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Ground conductivity

Jul 28th 2011, 18:36

W0OLL

Joined: Apr 4th 1998, 00:00
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I am installing a flafpole antenna and I keep reading about radials and "GROUND CONDUCTIVITY"

How can you measure ground conductivity?

I am in central Florida and I think the soil is mostly sand. I am using buried radials=8.

Thanks for comments.

73's
W0OLL Ray in Leesburg, FL
Jul 28th 2011, 23:55

W5CPT

Joined: Apr 4th 1998, 00:00
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Here is a link to a Home-Brew method:

http://amfone.net/ECSound/K1JJ12.htm

and here is a map that will give you a good approximation:

http://filebay1.home.comcast.net/~filebay1/ground.jpg

Now that you know, there is not much you can do about it except what you have already done, which is to put down radials. If you can measure the feed-point impedance of the antenna. If it is about 35 ohms you have a good radial field. If it is higher than that you will benefit by adding more radials. More and longer is better, up to a point.

Clint - W5CPT
Jul 29th 2011, 00:14

W0BTU

Joined: Apr 4th 1998, 00:00
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Quote by W5CPT
... there is not much you can do about it except what you have already done, which is to put down radials. ... More and longer is better, up to a point.


Well said. I would put my time and energy into laying down more radials, instead of measuring something that you can do nothing about. :-)

Adding more than 8 would likely help you; more than 60 is a waste of time and money, as this chart shows:

http://www.w0btu.com/Optimum_number_of_ground_radials_vs_radial_length.html

(That chart there only goes down to 16 radials. Does anyone have some figures I could add there for 8 and 4?)

And they don't need to be buried. Stapling them to the surface of the earth is not only easier, but most effective. After a few months, they'll disappear into the grass.

73, Mike
www.w0btu.com
Jul 29th 2011, 02:38

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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One can measure the ground conductivity at the surface, but one doesn't really know what is going on well below the ground. Especially at low HF, the skin depth of the ground as a conductor can be surprisingly deep.

In a discussion about this with Roy Lewellan, W7EZ, the author of EZNEC, he told me about work done by him and Tom Rauch on Rauch's veritcal system. No matter what ground conductivity models they tried, the could not match the feed point impedance and SWR curve of the actual antenna, even with a good radial system in place. Lewellan's theory is that there were layers in the ground, each with a different conductivity and that the phase shifts and other effects were acting differntly in the real world than the actions of a simple model. The model yielded useful results, but Lewellan was convinced that other factors were present.

For most antenna types, it is probably sufficient to estimate the conductivity and dielectric constant based on typical environments, such as average ground (old pastoral farmland), very good (wet, swampy) and poor (dry sandy, rocky).

Radials are only part of the ground issue. For a veritcal, a good radial system will reduce the ohmic losses that exist between the antenna and that ground that is down there somewhere. Radials will increase your signal strength. But the elevation angle performance of an antenna is detemined by the interplay between the direct ray from your antenna at upward angles with the ground relection that takes place from a ray from downward angles. For low angles of radiation -- ie, the "DX" angles -- the reflection may take place many wavelengths away from your antenna. Poor ground tends to lose the reflection and change the phase to be cancelling instead of adding. You don't have a radial system many wavelengths away, so the low-angle performance is determined by the ground that's quite a distance away and beyond your control.

73,
Ed Hare, W1RFI
ARRL Lab
Technology forums moderator

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