There are two problems with a severely tapered
full size element for 80M.
First, it becomes extremely flimsy at the top and thus
will not be self supporting. A 20M element using 0.058
wall aluminum needs to be stepped roughly every 30 inches
for a 30 psf windload (86.6 MPH).
Extrapolating this to 80M would require 24 to 30 lengths
from 1/2 inch dia to 3 inch dia. Most 40M full size beams
use 2 inch tubing for 20 ft followed by a reducer and tapered
sizes, and they have a lot of SAG, which I suspect is not
acceptable for a self supporting 80M vertical.
To arrive at a self supporting design you will need to perform
a bending moment analysis. This gets extremely complex
for a multiple size tapered element. I performed such an
analysis for 20M elements using a spread sheet with the
appropriate formulas for each section and checking each
piece to be sure that the maximum bending moment at
each section was within the ratings for that section.
You will need good computer skills, good math skills,
and a lot of time to program 20 or more sections!
If you go through with this analysis, please publish your
results. I am sure it will be of interest to many big station
owners.
I suspect a tapered aluminum tower would be a better
solution. See Heights or Universal Tower companies.
de Tom N4KG
On Mon, 08 Jan 2001 Arne Gjerning <gjerning@flash.net> writes:
> Good taper schedule for aluminum tubing is to use diameters
> differences
> of 1/8 (0.125) and to use a wall of 0.058 thickness. These telescope
> together very nicely. Note the difference betweeen the 0.125
> diameter
> differences and two times the wall thickness (2 x 0.058=0.116)
> allows
> for a 0.009 clearence of the innner to outer tuging. I use then the
> saw
> cut and stainless steel hose clamp technique to secure. Always
> remove
> ALL burrs when making any cuts. All dimensions are english.
>
> Previous posts gave wall thickness of aluminum as .0120 which is
> very
> thin and not a standard in the antenna world.
>
> My 18th Edition of the ARRL Antenna Handbook has a very useful table
> of
> aluminum tubing from 3/16 to 3 in diameters. Try Chapter 20 page
> 20-7.
> Chapter is titled Antenna Materials and Accessories.
>
> 73 de Arne N7KA
> NM
>
> Dan Levin wrote:
> >
> > Does anyone have data on taper schedules for building
> free-standing vertical
> > antennas from aluminum or copper? Is it in the Antenna Book, and
> I just
> > missed it? What I want is a table that says something like e.g.
> "if you
> > want to build a free-standing quarter wave vertical for 40 meters,
> then use
> > this taper schedule - 10' 2" o.d. .0120 wall 6061 aluminum, 10' 1
> 5/8" o.d.
> > .0120 wall, 10' 1 3/8" o.d. .0120 wall, 4' 1 1/8" o.d. .0120 wall,
> and it
> > will stand up in an 80 mph wind without falling over if the base
> is good."
> >
> > Thanks!
> >
> > ***dan, K6IF
> >
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