Topband: Ground conductivity discussions

[Grant Saviers]

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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