On Thu, 5 Jun 1997, Bill Fisher - W4AN wrote:
> I used unbroken guys on the bottom set of my 15M stack. The SWR on the
> lower antenna was different (higher) than on the upper antenna. When I
> switched this guy out for phyllistran last fall the lower antenna exactly
> tracked the top one's SWR curve. You can't convince me that unbroken
> guys are a good thing. But you can't convince me that broken guys are
> either. The only thing I trust for non-interaction is Phyllistran. I've
> heard stories about broken guys affecting antennas adversely also. From
> guys that knew what they were doing.
When we had only the harmonically related bands, finding guy lengths that
were relatrively non-interactive was easier than today, with the WARC
bands added. However, this note is about the focus on SWR.
Even an SWR curve that seems to track the specification for the antenna
may be misleading. If only the SWR is at stake, then some simple
adjustments to a driven element may correct the problem. However, the
real question is the effect upon the overall beam performance in terms of
pattern shape in all directions, gain, and front-to-back (or to-rear).
I have no magic answer to the question of broken or unbroken guys, but I
can suggest that one go beyond SWR checks and evaluate antenna performance
in all dimensions before being content with the installation. A
satisfactory SWR is a partial indication of proper antenna performance,
but not a full evaluation.
Modeling will provide a furtheer check on a proposed site and
installation, but its best use is likely on a case by case basis as one of
the many planning tools. The model will only be as accurate as the input
data and limited by program limitations. MININEC has trouble with angles
and a limited number of segments (remembering that guys must also be
segmented for each band according to program rules). NEC-2 has
difficulties with wires of differing diameters, and only linear elements
are corrected by a stapped-diameter algorithm. NEC-4 is better in this
respect and without segment number limits but it is not perfect. Hence,
it is a fairly good indicator of trouble, but when clean, does not
guarantee that reality is also clean. I have yet to see a usable model of
wet and dry pine trees and other site paraphernalia that surround most of
our antennas.
One general rule of modeling that I try to follow is this: for
generalized studies, I refer to the results as suggestive, not definitive.
For models to be built, I try to specify how far apart the model and the
constructed antenna are. Both have variables that introduce differences
between them.
Hence, even with the best modeling, every installation requires
post-assembly adjustment and refinement. The more complex the
installation, the longer the period of adjustment that we should plan into
the exercise.
Pardon the repetition of such generalized common knowledge, but I
sometimes get the impression that some folks would like antennas to be
"set-and-forget" exercises until a storm knocks them down for the next
one.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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