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Author Topic: Radials above and below ground??  (Read 2973 times)
KJ6TSX
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Posts: 382




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« on: June 16, 2019, 04:26:48 PM »

Can I add radials above ground circling the existing below ground radial field?? will it help or just be a exercise in futility ?
were talking about a small 40 x 40 yard

Thanks
George
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K7KBN
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« Reply #1 on: June 16, 2019, 05:04:57 PM »

Try it.  Your license makes it possible to experiment with changes to antennas for no other reason than to see if making a change in configuration makes any significant change in performance.  Take lots of notes and keep them safe.  You may need to refer to them later.

And only make ONE change at a time.
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73
Pat K7KBN
CWO4 USNR Ret.
G8HQP
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« Reply #2 on: June 17, 2019, 03:52:28 AM »

Radials well above ground are essentially resonators, which can be arranged not to radiate if there are more than one of them and they are arranged in some suitably symmetric way. They will have relatively narrow bandwidth, so have to be the right length or include reactive loading. You can get away with just two, going off in opposite directions.

Radials below ground are essentially earth couplers. They are heavily damped so have very broad resonance; the length does not matter too much. The length for resonance could be much shorter than you expect, because the earth is both a poor conductor and a significant dielectric. Each radial has a fairly high impedance so you need lots of them wired in parallel.

Radials near the ground are somewhere in between.
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KC9QBY
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« Reply #3 on: June 17, 2019, 07:25:47 PM »

Newbie to HF here, considering vertical antenna options. One of the multiband (traps) vertical options claims raidals not required, just a good ground.  I requested installation manual.  Turns out the antenna is sold with wire for "radials". 

For verticals, most of my reading, acknowledging my impressive inexperience, spoke to many radials on ground anchored with landscape stakes and let the grass over-grow them.

The vertical option in first paragraph above recommends "radial" of resonant length that "clears-the-ground" (not buried or staked is my interpretation) on event of poor ground conductivity (mostly clay in my case) and/or poor SWR performance.

I think I now understand what is/may be working here, so thanks for the commentary. 

That said, I'll start/experiment with staked radials and single change back-fill with resonant length "clear-the-ground" radial(s) if I can avoid trip-wire hazard.

I'd appreciate any wisdom/experience on this approach.

73,

Chuck
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RFRY
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« Reply #4 on: June 18, 2019, 03:06:25 AM »

The effectiveness of buried radials depends on their number, physical length, geometric arrangement, and the conductivity of the earth in which they are buried.  The graphic below illustrates this for the range of Earth conductivities shown there.

Relatively better performance is possible with a fairly minimal set of buried radials if Earth conductivity within 1/2-wavelength of the base of the vertical monopole is fairly high (~ 8 mS/m and more).

The lower the radiation resistance of the vertical and the greater the matching loss at its feedpoint, the lower the resistance of the radial path needs to be for "good" radiation efficiency of the antenna system.

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NK7Z
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« Reply #5 on: June 18, 2019, 06:04:49 AM »

This link:

https://www.antennasbyn6lf.com/design_of_radial_ground_systems/

will answer ALL your ground and radial questions...  Rudy is brilliant, and has no issue sharing his knowledge.
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Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
WA7ARK
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Posts: 608




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« Reply #6 on: June 18, 2019, 06:47:25 AM »

Radials well above ground are essentially resonators, which can be arranged not to radiate if there are more than one of them and they are arranged in some suitably symmetric way. They will have relatively narrow bandwidth, so have to be the right length or include reactive loading. You can get away with just two, going off in opposite directions.

Radials below ground are essentially earth couplers. They are heavily damped so have very broad resonance; the length does not matter too much. The length for resonance could be much shorter than you expect, because the earth is both a poor conductor and a significant dielectric. Each radial has a fairly high impedance so you need lots of them wired in parallel.

Radials near the ground are somewhere in between.

Stated clearly and succinctly. There is much confusion in the ham literature on this topic, but G8HQP clears it up...
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Mike, WA7ARK
N8YX
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« Reply #7 on: June 18, 2019, 10:37:40 AM »

I have a 6BTV @ 33ft with 4 tuned radials per band...except 80M, where it uses 3. Another "BTV" (which currently covers 11-12-15-17M) uses a similar setup.

Both work pretty decently. Not high-gain-directional-array decently, but I have no problem making contacts with them.
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G8HQP
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« Reply #8 on: June 19, 2019, 03:50:38 AM »

Quote from: WA7ARK
There is much confusion in the ham literature on this topic
Yes, it took me a while to work it out, trying to reconcile apparently contradictory comments. I suspect that there may be some confusion in the professional literature too, at least the early stuff which is what most hams quote.

There are counterintuitive aspects too. Assume, for example, that the local ground permittivity is 9. That means that buried radials will need to be only 1/3 as long as you might expect. Now put in a radial which happens to be quarter-wave long (i.e. 1/12 long in free space terms). Put a ground spike at the far end to 'help' things along; you have probably just raised the impedance at the near end because a quarter-wave will invert impedances. Far end spikes require radials around half a wave long, or quite short radials. Then you hit the problem that spikes too close to each other don't work so well; I think I saw a rule of thumb that the horizontal spacing of ground spikes should be no shorter than their vertical length.
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RFRY
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« Reply #9 on: June 19, 2019, 05:33:12 AM »

For further consideration:

The number of horizontal radial wires used in an elevated "counterpoise" can be far fewer than the 120 wires used by most AM broadcast stations when those radials are buried in the earth.

Several AM broadcast stations in the U.S. use 3 or 4 pairs of λ/4, co-linear, horizontal radial wires installed 10-15 feet above the earth, spaced equally from each other.

Monopoles using such elevated radials have radiation patterns and measured radiation efficiencies equal to (or better) than if that monopole used 120 x λ/4 buried radials (other things equal). Wire conductors connecting that antenna system to the earth are neither needed nor used to achieve this performance.

This is true because the source of the r-f current flowing on elevated radials is a direct, wire path through the 2nd conductor of the transmission line to/from the transmitter.

The source of r-f current flowing on buried radials is the r-f current flowing on and just below the surface of the earth within λ/2 of the base of the monopole, as the result of its radiation -- which necessarily must travel through a lossy conductor (Earth) to reach those radials.

The effectiveness of buried radials does not depend on their "resonance," but the degree to which they provide physical paths of low r-f resistance able to return those Earth currents back to the r-f ground terminal of the transmit system.  The sum of those currents is equal to the current that is able to flow along the vertical monopole, itself.

Ground Rods Used With Buried Radials

From the 1937 "benchmark" experiments and I.R.E. paper about radial ground systems by Brown, Lewis & Epstein of RCA Labs:

"Another set of measurements was made in which the ground system consisted of eight radial wires, each 135 feet long. These wires were laid on the surface of the earth. The ends of the wires were terminated in ground rods. We see that this ground system is about as good as an equal number of buried wires. These data are of interest since this is typical of the portable systems used for testing possible sites for broadcast transmitters."

The 135-ft buried radial lengths each equaled 0.41 lambda in free space, at the test frequency of 3 MHz.
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K3GM
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« Reply #10 on: June 19, 2019, 09:30:18 AM »

Returning to the original question, I'm willing to bet that the OP is not speaking of an elevated counterpoise, but surface ie. above ground radials.  He has a lot 40'x40'.  Assuming the radiator is dead centered in the property, he doesnt have enough even across the corners for 20m 1/4wave elevated radial wires.  Im guessing that George here is talking about adding additional  radial wires on the surface of the soil or grass in addition to the ones he has already buried.  If that's the case, have at it.  They will supplement what you have already installed.
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G8HQP
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Posts: 922




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« Reply #11 on: June 20, 2019, 04:18:52 AM »

Quote from: RFRY
The source of r-f current flowing on buried radials is the r-f current flowing on and just below the surface of the earth within λ/2 of the base of the monopole, as the result of its radiation -- which necessarily must travel through a lossy conductor (Earth) to reach those radials.
Assuming that the buried radials are connected to the feeder outer at the antenna base, it could be said that the current in the radials comes from the feeder (and so must be equal to the antenna current) but this current then leaks out into the ground via a mixture of capacitive and resistive coupling. This views the radials as rather lossy unterminated transmission lines.

Quote
The number of horizontal radial wires used in an elevated "counterpoise" can be far fewer than the 120 wires used by most AM broadcast stations when those radials are buried in the earth.
Yes, two may be sufficient.

Quote
The 135-ft buried radial lengths each equaled 0.41 lambda in free space, at the test frequency of 3 MHz.
They might have got better results with somewhat shorter radials, given the dielectric loading of the ground.
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RFRY
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« Reply #12 on: June 20, 2019, 05:02:02 AM »

Quote from: RFRY
The source of r-f current flowing on buried radials is the r-f current flowing on and just below the surface of the earth within λ/2 of the base of the monopole, as the result of its radiation -- which necessarily must travel through a lossy conductor (Earth) to reach those radials.
Assuming that the buried radials are connected to the feeder outer at the antenna base, it could be said that the current in the radials comes from the feeder (and so must be equal to the antenna current) but this current then leaks out into the ground via a mixture of capacitive and resistive coupling. This views the radials as rather lossy unterminated transmission lines. ...

Below is a quote on this topic from "benchmark" experiments and the resulting I.R.E. paper about radial ground systems for monopole radiators.

From pp. 4, 5 and 6 of:

Proceedings of the Institute of Radio Engineers, Volume 25, Number 6 June, 1937
            GROUND SYSTEMS AS A FACTOR IN ANTENNA EFFICIENCY
                by Dr. G. H. BROWN, R. F. LEWIS, AND J. EPSTEIN
               (RCA Manufacturing Company, Inc., Camden, N. J.)

... The considerations so far presented have been based on an antenna
system free from losses, and a constant radiated power. In actual practice,
we are interested in a constant power into the antenna. Then,
with losses occurring in the system, the radiated power no longer remains
constant. It is desirable to keep these losses as small as possible.

These losses are due to conduction of earth currents through a high
resistance earth and to dielectric losses in the base insulator of the
antenna. We shall next consider the earth currents flowing toward the
antenna.

The earth currents are set up in the following manner. Displacement
currents leave the antenna, flow through space, and finally flow
into the earth where they become conduction currents. If the earth is
homogeneous, the skin effect phenomena keep the current concentrated
near the surface of the earth as it flows back to the antenna along radial
lines. Where there are radial ground wires present, the earth current
consists of two components, part of which flows in the earth itself and
the remainder of which flows in the buried wires. As the current flows
in toward the antenna, it is continually added to by more displacement
currents flowing into the earth. It is not necessarily true that the earth
currents will increase because of this additional displacement current,
since all the various components differ in phase.
...
We see that the earth currents at points more than 0.3 wave length
from the antenna are practically the same for all antenna heights. Then
the power lost in the earth beyond the 0.3-wave length radius will
stay constant as the antenna height is changed, provided the radiated
power is maintained constant. Close to the antenna, the earth currents
of a short antenna rise to large values. It would thus appear that the
earth within the 0.3-wave length radius should be a very good conductor
in order to operate a short antenna efficiently. This situation
may be roughly approximated by a buried ground system consisting
of many radial wires.
...
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W9CN
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Posts: 133




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« Reply #13 on: June 20, 2019, 06:29:45 AM »

The Legendary AM Guru, Ron Rackley along with Tom King, Bobby Cox and Jim Moser of Kintronic fame presented a paper at the April 16th, 1996 NAB Show titled "An Efficiency Comparison:  AM/Medium wave Series-Fed vs. Skirt-Fed Radiators".

https://www.kintronic.com/wp-content/uploads/2016/01/An-Efficiency-Comparison-AM-MW-Series-Fed-vs-Skirt-Fed-Radiators.pdf

The section on measuring the ground system performance is very enlightening.
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G8HQP
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Posts: 922




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« Reply #14 on: June 20, 2019, 01:33:03 PM »

Conservation of charge (a property of the universe) means that the current entering the radial system where it connects to the feeder outer must equal the antenna current from the feeder inner, however good or poor the radials are. How the current then distributes itself between ohmic current in the radials, ohmic current in the ground, and displacement current back to the antenna depends on many things.

A short antenna has low radiation resistance and so needs more current for a given power. Hence the counterpoise/radial/ground arrangements need to handle more current. A distance of '0.3 wavelengths' in the ground can be misleading, because of the permittivity of soil - this could correspond to one wavelength in the ground itself. Take a wavelength of coax and fill it with dirty water; how much power will then make it to the far end? This is a crude picture of a buried radial.

We know that the ground need not be an important part of an antenna system, because antennas can work perfectly well with no ground connection at all e.g. just using elevated radials. An exception may be if you wish to excite a ground wave, as in local AM broadcasting.
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