On 1/8/17 12:53 PM, Keith Dutson wrote:
I like to climb. When up the tower and testing rotation (ground crew
operates), I do not like the feeling when the antenna starts and stops.
so that's a "feels uncertain under my feet" kind of thing, which is
totally rational.
Small Yagi antennas, such as a tri-bander are not a worry, but BIG antennas,
for 40 and 80, really do a number on twisting on my 45 and 55G Rohn towers.
I think this is not good for the guying system.
There, I think that's not really the case.. the loads on the guys are
probably not significantly varying. The guys are steel or composite,
and fairly low load, so fatigue failure isn't an issue. Maybe there's a
"wear on the attachment point" issue, but I that that wind gusts (which
happen all the time, much more frequently than rotor moves) would be a
bigger factor.
I think the take home here is that "more guys make it more rigid, which
reduces the pucker factor when climbing" is the dominant effect.
Even with the bracket, there is some twisting, so the rotator is not getting
much more torque.
73, Keith NM5G
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
jimlux
Sent: Sunday, January 8, 2017 2:00 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] torque arms or not?
On 1/8/17 11:39 AM, Keith Dutson wrote:
Most hams I know are using star guying to avoid excessive twisting
when the heavy Yagi stops rotation. I use the bracket from Norn.
Is that because of "nice to have" or is there an actual structural reason.
I wouldn't think the number of twist cycles is such that it would cause
fatigue failure, and if you make the tower more "rigid", then the stopping
forces on the brake and rotator housing are higher (maybe.. the "jerk" might
be higher, but the actual torque might be the
same)
As Mike says, for a high gain dish, with a beamwidth measured in degrees or
fractions, you need the tower to be rigid enough to keep the pointing loss
reasonable. But for HF Yagis, the beam width is 10s of degrees (if not
60-70 degrees), so a few degrees of twist is a non issue.
In these "cable stayed truss" structures you don't want them too rigid,
because that potentially causes stress concentrations in some parts of the
structure. (piano wire and rope in parallel problem: the piano wire takes
more and more of the load as the total load increases) But, nor do you want
it so floppy it isn't controllable.
73, Keith NM5G
snip>
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