Topband: trimming elevated radials
Jeff Blaine
jeff at ac0c.com
Sun Mar 31 22:48:00 EDT 2013
Thanks Dan, very excellent stuff (as you always do!!!).
Your observation of the large shift in currents with small changes in length
is exactly the problem I am concerned with. And the N6LF data shows the
problems with asymetrical radials.
So my question was how to make uniform the currents without having to
simultaneously measure and iteratively trim for uniformity. I have gear to
measure 6 radials at one time - but that brute-force method seems less than
elegant.
There have been quite a few comments back on this and while there are some
articles published on this (beyond the N6LF QEX stuff from last year), but
they seem to not be well known; KE7BT and K5UI.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Dan Maguire
Sent: Sunday, March 31, 2013 2:40 AM
To: topband at contesting.com
Subject: Re: Topband: trimming elevated radials
Jeff Blaine wrote:
> What is the preferred method of
> tuning elevated radials for uniformity?
> [snip] ...
> There are two methods that I thought of. Measuring the
> current at the base of the vertical/radial union and
> trimming lengths iteratively trying to get a uniform current
> reading on all elements.
> [snip] ...
Lacking a good answer to Jeff's question about the preferred method of
insuring uniformity in elevated radials I decided to look at the problem
from the other direction. That is, intentionally make the radials
non-uniform and then see what the difference in current magnitude/phase
would be at the innermost point of each radial.
I started with EZNEC sample model ELEVRAD2.ez. This model was developed by
W7EL to demonstrate the correct way to model radials close to ground, so the
first thing I did was raise the entire model by 120 inches. With a 1 amp
source the current distribution as shown by EZNEC is:
http://ac6la.com/adhoc/AsymRadials1.png
Looking at the same data charted a different way confirms the expected
symmetry. The yellow "info boxes" show the Wire number (W), Segment number
(S), current magnitude, and current phase for selected segments as marked
with the green dots:
http://ac6la.com/adhoc/AsymRadials2.png
Note that in the second chart the "shape" of the curve does *not* match the
physical position of the segments. That's because in this particular model
the segments do not have a uniform length. However, the magnitude/phase
results are as expected; 1 amp at the source (Wire 1 Segment 2 [W1 S2]) and
0.25 amps at the inner end of each radial (such as Wire 9 Segment 1 [W9
S1]).
Next I modified the model to make the length of the two adjacent radials
along the +X and +Y axes be 95% of the original length (1482" vs 1560" for
the radials along the -X and -Y axes). As expected the radiation pattern is
now a bit skewed. Here's the azimuth pattern at 24 deg elevation angle:
http://ac6la.com/adhoc/AsymRadials3.png
And here's the rectangular plot of the pattern instead of the polar plot:
http://ac6la.com/adhoc/AsymRadials4.png
The really interesting result is how much the current on the radials has
changed given just a 5% difference in length. Wire 9 Segment 1 [W9 S1] is
the inner end of one of the "shortened" radials, W 27 S 1 is the inner end
of one of the original length radials:
http://ac6la.com/adhoc/AsymRadials5.png
Jeff has some pretty fancy magnitude and phase measuring equipment developed
in cooperation with Greg Ordy, W8WWV. Given the substantial changes in the
current at the inner ends of the radials with just a 5% difference in
lengths it seems reasonable that he could detect much smaller differences in
"non-uniformity" of the radials. Of course, the part about "trimming
lengths iteratively" might be more challenging. :)
Blatant plug: Most of the charts shown above were created with the AutoEZ
program. See http://ac6la.com/autoez.html for more information.
Dan, AC6LA
http://ac6la.com/
All good topband ops know fine whiskey is a daylight beverage.
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