Neil, what frequency is this test rig running on?
Neil
Since your radials are 10-feet I believe that once you have a good set of measurement results, perhaps you might want to try a recipe I concocted for making part 15 radials a calculable length rather than random.
I reasoned that two factors should be merged into a formula:
The 1/4 wavelength, which would be typical of a real AM antenna, and 10-feet, the length of a part 15 antenna.
For 1680kHz 1/4 wavelength is 146.46576-feet.
Simply divide that number by 10, and you get the number 14.646576-feet.
If the antenna could be made that length, the ground radials of the same length would be what you intended to do by having your radials reflect the 10-foot length of your antenna, but there are two problems to deal with:
1) Antenna more than 10-feet would exceed the length under 15.219;
2) Your numbers have no relationship to the actual wavelength of your frequency.
Keep your 10-foot antenna, and add the difference to 14.646576.
Example: 14.646576-feet + 4.646576 = 19.293153-feet
Your radials should be 19.293153-feet in length.
I tried this formula on a simple two pole baseboard floating ground and it boosted my radiated signal 4 dBm over what I got with a longer random ground length.
I hope I have explained this in a clear description.
Carl,
In one of your posts I believe you mentioned "diminishing returns" and this may be what drives practical radial lengths. You pose an interesting thought on the relationship between the radial length and the antenna length and perhaps one exists but I am not so aware.
Let's pursue this carefully and it is with this in mind that I suggest the following and it is in no way meant to argue against anything you have posted.
The measurement of field strength may be influenced by the position of the detector with near field effects being different than far field effects. It would seem, however, that if all else is kept the same then measured changes in the field strength with changes in radial length should track the far field strength.
But another effect is the proximity of the detector to the radials. The radials are radiating signals and the cancellation of the fields depends on the detector being very nearly equidistant from each radial so the individual fields arrive at equal amplitude and 180 degrees out of phase. At small distances this may not be the case. While experimenting with my radial system I was able to detect the field from each radial with my loop antenna as I moved it around the antenna system..
It is a good thing you are doing by making measurements and sharing this with us and the above should not detract from this but rather be used to understand the possible limitations.
Neil
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Your response is very much appreciated and I realize at once that my readings have been offset by the fact that the Spectrum Analyzer is closest to the west wing of the bi-radial floating ground and farthest from the one on the east.
You are most courteous to consider my emotional investment in having fabricated the breakthrough of the year, but plain reality is absolutely fine with me.
What I'll do is use this formula for these experiments, so I can move on to other aspects of the testing. If this blend of ten-feet and 1/4 wave is not The Stradivarius Blueprint then I am secure in the otherwise practice of "some being better than none".
Although there is a lingering question brought to mind by PhilB's formula for the Ultenna, where he says the radials should be "a minimum of 30-feet". What I would like to hear is the reason for that being the case, i.e., how it was determined, and whether there might be "a maximum".
Once I reach a certain point I can do a test by centering the Spectrum Analyzer as you described, and comparing my present "special" length with a spliced-extra totaling 30-feet.
Having spent some time on ground radial experiments, of much smaller scale than Neil's radials in the subject of this thread, attention turns to coils.
I reviewed the photos and descriptions of the coil on acrylic form of Neil's installation, and what a beautiful work of art and electronics.
Does the coil on acrylic qualify as an "air-core coil"?
If not, then according to PhilB, "Ultimate Part 15 Installation"
www.part15.us/node/2728
"A big air-core coil is the best possible".
My own coil presently in experimental use is small and triangular, so I expect is very lossy.
The loading coil in the AMT5000 is compact, an iron-powder toroid wound with somewhat small wire, yet is efficient.
Getting all this into the head is a sacred duty of the part 15 religious life.
There are more questions to come.
Being neither an expert on terminology nor the English language but being an observer of both I offer that the term "air core" applies if there is no ferrite/iron slug or other material inserted in the coil to raise or change the inductance. Thus I consider the coil I used to be an "air core" coil.
Others may use the English language more literally and say "anything inside the coil which isn't air makes it not an air core coil" which would be literally correct but which wouldn't change the way it operates. I suppose that a true "air core" would require that all the insulation be stripped from the wire and it be suspended by air hooks.
Phil would be the one to define what he means by air core in this context since he supposedly has tied some operating characteristics to this definition. It is possible that his definition would exclude anything other than minimal support structures in order to eliminate possible dielectric effects though acrylic is known to be useful for RF work.
If it helps any, the measured Q of this coil is 147 which seems a bit low yet the measured inductance is within a few percent of the value calculated using the length, diameter, and wire size which suggests little or no effect from the acrylic. The coil turns were tight wound and maybe a better Q would result if the turns were spaced by about a wire diameter or so says the literature.
Neil
The loading coil in the AMT5000 is compact, an iron-powder toroid wound with somewhat small wire, yet is efficient.
Does anyone have any numbers (equivalent RF resistance or Q) for this coil? Toroid core material has losses and it would seem that the air core coil would have an advantage and it would take measured data to determine if and by how much.
Neil
Bruce,
Micro, Dog, Gnat, Flea, Slug Radio, Etc.
CD stack packs (100) make pretty good weather seals for coils.
Wish I had a lathe anna tractor 🙂
I haven't experimented yet, but wouldn't a close-mesh copper screen of, say, 3 or 4 ft. dia. help build near-field ground conductivity at the base then run out your radials from that?
IIRC there are some tables somewhere which help determine near-field FS radiation. Anyone know where?
The lower the freq (longer wavelengths) the stronger and more predominant is n-f FS. At higher freqs, n-f matters less because we're going for far-field distance.
In this way, we can also more easily see one factor why lower freqs are generally better for carrier current installations.
But, I was thinking a denser n-f ground grid would help at these puny power levels, even with higher AM bcb freqs, something slightly akin to skin effect (except in reverse) on the radiator.
Just musing ... 😉
The copper screen in the near-field with radials reaching out from its edges seems like it would increase ground contact near the antenna.
I had a somewhat similar idea, that would be placing two or three stacks of ground radials, each one deeper than the one above it. This might give a kick to the ground contact.
There is info in the library regarding near field FS. Look in the LPB carrier current techniques and notes (Tech Note #1). There is a chart, though not for all MW frequencies, but a measurement of near field intensity for the frequency of 540 Khz.
RFB
"we can also more easily see one factor why lower freqs are generally better for carrier current installations."
Not always the case. For example, a power grid system that relies on an Earth ground for the return path, the higher frequencies work better than on a system with a neutral return line. But there is a twist with this also. In some cases, the grid system with a neutral return wire may be a short run, say a couple of blocks, to which again the higher frequencies will work better than the lower frequencies.
In a typical crowded location such as in a city, where loads on the power grid are great, the lower frequencies will work better and be less prone to the inductance shifting which occurs on the lines between peak usage times and low use times.
Then throw in the environment factors..ie weather conditions from season to season, which drastically changes the loading inductance on ANY power grid systems. When ice builds up on those lines, that is like having ice on your 3 meter antenna, or small FM antenna, or even like ice buildup on a high powered multi-bay antenna. They never work well with ice buildup and produce a ton of reflected power and spurs, along with harmonics. The same thing happens with the power grid wires, which are fully exposed to the elements of weather 24/7/365.
CC is a system always in "flux". Meaning it requires about as much attention to peak tuning onto the grid lines as does a 3 meter antenna over a ground plane which is suspect to influence by even a bird perching on the top of the radiator, or a tree squirrel sitting on the outdoor box with it's tail bouncing back and forth across the bottom of the radiator, making the entire system's resonance "dance" back and forth with the squirrel's tail! Quite amusing to watch the meter dance up and down, the field strength bounce up and down with the tree squirrel's tail!
RFB