On 1/2/20 10:32 AM, Jim Thomson wrote:
Date: Wed, 1 Jan 2020 11:13:16 -0500
From: Gedas <w8bya@mchsi.com>
To: TowerTalk@contesting.com
Subject: [TowerTalk] Which Vertical is More Robust ?
##IF you know the yield strength of the proposed EMT,
plus the yield strength of the upper tubing, 6061-T6 is 40 ksi,
6063-T832 is 39 ksi. Then just stuff the proposed design
into DXEs handy ... yagi mechanical software. It will spit out
exactly how far it will deflect.....at any given wind speed, and
also what wind speed it will break at......and also WHERE it will
break.
.
## DXEs ..yagi mechanical is dirt cheap, easy to use, and dead
on accurate. Dont guess. It will also spit out deflection with
guys, like say 1 set of guys at 23 ft. Use the... no-spec windload
option,
thats what u get in a wind tunnel. U can also enter the yield
strength
for each individual section.
I suspect (but do not know) that DXEs program is a cantilever beam
calculation, but might not take into account the gravity load, which
Rick N6RK was talking about.
Once the antenna is "vertical" the loads from wind are probably going to
dominate, as long as the deflection is small (sin(small number) is a
small number).
However, there's a couple issues that I can think of (from practical
experience with long floppy poles, like the spiderbeam 40 footer, and a
bunch of 18 foot fishing poles) -
When it's being tilted up, there's more gravity load - one could use a
calculation like that for a Yagi element or boom to determine the static
loads and how close to failure you are.
The other one is the "weather vane" effect - It's unclear whether it
would have a significant "pattern" effect, especially if it's anywhere
close to 1/4 wavelength - most of the current is in the bottom, after all.
There's also a fatigue failure issue if it moves around in the wind.
You'd want to be *way* below the yield stress to avoid this.
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