Topband: trimming elevated radials
djm2150 at yahoo.com
Sun Mar 31 03:40:04 EDT 2013
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:
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:
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:
And here's the rectangular plot of the pattern instead of the polar plot:
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:
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.
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