> [mailto:towertalk-bounces@contesting.com]On Behalf Of Jim Hill
> Could be, but N6RK, what is the wall thickness of the tubing you
> used? Next time I'm in that area, I'll stop by, talk to them
> again and get
> more info.
> 73's Jim
>
> At 08:03 PM 3/21/2003 -0600, you wrote:
> >Hmmm,
> >The stuff I saw on the rainforrent Web site seems entirely
> unsuited to large
> >vertical antennas. They call it lightweight aluminum, which would suggest
> >its designed to cross horizontal spans and be supported at regular
> >intervals.
Standard irrigation tubing is 0.050 inch wall. "Heavy duty"
tubing is 0.065 inch wall. There is some "center pivot" stuff
up to 0.100 or so. For GUYED installations, I see no reason
to use anything beyond the 0.050 wall stuff. For UNGUYED installations,
I have done stress analysis on it and determined that 30 feet of
unguyed 2 inch is good for 70 MPH or so. I have actually observed
an unguyed 0.050 wall, 2 inch x 30 foot vertical in a "worst case" storm
and it bent over severely, but recovered completely after the storm.
If you increase the diameter to 3 inches, you increase the wind
handling for 30 feet to 80 or 85 MPH. Going to 4 inch increases
the maximum height to 40 something feet. You can't get much beyond that
by tapering because then it won't be stiff enough to be self supporting
even with no wind (ie gravity will make it droop). For 80 meters,
needing 60 something feet, there is a *HUGE* advantage in guying.
Even if you just guy the bottom 30 feet, you are OK because the top
30 feet can be self supporting (also proved that in a storm).
Unless you absolutely cannot have any guys at all, I would say to
forget about self supporting designs. Note that unlike real towers,
where guys cost $$$, for aluminum verticals, you use baler twine
which costs a dollar for 250 feet. Also, the irrigation tubing is
available used for less than scrap metal value, and even new is only
a buck or two a foot. Re: Rain for Rent. Only some stores have
aluminum and in some cases only "in season". I got mine at the Woodland
store.
Rick N6RK
www.n6rk.com
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