> Hello to All,
> The question of a full size 1/4 wave vertical element has been bouncing
> around for some time now. Many years ago I recall seeing at a Radio
Contest
> Club Meeting photos of a Four Square made with full size free standing
> aluminum 1/4 wave elements for 75 meters. I have never been able to get
the
> construction details. One would think with all the Professional Engineers
> that hold Amateur Radio Licenses a design would have been made available
to
> one of the reflectors by now. We will just have to keep hoping I guess.
>
> To put a 70 foot free standing element up and expect it to be there the
next
> day needs a little engineering thought to say the least. Even to design an
> element to be guyed has to be strong enough to be moved from the
horizontal
> to vertical position without bending on itself.
For an unguyed pole, the engineering has been done for you. Go buy a 65 ot
70 foot flagpole. http://www.flagpoles.com/ is the website for Concord
Flagpoles and they have dimensions, windload calculations, etc. They're
made of aluminum or steel. Be forewarned, they're pricey..A 60 footer will
set you back about $4-6K The 10" base, 0.250" wall unit is rated to 122
mi/hr (without a flag).
I'd find it hard to believe that you'd be able to successfully engineer and
build a similar unit without sticking fairly close to their dimensions. One
could probably make the walls thinner (Concord has a 0.188" wall unit for 98
mi/hr rating), or go with a more agressive taper (you probably don't need a
large diameter at the top for an antenna) to reduce weight, especially if
you're willing to let the thing flex a fair amount in the wind. Reducing
the diameter at the top isn't going to reduce your wind cross section a huge
amount, though.
If you are not mass constrained, an easy approach would be to just buy 70
feet of steel pipe (ungalvanized) and plant the bottom 10 feet in the
ground. Or a telephone pole. Run copper wires along it to reduce the
resistance. Since the pipe comes in 20-40 foot sections, welding would
probably be the connecting means of choice. Structural steel pipe runs
about $1/ft for 2-3" diameter and $2-4/ft for 6-8" diameter. Steel is a
wonderful material.. stiff, strong, cheap, and easily fabricated... too bad
it's so dense. You could probably build those freestanding 60 foot poles
for less than $100 each.
There are some design programs around for designing vertical antennas.. the
math is fairly straightforward, and it would be a pretty standard exercise
in a "structures" class (vertical tapered canteliever beam, and all). It's
one thing to have an engineer/ham do some back of the envelope calculations
on a structure you're going to put up on your 40 acre farm, it's entirely
another to have that engineer sign and seal them for the local planning
department.
Jim Lux, W6RMK
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