Below are plots showing how Part 15 AM system radiation efficiency is related to the resistance of the r-f ground connection, for the conditions shown.
The r-f resistance of a set of buried wires depends on the conductivity and dielectric constant of the earth in which they are buried, the number/length/orientation/location of those buried wires with respect to the monopole antenna, and the frequency.
Hardly anyone knows for certain what the conductivity of the earth is at/near their Part 15 AM antenna, and conductivity for e-m radiation is difficult to measure. An approach used by AM broadcast stations is to measure their radiated fields at several locations along a straight line from the transmit antenna, and find the best conductivity match for that change in value over that distance using charts supplied by the FCC.
The system analyzed in the graphic below shows more than a 5 to 1 improvement in radiated power between poor and good earth conductivity, other things equal.
It also shows that a buried ground rod is not a very low-loss connection to the earth at radio frequencies. Even with very good earth conductivity at the antenna site, ground resistance using that ground rod is about 25 ohms at 1640 kHz.
I took the liberty of posting this on our ham radio FB page as well.
Rich,
Very nice presentation. Is there a similar calculation that you can do for a system using buried radials with the changing conductivity so there would be a way to visualize if these are an advantage?
Neil
I took the liberty of posting this on our ham radio FB page as well.
The one below would be better for hams.
Is there a similar calculation that you can do for a system using buried radials with the changing conductivity so there would be a way to visualize if these are an advantage?
I did this one about two years ago for the 160m ham band, which shows how ground loss varies with conductivity for a set of bured radials.
AM transmitter antenna grounding is the bottom line of this post, but I need to talk about trees before getting back to antenna ground.
For several years my plans to build an "Ultimate Antenna," based on PhilB's post of that name, have been blocked by a massive pile of bamboo poles collected for someday building fences, towers and a generator building. The great pole pile stood squarely in the way of the antenna site, and in 15-years the poles had started turning to powder, so I spent months cutting them to size and packing weekly cans for yard-waste day every Monday.
Finally free of old poles, an intense windstorm brought down huge and medium size branches, with them now blocking the way.
To make matters seem hopeless, the polar vortex killed the entire standing grove of bamboo, and they place an ugly landmark that looks like a groteque Edgar Allan Poe Memorial dab in the center of an otherwise trim fancy neighborhood. (I almost said "Edgar Allan Pole.") Feeling the pressure of being the one stand-out neighbor with a "bad" yard, I've been sawing and removing what seems like 2,037 trees, and stacking them for further disposal, guess where? That's right. On top of the dream antenna site.
But I cannot be stopped! This ground system and Ultimate Antenna will go in together with a new AMT5000X, at which time comparisons can be made to the ground data in this thread.
If I remember correctly the impedance that the AMT3000 likes to see is around 800 ohms. You might contact Phil to verify if this is correct and if the 5000 is the same.
I'm reasonably certain that PhilB has posted about the matching of his two transmitters, and perhaps I can locate the info in my "data mine," the place where I keep "my data."
Just for starters I'll speak from rote, hoping to be accurate:
In its basic form the AMT3000 matches to a 3-meter antenna using the inbuilt inductors, with ad hoc grounding, i.e, jumpered to ground through the power wart, or a ground lead clamped to a water pipe or electric outlet ground.
Several internal modifications to the AMT3000 adapt it to a self-built loading-coil, outdoor 3-meter pole, and earth ground including radials... this is (I think lower than the 800-ohm figure).
The AMT5000 is intentionally designed to operate with a free-standing 3-meter pole away from obstacles and a dummy load can be constructed from a 30-ohm resistor and 30pF cap wired in series.
Carl Blare posted (in part): The AMT5000 is intentionally designed to operate with a free-standing 3-meter pole away from obstacles and a dummy load can be constructed from a 30-ohm resistor and 30pF cap wired in series.
1) Wasn't the AMT5000 designed to drive the ~3m flexible wire supplied with it?
2) The feedpoint characteristics of a "free-standing 3-meter pole away from obstacles" depend on the operating frequency, its elevation above physical Earth, the length of the radiating conductors in that configuration, and the r-f resistance against which that system is driven.
A dummy load consisting of a 30-ohm resistor and 30pF cap wired in series will not be appropriate for most sets of such conditions.
First, the AMT3000. For persons interested in more than the manual provides, there is a freely downloadable white-paper from sstran.com titled: "Constructing a Base-Loaded Vertical Antenna." On the page "Modifying the AMT3000" is an explanation about parts changes needed to convert from high impedance output to low impedance so as to match the loading coil and pipe antenna.
As for the questions raised by Rich(F) regarding the AMT5000 it would be best to order, build, and study your own AMT5000, as anything I might say would not measure up to technical reality.
Take a look at one of my previous post: http://www.part15.us/comment/30902#comment-30902
In that post I stated: "the efficiency varies by 2 percent over the ground resistance range of 10 to 70 ohms with a peak at 30 ohms. The efficiency drops almost linearly in the 70 ohms to 100 ohms region to 88%".
Since then I have made additional measurements at the low end. So I can now state :
The efficiency varies by 3 percent over the ground resistance range of 0.1 to 70 ohms with a peak of 99% at 30 ohms. The efficiency drops almost linearly in the 70 ohms to 100 ohms region to 88%. Above 100 ohms it continues to drop to about 59% at 300 ohms. In the important reguion from .1 ohm to 70 ohms, the efficiency will never be lower than 96% over the reange of ground resistance from 0.1 ohms to 70 ohms.
As always the "efficiency" stated is the MOSFET drain efficiency, which is the only important measure since all transmitters suffer similar coil loss and ground loss in the path from the transistor through the antenna.
Even a rudimentary radial system consisting of just 1 30ft wire laid out away from the transmitter over a poor conductivity 1 mS/m earth will bring the feed point resistance (input to coil) down to about 60 ohms.
The "challenge" was carried out somewhere in Fairfield County, Connecticut where the ground conductivity is only 1 mS/m. On the US map it's hard to spot the few areas in the entire US with such low conductivity. Rich's graph shown earlier in this thread shows the feed point resistance as 270 ohms for 1 mS/m conductivity using just a ground rod with no radials. This is a pathological case considering the extreme rarity of such low conductivity. The thing to take away is that if you are in an area with conductivity in the 1 ms/m or 2 ms/m nether land, then you must install some radial wires to get out of this pathologically extreme performance killing situation. Don't just settle for a ground rod. If you want meaningful performance, you will need ground radials.
If you are lucky enough to be located in one of the super high conductivity areas, a single ground rod may make you happy.