[TowerTalk] [Bulk] Re: Phillystran

[Jim Thomson]

Date: Thu, 22 Oct 2015 23:10:44 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
To: Grant Saviers <grants2 at pacbell.net>, towertalk at contesting.com
Subject: Re: [TowerTalk] [Bulk] Re: Phillystran


The 1200# test makes a good truss material except for very large 
antennas, or a shallow truss where the center  is relatively close to 
the boom. I've had no problems using the 1200# test well within its 
ratings.  I wish I had replaced the Al tube truss on the 7L C3i 6-meter 
Yagi with the 1200# Phillystran when I first put it up.  True, it's not 
a huge monobander, but at a couple inches shy of 30 feet it does need a 
truss.  A flock of Cormorants using the antenna as an over night roost 
broke the tube truss.

There was mention of pretensioning of the truss.   That would not be 
possible on most antennas, however once the cables are cut to length and 
properly terminated, they can be pretensioned in an external fixture, 
but as the tension in use will likely be less than the usual 10% 
operating tension I doubt there would be any need to do so.

73

Roger  (K8RI)

###  I believe in a lot of cases, for a boom truss, that the truss does not terminate
high enough above the boom !   Say you had a  40 ft boom, with a truss on either
side, and terminated 2 ft above the boom.   Lets take the same 40 ft boom, and toss
20 ft of it away.....one half.    Then lets flip the boom so it stands vertical..sitting on the
ground.   You would not guy a 20 ft tall mast, with guy lines that are only 2 ft out from the
base, more like  16 ft or 80% of the height.   Im not suggesting to raise the truss  16 ft above
the tower, but another 1-3 ft higher would reduce the stress on both the boom and the truss
line material.   In some cases, esp on long booms, you would be better off to use 2 truss lines
on either side, like 10 ft on either side of the tower, and again at 20 ft on either side of the tower. 

##  The old way of a double truss, with a short horizontal cross member mounted  a few ft up the
mast, with 2 truss lines on either side..terminating to the far ends of the boom.  The thought at 
the time was the assy would provide for some lateral support, from winds that are broadside to the
boom, they don’t.   The 3 point truss, like m2 and klm used,  will provide for lateral support.   Even them,
the 3 x   truss supports were several feet long each. 

##  I have never used philly for truss material.   5/32  winch cable in either galvanized or SS  works just
fine.   Then use nicropress crimps at the far ends of the boom..and cable clamps + turnbuckles at the
mast end of the truss. 

Jim  VE7RF