In a message dated 97-11-20 10:28:53 EST, jm@drsmesh.com writes:
> If one establishes that the antenna has all adjustments finalized at the
> actual mounting heigh and position then is not heli-arching all the
> aluminum joints the ideal? A great many commercial yagis are welded in
> position. Welded antennas look great as they never rotate, loosen, or
> move and they are always 100% electrically in contact. If a weld
> breaks you either have a bad weld or a mechanical harmonic problem to
> address. Welded joint antennas look great, last and perform in my
> experience.
You didn't ask me but...
You're correct. I don't ever remember seeing "amateur" hardware
(hoseclamps specifically) on any commercial antenna. The commercial
manufacturers also typically don't have the user measure or tune anything.
They make it as idiot-proof and reliable as possible by minimizing field
"adjustments".
Everything is a function of cost. I just bought a commercial 151 mHz 5
dB gain omni for a project and it was over $600.00. I don't know of any hams
who are willing to spend that kind of money for a 2M vertical. There is much
more labor cost in assembling the antenna and heliarcing the elements
together to say nothing of the increased shipping cost for bigger parts.
Every decision an antenna manufacturer makes is based on costs.
>
> What is your feeling on pinning elements to boom, pinning booms to
> masts and pinning the mast to the rotator? Telrex used to cross drill
> and pin all of these in some applications that I have taken apart.
> Nothing ever rotated in these pinned applications. How much pin is
> correct in terms of diameter and material becomes an interesting
> mechanical engineering question. In one application I found that
> dissimilar metals (a hard pin) lead to elongation of the aluminum. Some
> studies show that large diameter pins can lead to premature failure of
> the tube pinned.
I'm not a big fan of pinning masts to rotators. Whatever the wind-induced
antenna torque is on the mast, it's passed down to the rotator (and then to
the rotator shelf to the tower legs to the guy hardware to the guy wires to
the anchor - whew!). By pinning the mast and rotator, you just transfer the
force into the next weak point of the system - usually the rotator brake or
gear train. It's much easier and cheaper to go up the tower occasionally and
re-align the antennas because of a slipped mast than it is to replace a
broken rotator.
The points you made above (galling, stress riser holes, etc.) are other
good reasons not to pin the mast.
73, Steve K7LXC
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