Topband: Ground conductivity discussions … oops

[Dennis Ashworth]

> Hi Jim … thanks for the info. Rudy’s latest method is the one I was
> planning to employ. Brian, K6STI also mentioned Rudy’s method. Good
> consensus on which method to use!
>
> Yes, all other elements were either de tuned or on the ground. I have
> checked each vertical one at a time (with others decoupled) and read 25
> ohms on all 4 elements. Such a head scratcher! I’m beginning to the my
> aluminum elements are doped with Nichrome! :-)
>
> Dennis, K7FL
>
>
> On Mon, Dec 5, 2022 at 9:33 AM James V Redding PE <jredding at ieee.org>
> wrote:
>
>> 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
>>> _________________
>>> Searchable Archives: http://www.contesting.com/_topband - Topband
>>> Reflector
>>>
>>