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…what about on top of a non-metallic tower, building or wood pole? My interpretation of the rule is anytime you connect a “radial” ground system with a downlead of nearly any length onto a Part 15 radio station, the operator runs the risk of being in violation.
The issue isn’t whether or not the ground system is comprised of buried radials. The issue is the length of the conducting path leading from the tx chassis to the r-f ground plane at physical earth — no matter what the r-f ground consists of. That conducting path to r-f ground radiates. If it is connected to a good radial ground system, more r-f current flows in that conductor, and it generates a greater r-f field to add to the 3-m section above it. A poorer r-f ground such as provided by a ground rod (or several) has higher loss to the returning earth currents, so less current flows in it, and it radiates less. But regardless of how much it radiates, it DOES radiate. That’s why the FCC counts that length in the total 3-meter length allowed for Part 15 AM in 15.219(b).
Many of the manufacturers of certified Part 15 transmitters make a clear point of a single, short downlead, straight to a ground rod. And if elevated above ground on a building or pole, an elevated ground system around 20 feet in length can be used with a very short lead to the radial system.
That is the prevailing viewpoint among many commercial manufacturers of Part 15 txs (certified or not), and their users. It just isn’t supported by physics. A 20 foot conducting path from an elevated tx chassis to the r-f ground plane at physical earth by itself forms a radiator over twice as long as allowed by 15.219(b).
And there is no such thing as an “elevated ground system” unless you are talking about elevated, physical radials projecting horizontally from the base of the elevated 3-m vertical, and connected to the tx chassis. In that case, no conductor is needed from the tx to the earth in order for the antenna system to radiate efficiently.
It is incorrect from the viewpoint of physics to think that r-f ground exists at the top of a 20-foot wire leading up from a buried r-f ground. That conductor is actually a part of the antenna, as developed above. Yet this is the popular concept supporting the use of elevated Part 15 AM antennas, in the belief that only the short wire from the tx chassis to the top of the 20-foot wire “counts” in the calculation of radiating length for the antenna. But that is provably incorrect — the whole conducting path to earth counts: the “ground lead” PLUS the “ground wire” (or tower/flagpole/whatever).
Metal buildings and billboards are not necessarily sufficiently”earth” grounded to constitute a safety ground.
As in the FCC email I posted earlier in this thread, an elevated Part 15 AM installation that results in a violation of 15.219 is not acceptable to the FCC. Calling the ground lead a “safety ground” doesn’t change the fact that it carries r-f current, which radiates, making that entire conducting path from the tx chassis to its connection to the r-f ground plane at the surface of the earth part of the radiating length of the antenna system — and that radiating length cannot exceed 3 meters, altogether.
If an elevated installation of a Part 15 AM antenna plus ground lead/ground wire/safety ground or whatever you wish to call it results in operation beyond the limits contained in Part 15, then that isn’t an appropriate place to install it, if your goal is to observe Part 15.