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Author Topic: Radials for vertical antenna  (Read 3171 times)
K4EZD
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« on: July 13, 2013, 10:16:51 AM »

I have an area of my back yard that I use for a ground mounted vertical (DXE 24' multiband) and currently have 16 radials attached to the DXE ground plate at the base.  At one end of this lot is a fresh water lake and a wood bulkhead.  Would there be any advantage in extending the radials on that side into the lake or even driving metal rods into the edge of the lake and attaching the ends of those radials on that side? Locals I asked about this said that it would not hurt anything and that I should try it but I wonder if there is a technical reason to do or not to do this.
Thanks.
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KB4QAA
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« Reply #1 on: July 13, 2013, 11:17:29 AM »

Short answer:  No.

If you want to improve your antenna performance, add more radials.

Radials are not intended to be coupled to earth/water.  The purpose of radials is to provide a lower path of resistance for the return currents.  Copper is over 1000 times more conductive than earth, (I don't recall the comparison to water).  Fresh water is  not very conductive anyhow, but 'skin effect' at RF means that RF doesn't conduct very far anyhow (a few feet).

Modeling with EZNEC or other antenna software will show that adding or subtracting radials on one side of an antenna system does not significantly alter the pattern, and certainly does not give directionality like a Dipole, or Yagi.

Being within a few wavelengths of water can provide benefits to your antenna pattern (in the same way buildings or structures can detract from it), but this is unrelated to radials.

Rudy, N6LF has a treasure trove of easy to read info on antennas.
http://www.antennasbyn6lf.com/
« Last Edit: July 13, 2013, 11:19:31 AM by KB4QAA » Logged
WX7G
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« Reply #2 on: July 13, 2013, 11:19:09 AM »

Extending ground mounted radials beyond 1/8 wavelength does not significantly improve the performance of a vertical. If you operate 80 meters radials, or a lake, beyond 32' will add little gain. If you operate 40 meters 16' is the length.
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W8JX
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« Reply #3 on: July 13, 2013, 11:19:34 AM »

I see no advantage of driving a rod in at end of radial as it might actually detune things. If you want to do anything just add some more radials and maybe double what you have now but you will reach a point of diminishing returns.
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All posted wireless using Win 8.1 RT, a Android tablet using 4G/LTE/WiFi or Sprint Note 3.
RFRY
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« Reply #4 on: July 13, 2013, 12:22:25 PM »

Quote
...Extending ground mounted radials beyond 1/8 wavelength does not significantly improve the performance of a vertical.

The r-f currents on radial wires whose complete lengths either lie on the earth or are buried in the earth originate as displacement currents from radiation by the vertical monopole.  These currents enter the earth within a 1/2-wavelength radius from the base of the monopole, and circulate from there back and forth along the buried wires to their common-point under the base of the monopole, which becomes the "ground" connection for that transmit/antenna system.

For maximum radiation efficiency of the antenna system those buried wires need to be long enough and dense enough so that the r-f currents within their skin depth in the earth in that 1/2-wave radius do not need to travel very far through the earth before encountering buried radials -- which radials provide a much lower resistance path back to the transmitter ground terminal.

This can be seen in the data linked below, which also shows that if only a relatively few buried radials are used, they may as well be fairly short.

OTOH, as the number of buried radials increases they continue to reduce losses in the r-f ground system when using buried wire lengths to least 4/10 of a free-space wavelength.   The data for a 70-deg monopole shows that 30 x 0.4-wave buried radials loses about 23% of the applied power in r-f ground losses, but with 120 x 0.4-wave buried radials the ground losses drop to 2%.

http://i62.photobucket.com/albums/h85/rfry-100/GndSystemLosses_zps0b36c41e.jpg
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W4VR
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« Reply #5 on: July 14, 2013, 02:06:42 PM »

Short answer:  No.

Fresh water is  not very conductive anyhow....



Fresh water can actually be more conductive than soil...depending where you live of course.  The mineral content in fresh water would determine it's conductivity.  For example, the FCC conductivity map of the US labels the great lakes as having a conductivity of 8 mS/m.  Average conductivity in Maine, where I live, is 1 mS/m.
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KE3WD
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« Reply #6 on: July 14, 2013, 02:39:15 PM »

It is only pure water that is nonconductive.  And there won't be any pure water found in lakes, streams, rivers or saltwater oceans. 

However, trying to use the water as a counterpoise for a vertical antenna that is on land isn't good strategy, for other reasons. 


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KB4QAA
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« Reply #7 on: July 14, 2013, 04:18:39 PM »

The point of the discussion is that copper radials are a zillion (technical term) times better conductor than either soil or water.  Radials are not intended to be couple to either soil or water!!!   It far more efficient of time, $, and resources, to simply improve the radial system.
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K5LXP
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« Reply #8 on: July 15, 2013, 07:36:24 AM »

The point of the discussion is that copper radials are a zillion (technical term) times better conductor than either soil or water. 

Or even seawater.  Antennas located even in the proverbial salt marsh, while better than dry sand, still requires radials.


Mark K5LXP
Albuquerque, NM
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WX7G
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« Reply #9 on: July 15, 2013, 08:09:54 AM »

A salt water vertical at amateur power levels can perform well with no radials. The resistivity of sea water is 0.2 ohms/meter. The skin depth in sea water at 1.8 MHz is about 1/3 meter and a 1" metal pipe 12" long provides a spreading resistance of less than 3 ohms.

I use a 1" copper pipe for the ground connection for balloon verticals at the Great Salt Lake of Utah. For ocean RF grounding I use the keel of a sail boat.  

« Last Edit: July 15, 2013, 08:36:08 AM by WX7G » Logged
K5LXP
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« Reply #10 on: July 15, 2013, 10:10:09 AM »

A salt water vertical at amateur power levels can perform well with no radials.

But, it would perform "better" with wire radials.  While it may perform "well" with this novel balloon installation on 160M, the higher you go in frequency the worse it gets.  It's not the panacea HF ground plane many make it out to be.  To the point where you get guys that think just by tossing a wire in their backyard swimming pool it will actually make a difference.

In the sailboat scenario, even with a non-optimum seawater groundplane there is the benefit of having a clear shot to the horizon and the saltwater enhancement of vertically polarized signals.


Mark K5LXP
Albuquerque, NM
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WX7G
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« Reply #11 on: July 15, 2013, 11:32:53 AM »

You are correct that as the frequency increases so do sea water ground losses.

Using a 1" pipe as the ground connection the spreading resistance by band is:

160 meters, 3 ohms
40 meters, 6 ohms
10 meters, 12 ohms

Now use a 4" pipe just 3" long and the loss on the 10 meter band falls to 3 ohms. Use a 12" pipe - similar to several 12" radials - and the loss is 1 ohm. My use of the sail boat lead keel provides a ground resistance that is essentially zero.

« Last Edit: July 15, 2013, 11:35:55 AM by WX7G » Logged
G3RZP
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« Reply #12 on: July 15, 2013, 01:44:08 PM »

When I measured the feed impedance of my folded unipole, it didn't change as I added radials, either lying on the ground or elevated. Neither could I measure any current in them. The ground is very wet blue clay from about 6 inches down......So I don't use radials, just the four 8 foot ground rods.

Worked Heard Island on 160 and broke a few pile ups on 80, so I guess it's not doing too badly....

Any suggestions as to why they made so little difference?
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N3DT
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« Reply #13 on: July 16, 2013, 08:17:14 AM »

I still can't understand why anyone wants to put up with ground radials.  Look up C-Pole antenna and the article by Brian Cake, KF2YN, and rid yourself of the issue.  De-couple the antenna from ground and live radial free.  Plus it works on twice the freq too.
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