[TowerTalk] mechanical construction of 80m vertical

[Jim Lux]

----- Original Message -----
From: "Chuck Hartley" <hartley at smart.net>
To: "RCARIELLO" <RCARIELLO at si.rr.com>
Cc: <towertalk at contesting.com>
Sent: Saturday, March 22, 2003 8:39 AM
Subject: Re: [TowerTalk] mechanical construction of 80m vertical


> It seems like one should be able to use one of the programs for
> mechanical yagi element design to solve this problem. An 80m yagi
> element should certainly work in the vertical position since its design
> provides enough strength to support itself in the normal horizontal
> orientation (which also implies that it won't collapse while it is being
> raised). One difference is that the section joints have to support the
> vertical load of the sections above it, but that doesn't seem too
> difficult to deal with

. Can any mechanical engineers out there explain
> why the design principles for a horizontal element would be different
> than those for a vertical element?

I take a stab at it..
the design IS basically similar.. Differences that may or may not need
consideration are:
1) The Yagi design program will probably assume that the wind and gravity
loads are at right angles.  For a vertical, the wind load might line up with
the seismic load; although, if the Yagi program adds the loads (worst case),
then you're all set for a 1g seismic load
2) The assumptions about the base of the element might be different. For a
Yagi, you probably have a "center supported" beam that can (in theory, from
a loads standpoint) pivot around the boom. For a ground mounted vertical,
the bottom of the beam is "fixed" (it's a canteliever).  Of course, the
stress on the center of the yagi element (assuming it's a whole dipole,
supported at the middle) is basically the same as the stress of the
canteliever (imagine a plane cutting right through the middle of the dipole
element, making it two monopoles, base to base).. The only difference might
be dynamic behavior (flexing vibrational modes).

Off hand, I'd guess that designing a vertical mast is an easier problem than
a horizontal canteliever, and the Yagi program might lead you to a design
that is "too much"..  you don't have to worry about weight as much: steel is
an option, or size at the base isn't as big a problem: you can taper a lot,
making the base big, which greatly helps the stress, since it's, to a first
order proportional to  load at (top*length of mast/width of base)


On the other hand, I've seen pictures of some full size low bandself
supporting horizontal dipoles in ON4UN's book, and they bear a striking
resemblance to a standard lattice truss tower, on it's side.

Jim, W6RMK