Topband: Fwd: Ground conductivity discussions … oops

[James V Redding PE]

Here is a link to a method of actually measuring the soil complex
conductivity characteristics and it is focused on 80M:

https://rudys.typepad.com/files/soil-characteristics-qex.pdf

 Since the depth of the measurement is a function of frequency, the numbers
for 80M may be quite different than for other HF bands.

Was also curious whether the elements were detuned for their individual
impedance measurements like would be done with a BCB array or if the 25
ohms is a common point measurement.

Jim/VEZ



On Sun, Dec 4, 2022 at 9:21 AM Dennis Ashworth <dennis at ashworth.org> wrote:

> I meant to post this to the TowerTalk group. It still may be relevant to
> Topband ops.
>
> Dennis, K7FL
>
> ---------- Forwarded message ---------
> From: Dennis Ashworth <dennis at ashworth.org>
> Date: Sat, Dec 3, 2022 at 8:54 AM
> Subject: Ground conductivity discussions
> To: <topband at contesting.com>
>
>
> Very interesting and timely discussions on radials and ground conductivity.
> I’m currently rebuilding an 80M broadside array (with shortened, top loaded
> elements) in SW Utah that I’ve modeled at 12 ohms impedance. The current
> antenna was tested and the impedance measured was 25 ohms. Each element in
> the array (4 total) also measured 25 ohms. What accounts for additional
> system loss?
>
> Upon consulting the original builders, I learned they had also predicted an
> impedance of approximately 12 ohms. I’m not clear what methods or models
> they used for their prediction. There are 4:1 baluns at the base of each
> vertical which begs the question whether the array impedances were ever
> checked post-install. I suspect not … and I doubt anything has changed over
> the years that would equally affect the impedance *on all 4 verticals.*
>
> Where I don’t blindly trust models (antennas or otherwise), I do believe
> the 12 ohm figure is reasonable given the short, top loaded elements. I
> reviewed the FCC conductivity tables for the locale and they indicate 15-30
> millimos/meter. That’s pretty good! I would think the loss from a ground
> system of 32, 1/4 wave plus radials would NOT account for the 12 ohms of
> loss ground losses. But what if my ground conductivity is less than the FCC
> tables report?
>
> I’m going to the site again next week to install 26 additional 1/4 wave
> radials on one of the verticals and see if (and how much) the measured
> impedance drops. I’ll share my results here.
>
> This loss has to be a ground system issue. If so, adding radials and seeing
> a corresponding drop in impedance should confirm my suspicions.
>
> At some point, I’ll measure the ground conductivity, but it needs to wait
> for warmer temps (current temp at site is 19F!).
>
> Insight from the masses always appreciated.
>
>
> Dennis, K7FL
> Las Vegas, NV
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