[TowerTalk] supporting a 40 meter rotatable dipole with trusses

[Jim Thomson]

Date: Mon, 13 Aug 2018 16:40:04 -0700 (PDT)
From: terry burge <ki7m at comcast.net>
To: TowerTalk at contesting.com, terry burge <ki7m at 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