Date: Mon, 13 Aug 2018 16:40:04 -0700 (PDT)
From: terry burge <ki7m@comcast.net>
To: TowerTalk@contesting.com, terry burge <ki7m@comcast.net>
Subject: [TowerTalk] supporting a 40 meter rotatable dipole with
trusses
<Been working on my 40 meter rotatable dipole I plan to put up at 124 feet or
so. But I'm worried about making sure whatever I come up with to fix the 3/16"
dacron truss lines to the elements out about 20' or so are secure. Dealing with
the DXE tubing when <it gets down below 1" or so I don't want anything too
heavy. Also don't want to drill and weaken the dipole. I actually came up with
some 'rings' from Wilco just about the right size that would slip over the
0.5", 0.625", 0.750" to I believe the 0.875". There I <could use 1/2"-1 1/4"
hose clamps to make sure the ring doesn't slip further up. Just the dacron
wrapped around the ring might be enough to keep that from happening but I don't
trust it.
<I'm not sure I could trust just using the hose clamps to hold the dacron line
but I don't want to drill and weaken the tubing in any way.
<Any better idea? You can see a few pictures of what I'm building on my
QRZ/KI7M page.
<Terry
<KI7M
## Ken, W3JK, uses a 2 piece machined aluminum clamp, thick, rectangular,
that goes around the tubing in question.
On the top part of the clamp, a piece of 1 x 1 x .125 angle aluminum is
bolted, using the same pair of ss bolts as the
2 x piece clamp. The vertical part of the angle al is where the truss
terminates... with just a hole in the vert portion of the
angle al. Use a small thimble etc, so the dacron does not bear down on the
edges of the hole. Ken will make a couple of pairs
for you, if he knows the exact tubing OD in question. Then they wont slip.
## I looked at your pix. I would use better mast to rectangular plate
clamps.... vs the muffler style clamps you show in the pix.
Use a pair of the dx eng style muffler clamps.. the ones that have the SOLID
al saddle, then they have loads of surface area. The SS
u bolts that come with them are extra long threads, that will handle any
thickness al plate. You also
need a solid fiberglass center insulator. Without it, you have way too much
stress right in the middle.
## heres the real concern. Did you run the design through something like
the DX eng yagi mech software program ???
66 ft long and starting with just 1.25..and down to the typ .375 even
with thick, inboard sections, is going to have a tough
time with high winds... esp with dipole broadside to the wind. If we knew
the exact tubing schedule used, which means exposed
tubing lengths and also overlaps, and wall thickness, or if any inner liners
used etc, etc, then it can all be run through the dx eng mech
software very quickly.
## Your pair of over head truss lines wont buy you anything for increased
wind speed, only sag, and sag from snow and ice loading.
You also require horizontal dacron truss lines to handle increased wind
speeds. Horz truss lines will also minimize horizontal
deflection for XXX wind speed. If the ele deflects too much, then you
also get this effect called... wind induced swr.
## The dx eng yagi mech program will spit out how much vertical..and horz
deflection you will get..and also factor in both vert
and horz truss lines.
## At 1st glance, I just cant see your 66 ft long ele surviving...when its
only 1.25 inch OD in the middle. 1.25 is what you would typ
use for the center of a 20m ele. For a 66 ft long dipole, 2.00 inch is typ
used..then tapering down to .375 at the tips. Even then, the inboard
15 ft on either side of mast is typ heavy walled material, like .125. And
thats just to get it to survive a 85-90 mph gust. Without a horz truss,
the ele deflection is insane with high winds. Typ a single horz truss is
used, and terminates 15 ft from the mast.
Jim VE7RF
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