Topband: Ground conductivity discussions

[Jeff Blaine]

Grant, that higher Rr is the path I took as well.

My 160m antenna is a bit longer than 1/4wl - trimmed in length so that 
the resistivity component of Z was 50 ohms.  It's got Xl of course, so I 
use a series C bread slicer at the tower base to to cancel the Xl.

My thought back then was that whatever the ground loss was, it would be 
about 1/3 less if the native feedpoint R was 50 ohms instead of 
something more close to a true 1/4 WL of around 35 ohms.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 12/4/2022 11:19 AM, Grant Saviers wrote:
> Sounds like a great project.
>
> Isn't the reality that conductors (radials) in or near lossy mediums 
> (earth, even salt water) have loss?  And that the near field extends 
> beyond the 1/4wl of the radials?  The coupling among elements might be 
> the reason 4:1 matching was used.
>
> So while more & longer radials reduce the loss, there is a limit.  
> Note that my prior post for my antenna shows the feedpoint Z equals 
> the Rr value of about 14 ohms only when the radials are elevated 
> 100ft.  Often called a ground plane antenna.
>
> I played a bit in Eznec with my antenna and a lot (128) more radials a 
> few inches above your ground and saw essentially no change in Rg.  
> Your higher conductivity ground will improve the pattern - more gain 
> at lower angles.
>
> So I suspect adding 26 more radials to the existing 32 1/4wl will not 
> make much improvement in Rg.  I think Severns and Christman show this 
> in their papers.
>
> Other solutions to lower ground loss is higher Rr of the antenna to 
> improve the ratio Rr/Rg.  eg 1/4wl tall verticals. The center fed 
> 720ft BCB antennas eg KDKA are another solution. ie vertical dipoles.
>
> Grant KZ1W
>
> On 12/3/2022 08:54, Dennis Ashworth wrote:
>> 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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