For thought and discussion...
The link below leads to a performance comparison of two elevated transmit systems each using two horizontal wires as a counterpoise. The groundwave field intensity they each produce at a distance of 1 km is shown at the bottom of the graphic.
The horizontal wires add no useful far-field radiation of their own, because the r-f current on each conductor of the pair is flowing in opposite directions at each instant of time, and therefore their fields cancel.
Some interesting observations:
1. The system with no ground wire produces about the same field at 1 km that it would if located a few inches above the surface of the earth with no counterpoise, using a ground rod with 30 ohm loss.
2. The system using the 10-meter conductor leading down to a 30-ohm ground rod produces more than five times the field intensity at 1 km as the system using no ground wire (other things equal).
3. Both of the 3-meter conductors operate at the same height above the earth, but they don't produce the same groundwave field intensity at 1 km. This illustrates that it is not the height of the 3-meter whip that produces better signals, but the added radiation from the r-f current flowing along the ground wire of systems using such ground wires, and/or from radiating d-c power and program lines.
4. If the system using the 10-meter ground wire suppressed all radiation from the ground wire, d-c power and program lines using chokes/filters where those lines connect at the transmitter, then its performance essentially would be the same as the system shown with no ground wire.
Counterpoises, or radials, do not radiate, as long as they are perpendicular to the antenna (i.e., horizontal).
And, as Rich demonstrates, while an elevated transmiter/antenna/counterpoise system without a ground wire will not generate a field intensity any greater than one located at ground level, the primary purpose of an elevated install is to locate the antenna above obstructions. Obstructions are the bane of any Part 15 signal.
FCC inspectors have allowed elevated installs with radial systems in the past. But as we have all seen, their position on installations has changed over the years, so it is unknown what they would allow today. It would be interesting to find a recent example, if anyone has any info?
What I also found interesting is the calculated field strength at 1 km for the elevated system without ground wire. That's still pretty high, and translates to an actual signal strength of somewhere around 50-75uv to the receiver (based on mathematics in a previous thread).
I wonder, Rich, what the field strength would be without that counterpoise, just to give us a comparison number?
HERE are eariler notes discussed on April 28, 2012 of elevated and non-elevated antenna systems, though not including the 180* counterpoise.
RFB
as long as the elevated wires are 1) horizontal polarization and 2) equal opposite lengths of one another
the elevated counterpoise will not radiate any significant RF.
I wonder, Rich, what the field strength would be without that counterpoise, just to give us a comparison number?
Removing the 2 x 8-meter horizontal conductors used as a counterpoise on that modeled system converts the radiating wires to the form of an electrically short, off-center-fed dipole. The 3-meter whip is one side of the dipole, and the printed traces for circuit ground inside the transmitter are the other side of the dipole.
The capacitive reactance of that radiator is very high, in excess of 20,000 ohms. The loading coil needed to resonate it would be very lossy, and system r-f bandwidth would be small. The system would be hard to tune, and keep in tune.
But if the coil loss was 25 ohms, and there was no radiation from any other conductors connected to the transmitter, then the field intensity that system would produce at 1 km for conditions otherwise as shown in my graphic linked in the opening post would be about 137 µV/m.
I think it was worthwhile for Rich to post this information, considering that someone was asking about radials in an elevated mount.
Rich, I'm surprised that the field strength is so high with no counterpoise. That would still give a decent signal strength to a receiver at 1 km, no obstructions of course. While the counterpoise gives you about 60uv/m greater field strength, that is somewhat negated by the reduced signal strength at the receiver antenna (I use a factor of about 2.5). Significant, but certainly not the end of the world as we know it.
I agree with Mr. Artisan that it was good that Member Rich posted this Elevated Discussion.
It would be my contribution to say that a building, the same one holding the elevated installation, is an outgrowth from the ground; a veritable small mountain or hill.
Except that a building has somewhat different electrical properties than say, a bluff, or a mound of dirt.
Those differences become less in the case of dense architecture as found in high rise buildings, and especially skyscrapers, where the construction materials come closer to resembling a cliff, an actual chunk of earth rising above the average terrain.
There is more to say, and given the renewal of the world beyond Mayan rumors, we will be on hand.
Except that a building has somewhat different electrical properties than say, a bluff, or a mound of dirt. Those differences become less in the case of dense architecture as found in high rise buildings, and especially skyscrapers, where the construction materials come closer to resembling a cliff, an actual chunk of earth rising above the average terrain.
Yes, the metallic conductors within a high-rise building have much different conducting and radiating properties than a mound of earth or a bluff of the same height.
For example, an elevated Part 15 AM system using a 3-m whip with a "ground" connection to the building steel in that high rise building will produce radiation from that building steel.
OTOH, a Part 15 system using a 3-m whip installed within a few inches of the earth on a hilltop of the same height as the high-rise, and grounded to the earth via a ground rod or buried radials produces radiation only from 3-m whip itself. The r-f current in the earth does not produce radiation.
The difference in radiated power and received field intensity between these two installations can be great, in favor of the system mounted on top of, and connected to the vertical conductors in the building (steel, conduit, wiring etc).
What I am about to say may seem absurd at first, but I think it makes the point at this moment.
If you filled a building with mud, you would have a virtual earth rise.
Otherwise, all the differences described by Rich hold true.
What have I overlooked?
Consider me green and stupid here, but, seeing some issues.
The antenna appears to be a 3 meter whip.
The "counterpoise wires" are 10 meters above earth... Are these wires radials ran above the ground or placed on the ground?
The placement of the grounding rod seems to be adjacent or under the antenna. Is this a single grounding rod and what is the depth?
The performance difference is massive 🙂
The horizontal counterpoise wires in my chart are located 10 meters above the surface of the earth. They are not placed on, or in the earth.
The ground rod for the system using the 10-meter ground wire is buried in the earth directly beneath, and 10 meters below the vertical axis of the elevated transmitter and its 3-meter whip.
The 30-ohm r-f resistance I used for it is about typical for such a single ground rod that is buried vertically to a depth of about 8 feet in average soil.
"The antenna" of a Part 15 AM setup necessarily includes all conductors above the surface of the earth that carry r-f current produced by that installation -- NOT just the 3-meter whip attached to the transmitter
In the case of elevated Part 15 AM systems using long conducting paths to a functional r-f ground (something buried in the earth), the radiating length of that antenna system can be much greater than the 3 meters authorized to unlicensed operators under FCC §15.219.
And therein lies the problem.
If you have an elevated mount, AND use a long ground wire, then the ground wire acts as part of the antenna and radiates.
If you don't use the ground wire, then you get reduced performance, but you are legal. That reduced performance, however, still gives you a pretty good field strength at 1 km out, and you are minimizing signal absorption by obstructions. You do not have a safety ground, however.
It all boils down to where you are located. If you are in a relatively open area, it is probably adviseable to have a ground mounted setup, with radials. If you are in a built up area, and you can use an elevated counterpoise (radials), then that is probably preferable. But if you can't, then you will have to experiment between the elevated mount with no ground (minimizing the effect of obstructions, but keeping safety in mind) vs a ground mount (which will have better performance, but potentially also more signal loss due to obstructions - it will also be safer).
If you are in a built up area, and you can use an elevated counterpoise (radials), then that is probably preferable.
Wouldn't such compliance with Part 15.219(b) depend on current FCC policy, and the assessment of the FCC field agent inspecting such an installation?
But if you can't, then you will have to experiment between the elevated mount with no ground (minimizing the effect of obstructions, but keeping safety in mind) vs a ground mount (which will have better performance, but potentially also more signal loss due to obstructions - it will also be safer).
Just to note that radio waves in the AM broadcast band are not much affected by typical "urban clutter."
For example, the signal strength displayed on my Tecsun PL-310 portable receiver is the same just outside my house on the side closest to the transmit source as it is just outside my house in the opposite direction.