You said: "But nothing anyone says on this forum or HB physically stops me from installing a transmitter in some potentially non-compliant way."
I said: Rich Fry has been saying that for years.
To which I, in turn, said: And I have been knowing that for years.
The only intent the FCC had when it drafted the Part 15 rules was to restrict transmit range.
I'm sure that the FCC knows the approximate field strength of a fully compliant transmitter installation. Not only do they have field experience, but I'm also sure that they have their own modelling software (such as NEC 4.2). Measuring field strength is the only way to initially measure compliance, prior to an actual inspection of a station. So if you're over (and I suspect it has to be well over in most cases, not just a few percent), then they will inspect you. If they don't like what they see or hear, then, as theLegacy states, they will find the closest thing in the rules to shut you down.
There are no radiating ground clauses in Part 15.209 or 219 and no mention of elevated installs. Just the factors that go into restricting field strength (i.e., range), such as input power along with your antenna & ground system length.
So even with Part 15 AM, we're back again to field strength, much like Part 15 FM.
If you look at FCC enforcement actions from that viewpoint, then several examples often discussed here start to make more sense. That case with the Talking House, which was supposedly righteous? More than likely, it had a field strength slightly greater than what the FCC considers OK (for whatever reason - it could just be because it was elevated, as it was on a balcony); add a very aggressive FCC agent and you get the result that we saw.
The field strength of the KENC transmitters was well above an install where the antenna+ground+feedline was limited to 3 meters. So it got shut down.
It really doesn't matter what part of the rules was used in either of those cases. The FCC has the ultimate say, and those installs went against their intent.
If only we could get what the FCC thinks is the maximum field strength of AM (given the current rules). For while we may rail against the field strength limitations of Part 15 FM, there is no confusion. It's all there in black and white. If you have a field strength over the maximum allowed, you're not in compliance with the rules.
If only it could be so with Part 15 AM.
If only we could get what the FCC thinks is the maximum field strength of AM (given the current rules).
The measured field will depend on the frequency, length of the radiating conductors, the r-f resistances of the base loading coil and connection to earth ground, and earth conductivity at/near the antenna site.
Below are the field strengths for 1650 kHz at a horizontal distance of 30 meters from an unlicensed AM setup strictly meeting FCC § 15.219 (system base within a few inches of the earth).
The calculations are based on 85 mW of tx output power at the input to the base loading coil, 15 ohms of r-f loss in the loading coil when tuned to system resonance, and a 3-meter monopole.
This path length is so short that earth conductivity has a very small affect on those fields -- so they won't be much less than for a perfect ground plane (which was used here).
The first column below is the r-f resistance of the antenna system connection to earth ground. The second column is the field intensity at 30 meters, in millivolts/meter.
Note that these numbers did not originate from any FCC source, but from mathematical analysis.
10 6.2
20 5.2
30 4.6
40 4.2
50 3.8
60 3.6
70 3.4
80 3.2
90 3.0
100 2.9
Probably we can assume that if the FCC were to measure anything reasonably close to these values for these conditions they could consider the system as compliant with §15.219. If the fields are considerably greater then probably not.
The issues here are that neither the FCC or the user probably will know the r-f resistance of the loading coil, the r-f resistance of the connection to earth ground, or the power at the loading coil input, for that system.
Note that the fields above are considerably greater than allowed by §15.209.
The FCC looks at two things when they inspect a Part 15 AM Site
1.) Does the Total Length of the antenna and ground lead exceed 3 meters?
1a.) If so the station is operating under 15.209 check field strength.
1b.) If not, station is 15.219 check input power if needed.
FWIW AM stations typically do not have elevated antennas. So it is possible that the FCC did not anticipate elevated installations with ground systems.
Yes, but what comes first - the inspection or the field strength reading?
Presumably, there is a reason to inspect your station in the first place. I imagine that a complaint alone might suffice, but everything I've read in NOUO's indicate that they first take a field strength reading (most read something like - a complaint was received, a field strength reading of x was taken, they traced the signal to address y, no license has been issued for that signal, station is inspected). I'm guessing here, but if the initial field strength reading is around what they feel a compliant installation's field strength is, then they might pass up the inspection, unless interference to another station was involved as well.
If you have an elevated antenna, then your ground lead (wherever it comes from) + antenna is going to be greater than 3 meters. Although the FCC inspectors obviously have some discretion as to interpretation, as some have passed elevated installs.
Considering that the field strengths involved in Part 15.209 are close to noise level in most locales, if you can hear the station at all you're not going to be compliant.
My guess is that the FCC has got to "sniff the air" before taking action, to make sure there is a station there. If it's not operational when they go looking there would be nobody and nothing to contact.
They would need to determine what the field strength is; who knows how many fish get thrown back in the water because of compliant part 15 signals?
Any one of us might have been checked out and will never know about it because we're so squeeky clean.
Rich gave us what might be a typical range in his post. Anything over that by whatever fudge factor they use would probably be enough for them to want to inspect a station. That fudge factor would be important, as I'm sure they don't want to waste their time tracking multiple Part 15 compliant stations (which they have to do before they can inspect). Except for the most exuberant (or idle) FCC inspector, I suspect that it could be rather large.
basically congratulations to the people who were splitting hairs on the subject. because of your nitpicking you basically ruined a gray area. america was built on gray areas. we're talking milliwatts here. so even if someone might come along and say eh, no, if you shut er down yer usually fine.
Everytime someone says "fudge factor."