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Re: [TenTec] New and Improved Terminology (NVIS origins)

To: tentec@contesting.com
Subject: Re: [TenTec] New and Improved Terminology (NVIS origins)
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
Reply-to: geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sat, 08 Jan 2011 18:29:27 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
My dad and I did that to a rotating pipe mast in the late 1950s. 60 feet 
of 4" galvanized pipe with a bearing and guy wires at about 565 feet. It 
stood (except for the times we took it down to work on antennas) until 
about 2004 when a contractor that coveted the land cut a guy wire. 
Unfortunately my dad had moved his licensed address away from home so I 
couldn't sic the FBI onto the contractor and I was wondering how I was 
going to take it down anyway.

The bending strength a tube is proportional to the diameter to the 4th 
power. So a little loss in height costs a big loss in bending strength. 
A 10% loss in diameter results in a bending strength down to about 63%. 
So in a tube its vital to maintain full diameter. Many times a wooden 
dowel is fitted where the bending stress is highest, not nearly so much 
to add the stiffness from the dowel but just to keep the tubing from 
collapsing. A tube collapsed to half height has 6% of its original 
bending strength and soon will have collapsed to two metal thickness 
were it bends (and sometimes breaks). Column load is more complex but 
bending is a part of failure if it doesn't buckle. Wood inserts help 
slow buckling too. I'm not so much a fan of foam, I don't think its 
strong enough in the tube to keep the tube round and full diameter, 
though foam faced with very thin metal shows a lot of strength, I think 
that structural foam is a lot more dense than foam in place out of a can.

Stay wires may contribute to the endwise crushing of the tubing to lead 
to a buckling failure, but when I've had stay wired masts fail, its been 
because the stay wire opposite the wire antenna load failed in tension.

73, Jerry, K0CQ

On 1/8/2011 5:59 PM, Richards wrote:
> Cool.    That is a new one on me.   The photo really helps me visualize it.
> A local ham suggested I fill the tubing with that expanding foam
> insulation stuff that comes in a aerosol can... thinking it would
> expand, and become stiff and dampen any vibration from flexing.   I had
> not thought of that... and am not too sure it will do what we intend...
> but it also is a different approach.
> DX-Engineering agrees with Rick...  I need heavier, larger sized tubing.
>    (But that is  1) too easy,  and 2) fails to use what I have on hand.
> (I understand that scrounging and using parts on hand is a venerable old
> ham tradition!)
> Thanks for the idea.   Kind of a "pull-yourself-up-by-the-bootstraps"
> solution... but one that works.
> ================  James -K8JHR  =========================
> On 1/7/2011 10:33 PM, Dr. Gerald N. Johnson wrote:
>> You can solve your willowy tubing with stay wire braces, probably want
>> to use a poly rope instead of wire. Stay wire braces have been used on
>> slim masts, elevator legs (for grain transfer at an angle), crane booms,
>> and vintage aircraft wings.
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