k1xx@cfl.rr.com wrote:
> Julio:
> I don't totally agree with all your statements regarding top rails. I've
> used them very successfully in the past. Since this goes back a few years,
> my memories a bit fuzzy. But, I seem to recall that there might be a heavy
> duty top rail which is "just slightly" under 2 inch OD and not much smaller
> as you mentioned. I'd have to go out back and measure one to determine the
> actual size.
>
The top fence rails/pipe vary widely in strength but all I've seen had
much more spring/give than structural steel tube. IOW they would spring
much farther than the steel without taking on a permanent set/bend.
Around here what ever size fits the top of a 25 G would comfortably
handle a small to medium size tri-bander as long as it was no more than
about a foot above the top of the tower. It'd also handled an array of
144 and 440 arrays if they weren't large, BUT they would move around a
*lot* when it'd get windy. I use a piece of 1 1/2" in my gin pole and
its handled up to 200#. I've seen the top of that at least 2' out of
plumb but I take great pains to avoid that situation.
It's not a material I'd recommend but it can be used with forethought as
you've shown.
Speaking of unsuitable materials, I've even welded up "H-frames" for UHF
quadrature arrays out of EMT and they held up just fine even in 60 Plus
MPH winds.<:-)) OTOH they weren't overloaded and I'd not resort to doing
that for large arrays using long 2-meter antennas.
> In several cases, I used 12'ish foot lengths for yagis mounted at the top of
> the tower with the rotor recessed 10 foot into the tower. In one case, I had
> a full 20 foot length with a custom-turned insert placed down into the top
> rail. This one had a big 6 element 10 meter beam on a 3" boom at the very
> top (10 foot out of the tower). I can't count how many ice and snow storms
> that combo survived.
>
> If any mast material's overloaded, it will bend. Doing the analysis is the
> first best step.
>
> 73 charlie, k1xx
73
Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)
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