[TowerTalk] Short Ground Rods??

[Jim Lux]

At 05:29 PM 5/19/2005, Red wrote:
>Hi, Jim;
>
>Are you referring to the measurement method published by George Hagn?

That's the one.  The OWL (Open Wire Line) kit.

>
>Are all the necessary details in the more recent Antenna Books?  My 19th, 
>15th, and 13the

The 2004 edition (the one that came with HFTA) has the dimensions and such 
in it.
I don't remember if it had the equations you need.  Basically, you build a 
transmission line that has an impedance of around 300 ohms (in air).  You 
can use the standard open wire line equation.

You need to make several measurements with different length rods to get a 
good measurement.  Mostly to check for surface layers, etc.

The other problem is that the impedance isn't close to 50 ohms.  I've 
discussed this a bit with George, and when last I talked to him, he thought 
that some sort of 4:1 or 9:1 transformer might get close enough that a MFJ 
259 type analyzer can read it. He used an impedance bridge.

One interesting point is that you don't really need to do it at a lot of 
different frequencies, because over the HF range, the properties don't vary 
much (the impedance will, but sigma and epsilon don't)

>editionsoffer only a general description and do not include details.
>
>The method published in each of those books is that published by J. 
>Sevick.  As published, it won't work if plugged into a GFI protected 
>circuit, and it measures only conductivity, not dielectric constant.  I 
>modified it by adding a 24 volt transformer to isolate input power from 
>ground, by omitting the light bulb, and by using conveniently available 
>resistor values, with the necessary change in the scaling constant to 
>calculate conductivity.

That method just gives conductivity (it's basically like a 2 or 4 probe 
Wenner measurement), and you really also need the epsilon, especially if 
you're working with vertical polarized waves.



>I'm interested in learning more about Hagn's method, as implemented with 
>instruments readily available in the ham shack.


I've got a copy of Hagn's paper from the APS conference, where they 
reported making measurements at LLL (interestingly, to be used in 
validating NEC).


>Can anyone share more?
>
>73 de WOØW