which most users don't, you won't know.
Range might give you some indication, but it certainly doesn't tell the entire story, as, again, there are numerous factors that affect range, such as radio sensitivity, ground conductivity, electrical interference, obstructions, even weather.
That's why I believe it's important to run a certified FM transmitter. I'm sure the FCC agents understand all of this, and it indicates to them that you intend to be legal, no matter what your actual field strength is measured at.
There may be a question about how that got certified but it's hard to believe the lab in China that did the certification or the company did some shady stuff as it would come back to them as the lab no matter where in the world is authorized by the FCC or ISED to do the credible testing and the company can get into trouble if they cheated to get it done and then manufactured something different. There can be big penalties for that. Who knows what to think.
This is a little off subject, short story.. I know a guy that has an illegal 15 watt Chinese fm transmitter. He asked me if it would over power and jam a commercial 100 kilowatt radio station? I told him No it can't. But then I said to him he might be able jam a short range area. Like in the parking lot of radio station studio where the D.J. is. I didn't think he would really do that. But a few days later I heard on the news that someone jamed and tried to hijack the local rock radio station. I never talked to him about it, but I bet it was him that did it. I don't know why anyone would want to do something like that.
This kind of activity is why there was an act of congress a few years ago . People broadcasting over top of licensed stations.
@mark Great logo on your page. What did you do while working in radio.... There's a station in Baltimore and Hampton Roads VA that fired almost everyone,no DJ's.
@1620am-w9xaz I never worked in radio. This is just a hobby that started about 14, 15 years ago when all the commercial stations left me behind and I found out that I could do it myself (and both our countries allowed it) to keep rock and roll and recently old time dramas on the radio.
@mark Radio has left a lot of people behind. That's another reason I'm doing it. I have worked in radio as I have said perviously. It's pitiful what has became of radio. And I hope the other member stays in the forum. It was a valuable discussion. I see no reason for him to leave.