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[TowerTalk] need advice on 80m delta loop

To: "Tower (Tower)" <>
Subject: [TowerTalk] need advice on 80m delta loop
From: "Dick Green" <>
Date: Sat, 8 Nov 2003 23:15:52 -0500
List-post: <>
I've had an 80m delta loop suspended at about 75-feet from a tall spruce
tree for three years and the wire keeps breaking at the corners or at the
apex. I'm probably putting too much tension on it, so when the tree bends in
the wind the wire breaks. But I need to put a fair amount of tension on the
loop to keep the base wire from drooping too close to the ground. I've been
using #12 stranded insulated THHN, which seems like it should be strong
enough for the application. I'm wondering if I would be better off using a
different kind of wire -- perhaps #12 insulated Flexweave or # 10
Copperweld. Does anyone know the relative breaking strength of these wire
Another approach might be to find a way to keep tension on the wire, but
provide enough give to compensate for tree movement. I'd be interested in
any recommendations or ideas from people who have successfully kept
tree-supported delta loops up for a long time -- what kind of wire did you
use and how did you support the apex and corners?
Here are the construction details of my loop:
Originally I had a friend shoot a pull rope over a high branch of the spruce
tree -- about 75 feet up. I pulled the # 12 THHN up and across the branch
with the pull rope (i.e., the apex was supported by a branch. Per W4RNL and
others, the feedpoint balun is inserted in the loop about a 1/4-wave down
from the apex, about 12 feet from one of the corners. I supported the
corners with glass dogbone insulators attached to 1/4" black dacron rope
which runs to pulleys that are mounted in trees about 15 feet up. The loop
doesn't hang quite symmetrically: one corner is higher off the ground than
the other. 
In this configuration, the loop broke at the lower corner several times over
a period of three years. It was easy to lower the insulator, grab the rope
ends, slip the insulator back on, and solder the wire back together. At one
point I got tired of this and replaced the dogbone insulator with one of
those heavy black rubber bungee straps. This provided a little strain relief
and a gentler interface with the wire. I have successfully used these straps
with my 160/80 inverted vee, which is also suspended from a tree. This
configuration lasted about a year, then a month or so ago the loop broke at
the other corner (still had a glass insulator there.) It was not possible to
reach the free end of the wire -- it was too high up. I tried to snag it
with a pole saw (blade removed) and pull it down, but the wire was stuck in
a crotch of the branch. I had pulled a backup rope up with the wire, but
pull rope was stuck in the crotch, too!
Determined to provide a reliable backup pull rope and minimize the
possibility of breaks at the apex, I hired a tree climber to go up there,
cut down the old rope and wire, install a double-wheel pulley, and thread a
couple of pull ropes through the pulley -- one to pull the wire and another
for backup. I used one pull rope to raise a replacement wire and connected
it to the balun. I used a single-wheel pulley at one corner and the same
heavy black rubber bungee strap at the other corner. I figured a pulley at
one corner would provide some give for the wire and a gentler interface, but
thought that pulleys at both corners and the apex would allow the wire to
reorient itself too much -- it might start rotating around the tree! So, I
used the bungee strap at the other corner to keep the wire from slipping.
BTW, I actually got the tuning right on the first try!
This configuration lasted all of one week. At the first moderate windstorm,
maybe 25 MPH, the wire broke at the apex. I guess when the wire ran over a
branch there was more give. The pulley seems to have eliminated the strain
relief at the apex, so I need to either provide more strain relief at the
corners or use stronger wire, or both. What's a good way to provide some
give at the corners without letting the loop sag too much? Springs? weights?
Is THHN not a good choice for this application? Any other ideas?
I'd appreciate any comments from those who have been down this road.
73, Dick WC1M

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