>
> Remember seeing a chart an old Rohn book, giving specs on self-support
> for 25/45/55, but can't find anything in semi-current ones. ???
> My original post was to start the discussion, hoping for all of us to
> hear from an expert. My guess is to put'm up the way they were designed.
> If it's free-standing, it's designed to flex and put the stress and
> movement different from a guyed system....
>
> --
I have a Rohn book old enough to have that information, and I'll look
it up if you really want me to, however I can tell you from memory
that 25 specs were for 2 sq ft of antenna and about a 30 foot limit
in a relatively low wind area. It really wasn't worth it. The 45
specs were only a little better; a little more height, but no
increase in antenna capability.
The thing to remember about "free standing" towers, is that they get
their strength by distributing the horizontal moment of force
through the legs to the base, and then the ground. There is almost
always a taper in the cross section of the individual tower sections,
thus distributing the load as if each section were a mini array in
itself. The final load is distributed to the base. That's why there
is always a very large base on a self supporter.
In contrast, stick towers are designed to have the horizontal moment
distributed through the guy wires to the guy anchors. The base is merely
support for the weight of the tower and arrays plus the vertical
component of guy wire loads. That is also why stick towers can be
built on a pier pin, which was the subject of another thread a while
back; let's not rehash that one.
When installed as a free stander, the stick tower acts as one long
lever with the fulcrum at the base. Those piddly little legs and
bolts can't do much in that arrangement.
73, Rod N4SI
The DXer formerly known as N9AKE
(c) 5 November, 1996
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