Using just common sense and no scientific analysis whatsoever, probably all of us would/should agree that at least one end of a "ground lead" must "lead" to "ground."
A frequent belief posted on Part 15 boards is that connecting a short conductor identified as "the ground lead" from an elevated transmitter to a long, metallic, grounded conductor such as a "massive" wire, a billboard frame, a flagpole, a tower, a "lightning ground wire" etc connects that transmitter to ground.
However that belief is not valid for the frequencies permitted by the FCC for unlicensed AM/FM broadcast band use under Part 15.
The r-f characteristics for such frequencies at the top of such conductors are NOT the same as at the bottom of such conductors, which means that such conductors will radiate along their entire lengths/heights.
IOW, the tops of the conductors identified in my paragraph two above are not at r-f ground potential. So those tops do not provide a functional r-f ground for "the ground lead" of an elevated Part 15 transmitter.
Assuming that they do so already has resulted in FCC NOUOs.
PLEASE note that this information is posted only for the consideration of the operators of unlicensed AM/FM setups subject to Part 15, and not as a "directive."
Such operational choices are those of such operators.
That may be, but in the past FCC inspectors have accepted such installations as conforming to Part 15.219 rules. In recent memory, not so much.
That just shows that there is some discretion on the part of FCC inspectors when they inspect a station.
Rich, the part of your presentation that I find false is this: "A frequent belief posted on Part 15 boards is that connecting a short conductor identified as "the ground lead" from an elevated transmitter to a long, metallic, grounded conductor such as a "massive" wire, a billboard frame, a flagpole, a tower, a "lightning ground wire" etc connects that transmitter to ground."
I have never seen a single claim of the sort.
Other than that, you may have something.
Uh Carl... Did you forget how super common it was, even in this very site, to ground the transmitter to a flag pole or mast? A ton of Part 15 stations did that, there was a whole thread about it when that station in oregon was shut down.
Also, ground disconnected or not, that transmitter WILL find ground. It wouldn't work without it 😉
"a dangling ground lead not connected to ground."
Q: How does it act as a ground lead if it doesn't lead to ground?
A: It doesn't. It only acts as one side of a (probably off-center-fed), electrically short dipole antenna.
Note that dipoles (even if off-center-fed) are ~ balanced antennas. Balanced antennas need no r-f ground to generate radiation.
In the case of the Oregon station, the short wire referred to as the "ground lead" connected to the top of a ~40-ft tower, which was connected to the earth at its base.
So the short lead from the tx to the top of the tower was just one part of a long, conducting path to the actual r-f ground, at the bottom of the tower.
Mighty 1650 said: "Did you forget how super common it was, even in this very site, to ground the transmitter to a flag pole or mast? A ton of Part 15 stations did that"
Somehow I didn't pick up on how common it was to ground to flag poles or other metallic masts. It does make sense.
A wire attached to the RF ground portion of a Part 15 AM transmitter circuit is called a ground lead whether attached to ground or not. There has been much said about ungrounded ground leads.
I seem to recall it was common until a string of seemingly several NOUOs were issued to elevated transmitters with long grounds.
Yes Mighty 1650, that's what we're talking about.
And the hero who brought about that crack-down on long ground leads?
Get ready to take a bow!
Mr Fry said: "However that belief is not valid for the frequencies permitted by the FCC for unlicensed AM/FM broadcast band use under Part 15."
I said: I did not know that there "ground lead" specifications for devices operating in the frequency range of 88 to 108 mHz.
"the frequency range of 88 to 108 mHz."
That's a really low-frequency band, John.
But as shown in the ongoing field measurements by Tim in Bovey posted in other threads here, a wire by any name that is connected to a commercial Part 15 FM transmitter can produce fields that (greatly) exceed FCC 15.239.
One minute it's about ground leads, the next it's wires.
Should we take a minute to recover our logic?
Take all the time you need, Mr Blare.
Of course a radial ground wire is not the same thing as a ground lead, and therefore is not measured as part of the 3-meter limit for AM radio.
This authoritative information from "Radio Handbook" by William I. Orr, W6SAI, provides a useful description of a ground radial for an unbalanced antenna system, which of course is the normal type of antenna used for Part 15 AM:
"The ground termination for an unbalanced antenna system can be improved by the addition of a radial ground wire which is connected in parallel with the regular ground connection. The radial wire consists simply of a quarter wavelength of insulated wire connected to the ground terminal of the transmitter. The opposite end of the radial wire is left disconnected, or "floating." The radial wire may be run around the baseboard of the operating room or out the window and a foot or two above the ground."
With a field strength meter for AM a good friend with such equipment has done ground tests with a Talking House and iAM Transmitters and the was no flux in the field strength when ground wires were connected and removed. Some of these part 15 AM transmitters ground terminals will not radiate any RF. Plus Today I tried to see what would happen if I simply connected a long piece of 75 ohm cable to the ground that was not connected to an actual ground. If anything the range was decreased. So it can actually hurt you with an excessive ground lead contrary to popular belief for part 15 AM. It floored me when I did the test myself but this is very true here. It makes me believe that these transmitters have been tuned for mono pole operation. This too would explain compensation for a VSWR (SWR that swings when any thing is around the TX's antenna when not grounded). Have you ever had a CB Radio with say a magnet mount antenna sitting inside your house on a desk and connected to an SWR meter? Try it you'll see the SWR be flat and raise up as you even get near the antenna while there is no ground plane. Even with an antenna match unit where you tune two capacitors to simulate a perfect antenna for the Radio to transmit into. Some antenna match units work better. That is what an ATU tries to do. Since there is no solid ground the capacitors you tune tries to lower the SWR for your transmitter so as to fake it into believing its transmitting into a ½ or 5/8th wave AM antenna. I think when they made the ground rules that the FCC had home made AM transmitters in mind where as they were more fit for a balanced AM antenna. This is one good solid reason to lift the ground rules. These little transmitters we're using are not meant for a full wave AM antenna as a consumer could never have enough real estate for even a ½ wave balanced AM antenna. These rules are so out dated and written most likely when AM Radio was first born. Now inductance is possible to a certain very low level with this too.