Thank you for your responses to the polls. So as not to clutter up the landscape, they have been removed.
The poll regarding non-radiating grounds indicates the majority believe effective RF grounds in addition to earth exist. The most obvious which comes to mind is a properly designed radial system or counterpoise. Discussion on this will be interesting as a previous thread mentioned using a radial system which is not symetrical. Seems as though that should radiate.
The poll regarding the FCC certifying non-compliant equipment was a unanimous NO. But the point was brought out that if the FCC certifies a device it could still be non-compliant due to testing oversight. The example being the Talking House using the AC ground as an RF ground. Rich has made the observation that a typical AC ground will radiate as the distance to a true RF ground would violate the 3 meter rule. One would have to isolate the connection at the receptacle and provide a true RF ground point to remain compliant. This would be true for any device using the AC ground connection as an RF ground. So even though the device was certified there is no practical way to put it in service and remain compliant.
The poll regarding the need to duplicate the OET test methodology when placing a certified transmitter in service showed the majority felt NO. See previous poll explanation.
Well, of course these are my opinions based upon lengthy if not heated discussions. I think each situation is worth further discussion as many of our installations rely on radial systems, RF grounds and trying to maintain compliance.
Be on the lookout for future polls.
Grounding an AM transmitter to AC ground can SOMETIMES cause added radiation. But this is not always true.
I have a situation which compares to using the AC ground, although I'm doing it slightly differently.
My AMT3000 with loading coil and Wintenna is located on the 1st floor of the house, and the ground lead drops down through the baseboard and clamps to the steel I-beam, which is connected with all house ductwork and electrical ground.
Although the grounding enables reaching the needed resonance for the AMT3000 being correctly loaded, the back-and-forth meandering of all the metal does NOT add to the radiated signal and the field is only strong for a very confined area that just matches the property boundaries, with another half-block on the car radio.
Still, I fully understand that other AC grounds might be arranged differently and contribute to radiation.
The method of grounding to frameworks has been discussed. Some felt it was non-compliant as the structure, just like a long wire, will radiate. The typical situation is a transmitter attached to a metal billboard. Granted the billboard is above ground but your meandering I-beam isn't in the ground.
Perhaps symetry plays into this. If you have attached at the mid point of the I-beam the fields from opposing halves may cancel out. But if you have attached off center, do the fields cancel out or will there be some radiation? And then there's those few feet of wire leading from the transmitter to the attachment point.
It will be interesting to see how this plays out.
My ground clamp from the AMT3000 transmitter is at the north end of the I-beam, but the I-beam is criss-crossed with metal duct work and a copper electric system ground wire which passes from the electric panel over to the water pipe, but is also earth grounded at the panel. All this back-and-forth interuption keeps the I-beam from being a horizontal radiator.
That is my opinion, but the actual performance of the small but strong within itself field seems to agree.
I suppose we need someone to weigh in with an NEC or some such model to analyze the non-symetrical counterpoise, if that is the correct description for a random path grounding system.
I've heard of fractal antennas, I suppose this would be a fractal ground...
I have read articles that suggest using just two radial wires, equal in length and opposing each other in direction, to be a workable solution for quick, simple installations.
Another article states for electrically short vertical antennas, a ground mat or many radials similar in length to the antenna incur less ground loss than a few very long radials.
I suppose we need someone to weigh in with an NEC or some such model to analyze the non-symetrical counterpoise
Here is one I did for a "ham" operator who wondered what the radiation pattern of a monopole might look like if he used buried ground wires arranged as shown. He was thinking of tunneling one wire under a patio to get to accessible earth where he could bury radials over a 180-deg sector.
The offset radials really don't do much to the shape of the radiation pattern, but the radiation efficiency of the system would be better if the radials converged at the base of monopole, and covered a 360-deg span.
This monopole at 14 mHz would be 1/4 wave. As the description notes there is no loading coil as the radiator is self resonant at the frequency of operation although the info indicates a reactive component.
It's interesting to see the plot shows a nearly perfect omnidirectional pattern with the offset radial system. Would this be a result of the 1/4 wave radiator or is that not a factor? A plot for our typical base loaded 3 meter antenna with offset radials would be very informative.
Articles I've read typically relate to a skewed pattern when the radials are not symetrically placed around the base of the antenna.
The useful radiation "launched" from a monopole system originates from the current flowing along its vertical conductors located above the surface of the earth, and the reflection of that radiation from the surface of the ground plane at distances beyond 1/2-wavelength from the vertical conductors.
The buried radials are part of the series path for the r-f currents induced in the earth by radiation from the vertical conductors within the radius of 1/2-wavelength from the vertical, and the "ground" terminal of the transmit system. If no such path exists, then a monopole will radiate almost nothing.
If the buried conductors are offset from the base of the monopole then they will not be as effective as a source for those earth currents. But whatever they do gather will flow along the vertical monopole, and produce omnidirectional coverage.
So while it might seem like it shouldn't happen, there is no physical reason why a monopole would radiate a noticeably skewed pattern in such circumstances.
Carl will probably point out his buried wire that "helped his signal go uphill," but that would be a localized effect. Most likely the average signal strength a few blocks away was/is fairly similar in all directions, except for other localized effects.
The uphill ground radial serving my AMT5000 points south, and it does succeed to deliver a reliable local signal to the top of the yard. However it's surprising how much farther the signal goes.
There is a street heading south only one lot west of me, but I alsmost never drive there because it isn't an efficient way to get anywhere, but one day I did drive south and was astonished at the distance of my signal's reach.
It's also interesting that the uphill incline continues beyond my yard for about another block, yet the signal was present for (guessing) four to six blocks!
It appears that the signal goes farther in the direction where the radial points.
I have no east west radials, and the signal in those directions is good but not as good as the southern lobe.
The northern side is not served by much of a ground radial, and the northern ground radial runs across the basement rafters to the front of the house. I have yet to actually measure the radials.
Being that whatever current flows along the vertical portion of the antenna must be returned via the ground system, if the radials are not symetrical will they radiate?
If that is the case would they be considered as part of the 3 meters allowable for the antenna system?
Granted, radials reduce ground loss by improving the current returned to the circuit but if the distribution of those radial currents is not symetrical (equal and opposing) it would seem they would not cancel, becoming part of the system radiation resistance not unlike a long wire to an RF ground point..
Granted, radials reduce ground loss by improving the current returned to the circuit but if the distribution of those radial currents is not symetrical (equal and opposing) it would seem they would not cancel, becoming part of the system radiation resistance not unlike a long wire to an RF ground point.
The function of buried radials used with a vertical monopole antenna is to return the r-f currents induced into the earth by radiation from that monopole and which exist within a radius of 1/2-wavelength from the base of that monopole, back to the "ground" terminal of the transmit system.
The r-f resistance in that 1/2-wave radius is a circuit element of that antenna system, and comprises a loss to the radiation efficiency of that antenna system.
A small amount of radiation is produced by the induced earth currents flowing along each buried radial within that radius, but that radiation is both localized, and horizontally polarized. It can be detected using a current probe, or a field intensity meter whose antenna is physically close to that radial, in its very near field.
But there is no useful benefit to far fields produced by a vertical monopole due to that radiation, because in the far field, horizontally polarized groundwave radiation in the AM broadcast band has very high propagation loss (theoretically infinite).
This is true whether or not the buried radials are symmetrically arrayed, and/or converge at the base of the monopole.
The comment just published by Rich is informative to me, because I'd thought, as MRAM noted, that the opossite legs of the radials needed to be of equal length, and I'd planned to eventually match them accordingly.
The evolution of the radials in my system explains why it is the way it is currently configured.
1. It started with the problem that when up the hill on the south end of the yard I lost reception from my transmitters.
2. I strung a single copper wire from the transmitter, located on the back house wall, all the way to the top of the hill with no oposing radial. This totally delivers full transmitter reception right up to the top.
3. Believing that compliance required a minimum of two radials of equal length I added a south pointing radial under the house and through the basement but so far it's only about 1/3 the length of the northern radial.
4. At present I have plans to match the two lengths by taking measurements, as well as adding more radials to cover a circular pattern.
The most important thing about the experience is that it woke me up to the need for having a radial ground system.
Rich, please elaborate then on the fact that the horizontal component would seem to be insignificant. If that is the case, why is the AC ground we discussed such a problem.
Assuming that most of the AC wiring in a single story building runs horizontally, to terminate at the circuit breaker pannel which normally has a ground connection to a cold water pipe or ground rod, why would this horizontal field contribute so much to the field strength that it would draw the ire of a rouge field agent?
I fail to differentiate between offset radials and random AC wiring ground when you stress that "the field is detectable using a current probe or field intensity meter whose antenna is physically close to that radial." It seems as though this is double speak on one hand stating radiation from these offset radials is insignificant yet the offset random AC wiring ground contributes to a strong field.
The only difference seems to be your mention of "buried radials" yet, effective counterpoise systems buried on not are said to be non-radiating, only acting as a return path capacitivly coupled to these earth currents.
please elaborate then on the fact that the horizontal component would seem to be insignificant. If that is the case, why is the AC ground we discussed such a problem.
Even though they add little to the useful groundwave, using horizontal, unburied, a-c "ground" wires is a problem to Part 15 AM operators trying to follow ¶15.219(b), because the lengths of those wires are not excluded by ¶15.219(b) if they are oriented in the horizontal plane.
The only difference seems to be your mention of "buried radials" yet, effective counterpoise systems buried on not are said to be non-radiating, only acting as a return path capacitivly coupled to these earth currents.
Elevated counterpoises using pairs of horizontal wires behave much differently than when those wires are on, or just under the surface of the earth. The current in the elevated wires comes directly from the transmitter output terminals. A monopole driven against an elevated counterpoise needs/uses no currents collected from the earth around the monopole, in order for it to be a very efficient antenna system.
"Elevated counterpoises using pairs of horizontal wires behave much differently"
So in Carl's case, where his I-beam-air duct-random wire counterpoise is not buried or in close proximity to the earth as it is within his home, his grounding system would significantly add to the radiated field strength as would an AC ground system "because the lengths of those wires are not excluded by ¶15.219(b) if they are oriented in the horizontal plane" or any other plane for that matter far enough above ground.
However, his uphill ground wire and other such wires near or in earth contribute little if any to the measureable far field but serve to improve the far field by reducing ground loss.
So where as both situations result in improved far field signal, one method is and the other method is not OK.
Hmmmmm...
Also interesting to note is the fact that these gound currents which are so feverishly sought after with elaborate ground rod systems, sea salts, etc. are not even required when "A monopole driven against an elevated counterpoise needs/uses no currents collected from the earth around the monopole, in order for it to be a very efficient antenna system."