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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