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15.23 says use good engineering practices to make sure equipment meets the standards to the greatest extent practicable.
I would guess the FCC knows the field strength 100 milliwatts can produce into a perfectly matched 3m antenna over perfect ground for the entire range of 510-1705 kHz. If they are investigating an unlicensed AM signal trying to determine if it is a pirate or a part 15 transmission, a reasonable first step would be to see if it is even physically possible for the measured field strength to be compliant with the constraints of 15.219. If it is possible for a part 15.219 compliant setup to generate the field, they may go on their way. If it is not, they will probably investigate.
So someone can have an elevated 15.219 install, but if they do anything that could be considered “gaming the system” they are not using good engineering practices to make sure their install meets the standard. The FCC does not care about electrical code grounding requirements; if a 15.219 compliant install and a NEC compliant install are mutually exclusive the FCC has no obligation to change 15.219. (For an example of the FCC not changing rules to help a sub set of people look at the FCC’s over the air reception rule, people with north facing balconies can’t receive satellite service but the FCC is not changing the rule just to help them. For example of the FCC not caring about grounding, the limited protections of the over the air reception rule do not allow a code compliant ground of satellite dishes installed on balconies unless the power entered the apartment/condo on the balcony.)
The AM elevated NOUO’s I have read all involved obvious (at least to the inspector) use of the ground/support to radiate. I have never seen a NOUO for an elevated install with the ground RF choked. So as long as someone is not trying to get extra range with an elevated install, they can probably show best engineering practices.
Here is an example of trying to game the system: Installing a range master at the top of a tower in a back yard. At the top of the tower the transmitter is not grounded to the pole, but at the bottom of the pole the ground side of the DC power cable and the audio shield are tied to the tower’s lightening ground radials. There could be radiation from the DC power and audio cables that would enhance the field strength. If the person has enough yard to put up a tower, they have enough space to do a unquestionably compliant ground install so the tower install is probably an attempt to get more range. (Note, the FCC does not care if your signal can clear obstacles around your property; they are just looking at the technical requirements of 15.219. Nothing in 15.219 or anywhere in part 15 gives you any right to your signal being receivable at some distance from your transmitter.)
Here is an example of best engineering processes for a balcony install: A talking house is setup on the balcony in a weather proof box. The supplied wire antenna is used, extending up as high as practical while staying in the space exclusively under the control of the balcony user and as allowed by the property owner. The supplied power supply is used and plugged into the closest convenient outlet preferably without an extension cord; if an extension cord is used it should have an RF ferrite core on it. Audio is fed with a standard audio cable; it should probably have an RF ferrite core.
If the only place someone can put an AM transmitter is on the roof of a building they cannot comply with 15.219 and NEC grounding requirements at the same time. In that case, the best option is to install the transmitter as close as possible to how the manufacturer recommends, put RF ferrite core’s on the power and the audio leads (to demonstrate an attempt to use best engineering practices), and ground the unit only through the audio and power leads with a surge suppression before the cables come inside. The goal would be to show that while lightening energy would get shunted off the lines to ground no RF would be radiated by the ground.