de K6NA 2 April
Towertalkians interested in guy wire effects on sidemounted Yagi antennas
might like to read of my experiences. I've been conversing with Steve,
KJ7CH about Nico-Pres tools, etc. and decided to mention a couple of related
things to him, hence this note to TT.
Good deal Steve, glad you are a swaged-fitting convert. You will be happy!
One note: practice pushing the sleeve just "the right" distance along the
wire, next to the ceramic insulator. If it ends up too loose, the insulator
can flop or get partially turned before you tension the guy. If it's too
tight (too close to the insulator), you can actually break the insulator as
you install the sleeve, because the material creeps or elongates a bit while
you make the multiple presses required, and the wire loop can get too tight
around the ceramic. "Just right" is snug, but not broken.
Ok about the JA version. The Nico-Pres brand name is used by I think the
American Telephone Supply Company from Ohio. And it is absolutely great
quality... I don't think the JA's can improve on it, but could certainly
equal it.
Only thing is, be absolutely certain that their sleeves are CORRECT for your
wire size... wouldn't they be making a millimeter-type tool whereas we spec
all our wire in inches?
An electrical note: Everyone including the Antenna Book says we can make
the wire guy sections 27-29 feet long between insulators. I built my two
140-foot towers using that method. I had a 3-foot pigtail from the tower
guy bracket to the first insulator, and then 28-foot sections all the way to
the ground. Well, my 20 meter stacked array of two 5-EL yagis did not work
correctly. The top antenna is at 140 feet, and the lower identical one is
just above the 60-foot guy level on a rotating sidemount. The guy radius is
105 feet. This means the 60-foot guys are very "flat-looking", i.e. almost
"horizontal". The KLM driven elements of the lower beam are right out over
these guys, and after observing what was going on, I had to make changes.
Turns out the beam was really coupling to the 28-foot wires right under them
(like inefficient directors!). This messed up the pattern, and there were
three broad but distinct bulges (every 120 degrees) in the VSWR curve of the
lower antenna as I turned it around the tower.
The fix was tedious but successful. I rebuilt the upper portions of those
nearby guys, one at a time. I installed more insulators, so that each
28-foot wire became three nine-foot wires, in the area near the beam. I did
this on both the 60- and 90-foot guy levels. The lower beam, and thus the
stack, then worked correctly. There is also a 15-meter beam on a sidemount
just above the 90-foot guy bracket, so the additional break-up of the
90-foot-level guys is probably helping that antenna also (it really works).
Note that 28 feet is something over a half-wave on 15, so there is a big
chance of coupling there too, I think.
I related my experiences to K6STI a few years ago. He became interested in
the problem, and ended up writing his guy wire analysis program. I had
fixed the problem already, but he told me that the model did in fact show
the problem.
Bottom line: Within about, say, 50 feet of the tower legs I recommend guy
wire insulators every nine feet if you are using side-mounted yagis, alone
or in stacks. Beyond about 50 feet out, I don't think it makes much
difference, 28 foot lengths are probably fine, or even no insulators at all
are probably o.k. in those lower areas far out from the antenna (unless you
also have some vertical arrays in the vicinity...but that's another matter).
Sidebar: Slope angle of the guy obviously has something to do with the
amount of potential coupling. That's why the top antenna on a tower usually
works fine even with no insulators at all in the guys. The top set of guys
slopes steeply away from the vicinity of the yagi and coupling would be
minimal. Modeling fans can look at these things and decide.
73, Glenn K6NA
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