A couple of problems with dacron (which may really be polyester)
1. stuff grows on it, my Synthetic Textile lines slowly turn moldy,
faster than in CA, slower than some of the knockoffs. Probably a better
conductor as a result.
2. stretch - expect about 3% at 20% of break strength
Very good dacron/polyester such as New England Ropes Sta-Set is
prestretched so has less initial stretch, but still has prob 2.
Dyneema and Kevlar core lines have 1% stretch at 20% of load, but still
have problem 2
I can't find any Phillystran specs for the current stuff that works with
grips. When I test stand measured the parallel strand "old style" it
was within a few percent of the raw kevlar tensile modulus of elasticity
- essentially zero stretch: 16e6 modulus vs steel at 30e6 psi. (NCJ
Nov-Dec 2014) I expect since the new Philly has twist it will stretch a
bit more, but not as much as braided dacron/polyester etc.
I think an 80m dipole antenna will be slightly influenced by a big grip
electrically connected at the truss attach point. An easy thing to model.
All of my trusses are Philly.
Most every guy material is not expected to by subjected to more than 40%
of break strength. Even EHS. Almost everything that can be bent starts
to permanently stretch at that tension.
Grant KZ1W
On 3/31/2017 12:38 PM, Jim Thomson wrote:
How good is dacron rope, as an insulator for HV? I am planning on using
.3125 dacron rope as an overhead truss for the shortened 80m rotary dipole.
One length going out each side of the mast. They will terminate on the mast,
aprx 4 ft above the 80m dipole, with a small turnbuckle on each side of the
mast.
The other ends will terminate 15 ft out each side of the main trunk section,
which is 2 inch OD tubing. This will also be just a few inches inboard of
the T bar
capacity hats. Balance of loading is done with the tornado drive, which
consists of a pair of compressible 6-12 uh coils at the feedpoint.
Going out 15 ft will be at a HV point, esp since thats also way outboard of
the 6-12 uh coils at the feedpoint.
Will 15 ft be enough to stand off the peak V....esp when soaking wet ? IF
each side is opposite polarity, then it gets even worse, as the potential
difference
right at the mast could be substantial under some conditions. Or do I need to
install an insulator on either side of the mast, right at each turnbuckle ?
Id rather not have to install insulators if not required, but have no
experience with dacron + HV.
Jim VE7RF
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