Lefty Gomez said:
Rich could I get you to run some Computer models of the TH/IAM ATU on the ground with four radios or on the roof at 16' with no radios. (etc)
Lefty, the best I can do is to answer in general terms, as there are too many unknowns here for a computer study.
Most likely the ATU and attached whip mounted on the ground, and using four horizontal radials attached to the ground terminal of the ATU would have lower fields at a given distance than if was mounted at 16 feet above the ground, and the r-f ground was supplied only by the shield of the coax cable via its transmitter chassis connection to a buried ground rod.
The reason for the better performance from the elevated whip and ATU system is not because of height, but rather the result of radiation from the coax shield between the transmitter and the elevated ATU+whip (unless some means is supplied to prevent that radiation).
If the coaxial cable is not prevented from radiating, then as far as physics is concerned the elevated system probably would not meet FCC §15.219(b).
So even though the TH/iAM system connected to a remote ATU using a coaxial cable may be FCC certified for Part 15C, an FCC site inspector might consider an elevated installation of the ATU+whip to be non compliant with §15.219(b), just as they have done for such elevated installations using certified transmitters of other brands. The only difference is that with the TH system, part of the long, radiating path to an r-f ground is provided by the shield of the coaxial cable, rather than by a separate, dedicated conductor.
There have been discussions on another web site as to some of these issues including whether or not the coaxial cable in this system functions as the "feedline" included in §15.219(b), and the accepted definition of "the final radio frequency stage (exclusive of filament or heater power)" stated in §15.219(a). I think you have read some of those comments already. AFAIK the FCC has yet to weigh in on these issues in any official way.
Sorry if his answer isn't exactly what you hoped for, but I did want to make it as complete and accurate as possible.
