Earlier today I posted the text following the line below in response to a question on a ham radio board. This topic shows up on various websites occasionally, such as using a single buried radial leading to, and submerged in a creek, lake or even the ocean. So maybe the following study might be useful to readers on Part15us.
The frequency and wire lengths used in this NEC model were not based on a 3-m monopole used in the AM broadcast band, but the conclusions about the location and effectiveness of a buried radial ground system still apply.
This post is rather technical, but hopefully the meaning will prevail even if all of the terms there are not completely understood 🙂
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The graphic below shows an analysis of a 20m, 1/4-wave, base-fed monopole using a buried radial field having an offset of 4.9m from the base of the monopole, with radial wires over a 180-deg sector. The radiation pattern is symmetrical around the monopole, and for 5 mS/m, d.c. 13 earth has a peak gain of -6.2 dBi at 28 degrees. Its feedpoint Z is 139 -j 28.1 ohms. The radiation resistance of a 1/4-wave monopole over perfect earth is about 36.5 ohms, indicating that the loss in this r-f ground system is about 135 - 36.5 = 98.5 ohms.
A similar model where the monopole was centered over the common-point of 16 buried radials showed a peak gain of -0.8 dBi at 28 degrees. Its feedpoint Z was 41.6 +j 0.09 ohms, indicating the r-f ground system loss to be about 41.6 - 36.5 = 5.1 ohms.
So the system with offset, partial radials has 5.4 dB less peak gain than the other one. The reasons for that are the fewer number of radials used, the increased distances that r-f earth currents need to flow through lossy earth to reach those radials, and the fact that those currents are concentric with the vertical axis of the monopole.
(These NEC models reported no segment or geometry errors.)
Reading the interesting exposition twice, it appears to me that no advantage is gained by off-setting radials in the way shown, and in fact such an antenna is less effective than using the classic symmetrical radials with centered vertical monopole.
Why then, would someone install such an offset system?
Also, I would expect the radiated signal to be directional on the radial side.
Yes, some antenna sites are not open flat land, and it's good to know special approaches to make an installation fit the space.
I've got an irregular situation coming up, consisting of a 3-legged bamboo tripod holding up a 3-meter antenna, right at the edge of a 4-foot drop down to the driveway. The ground radials will be flat on three sides of the tower, but the drop-off will have radials buried at a down-hill angle and buried under the driveway over to the edge of neighboring property.
The driveway side radials might be shorter than the hill-top flat radials, depending on the lengths finally used.
Can you make a prediction about the performance of such an antenna design?
"Can you make a prediction about the performance of such an antenna design?"
NEC has no means to model terrain of differing elevations, but probably the elevation drop described would produce little difference in the r-f loss of this buried radial system, for about the same wire lengths in all directions.
Reportedly the TH system uses a path from the transmitter chassis and power supply via the a-c wiring in the building to reach an r-f ground supplied by a vertical rod buried at the a-c service entrance of the building. This situation is similar to the NEC model shown above.
From the analysis in the opening post of this thread, it can be seen that there would be much less loss in the r-f ground connection if the TH was used with its external ATU installed outside, a few inches above the earth, with its r-f ground terminal connected to a symmetrical set of buried radials at their common point under the base of the 3-m whip.
But using an elevated installation of that ATU with a lengthy conductor leading vertically down to the common point of the buried radials or just a single ground rod buried there would be non-compliant with §15.219(b), which might result in an FCC NOUO, however unlikely some consider that to be. The installation configuration is the operator's choice.
That was quite a shift away from the original message of this thread, offset buried ground radials for near ground transmitter/vertical installs.
Now, abruptly and without smooth transition, we are face to face with the illegality of a long ground lead from an elevated install.
All roads lead to long ground leads.
If we resolve the radial currents in Carl's example into the horizontal and vertical components there would be vertical components produced by the sloping radials. Would not this cause some directional effects due to the field of the vertical components adding to that of the monopole?
Neil
Each conductor in a set of symmetrically arranged, buried radials around the base of a monopole collects and delivers an r-f current of about 1/n of the current flowing into the r-f ground terminal of the transmit system, where n = the number of buried radials.
That small current flowing along moderately sloping radials would produce very small vertical fields, but those fields would be attenuated by the depth of the earth they were buried in, in order to add in space to the vertical field radiated by the 3-m monopole. The great majority of such radiation would be horizontally polarized -- which would not be useful for groundwave propagation in the MW band.
Many licensed, omnidirectional AM station antenna systems are installed on sloping terrain with their radials buried at a uniform depth, and I have never found any references showing that their radiation patterns had significant directional effects in the horizontal plane, because of that.
Maybe after its installation Carl can take some measurements of his system using his PL-310, and let us know what he finds.
The hillside tower is one of several outdoor projects being planned for a long time, and the slow progress can mostly be blamed on nature, especially weather.
But the hillside site is a long awaited project, where a previous tower got top heavy with flowering vines and blew over in a windstorm.
Coupled with the plan to build the outdoor hillside antenna, is the building of remote controlled carrier-up/down control so that the present Wintenna, which has served so well for a long time (metal window frame as an antenna), can be compared to the outdoor system.
Not only will readings on the TECSUN PL-310 be observed, but also those on the spectrum analyzer.
I thank you Rich, for presenting your advance assessment of the project.
(I am not sure that an "assessment" can be made in "advance," in that the word "assessment" might exist in the past tense..? )
At 4 A.M. from under the quilts in near zero weather I spent some time visualizing the monopole antenna being planned for installation this spring in the rear yard.
The tower will be a 3-legged bamboo tripod suspending a 3-meter 1"-diameter copper pipe, fed from the base by an AMT5000 in weather-box, surrounded by ground radials per PhilB's post: Ultimate Anetnna.
Comes now the tricky part for which I am seeking comment...
I want to mount any number of things at the top of the tower, above the copper pipe, including a bird house, a video surveillance cam, an STL receiver housing, and a lamp to light the area when desired. The trick will be getting power up to the top of the tower.
I want to feed video, audio, and DC power through the inside of the copper pipe.
I want to avoid running wires to the top of the tower in open air, or external to the pipe.
No doubt the wires inside the pipe will detune the antenna, but can this be compensated by retuning?
What other observations or predictions can be made?
Thank you.
I plan on using Ethernet cable (CAT6), you can run power and data over 300 ft. It's also really cheap, if you know where to get it.
The intended radiator operates in a high Q mode with a high impedance so any conductive object nearby will affect the tuning. This could be compensated by retuning but there will be losses due to capacitive coupling and less signal radiated.
"Hiding" the wires inside the pipe will also cause capacitive coupling losses.
One thing which may work would be to use RF chokes at the bottom and top of the extra wires. The radiating element would still couple to these wires but the currents would be choked off at the top and bottom and since the RF currents would have nowhere to go this may reduce losses.
It will be best to start with only the radiator on the tower, get things tuned and working, and then if you add other things you will have a baseline from which you can determine the effects of the add-ons.
Neil
Thank you Neil I knew you'd have a possible solution, and I will try exactly what you suggest and report on the results.
Meanwhile I just spotted a small article in the Mar. 1 edition of Radio World about a very unlikely AM tower location.
In "GM Journal," page 23, Dan Slentz describes a 1kW AM tower with business buildings on either side, the neighbor is a bar with an outdoor patio right at one foot of the tower. The item is lacking detail and doesn't tell us the call letters or location, but having a tower sandwiched between buildings is unique and we must wonder how ground radials are managed... perhaps through the basements of the buildings?
WDCF in Dade City also has an irregular distribution of its ground radial system. The building that house the studio and transmitter is 30 feet from the main tower. The ground systerm is fashioned from 4 inch wide copper strap that is bonded at the base of the tower.
"Why then, would someone install such an offset system?"
Such an offset system might be considered / installed due to physical and/or HOA constraints at the installation site. In the case I modeled, this was the reality being considered by the poster to which I responded.
"Also, I would expect the radiated signal to be directional on the radial side."
Others there did also, at first. But buried horizontal wires by themselves when used as an r-f ground produce no useful v-pol, far-field radiation -- no matter their physical geometry, or where they are located. All of the useful groundwave radiation from such antenna systems is produced by the vertical conductor extending above the physical earth.
So such buried conductors cannot produce useful directional characteristics to the groundwave fields radiated by a vertical monopole, no matter where those buried conductors are located with respect to the base of that monopole.