I cheat: http://ac6v.com/swrmeter.html
On Tue, Aug 14, 2018 at 9:56 AM, Jim Thomson <jim.thom@telus.net> wrote:
> 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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--
- 73 -
- Joe W6JWP
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