TopBand: Re: Rohn 25

[Earl W Cunningham]

On Thu, 14 Aug 1997 12:41:37 -1000 Jeffrey Herman <jeffreyh@hawaii.edu>
writes:
>Hi Earl,
>Does your computer program allow you to input soil conductivity for
>the region in which the antenna will be erected? How would the various
>soil moisture contents effect the results you computed?
>73,
>Jeff KH2PZ
===============
Hi, Jeff,
Yes, ELNEC does allow you to change the soil conductivity (which extends
from the base of the "tower" to infinity).  I use "average" soil
conductivity (5 millimhos/meter) when I model.

ELNEC always calculates impedances over perfect ground, no matter what
type of ground is input for gain calculations.  I also have EZNEC, which
does not operate that way when calculating impedances, however, EZNEC is
primarily for modeling wire antennas, where all "wires" are the same, or
close to the same, diameter, so in the tower models where there's a large
"diameter" tower and a much smaller shunt-feed wire, the results would
not be accurate.  That's why I used ELNEC instead.

------ I just took a break from writing this to see what EZNEC would do. 
I modeled a shunt-fed 1/4-wave "tower" with all conductors the same size
(1") and tried different types of soil.  The impedance results changed
drastically from one type of soil to the next, with the resistance part
always several hundred ohms, rather than the 50 ohms I tuned the shunt
feed to over perfect ground. (Reactance over perfect ground was an
inductive 430 ohms, i.e., 50 + j430  ---  about 202 pF in the gamma
capacitor would tune out the reactance at the 1830 kHz I modeled at.)

I got to thinking that it's the near-field soil conductivity (rather than
far-field) that's affecting the feedpoint impedance, so I added 4 radials
to the system (I have the new EZNEC v2.0, which makes modeling radials
easy) and voila!!  From all ranges of soil conductivity (from "very poor"
to "very good"), the resistance and reactance varied only 4 ohms, with
the resistance centered at about 53 ohms and the reactance centered at
bout an inductive 439 ohms (53 + j439).  I didn't try it, but I'm sure
that adding more radials would have made it even better.

Without any radials, EZNEC looks at the ground at the base of the
vertical as if it were just a simple rod driven into the lossy soil.  I'm
sure most hams use a better ground than that with their verticals.  That,
coupled with my experience in the real world with shunt-fed towers,
strengthens my faith in the values I arrived at by using ELNEC.

I hope this answers your query, Jeff.

73, cu on 160, de Earl, K6SE


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