Topband: trimming elevated radials

[Jeff Blaine]

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.
_________________
Topband Reflector