I had a homebrew 90+ foot 80m dipole over a decade ago so it's hard to
remember the details. I think the elements were 2" diameter and tapered down
to 1/2 or 3/8. The element was split and the stainless steel wire truss was
also the loading the element so it was a V truss. Even with the truss there
was considerable element sag however it was not really noticeable from the
ground. The antenna was up for a number of years in the Chicago area and it
never failed. Actually it worked quite well, but it did not have enough
bandwidth to cover both CW and SSB. What grade aluminum are you using? I use
6061-T6 which does not bend easily. How far out is your truss? I am
guessing (depending on your aluminum taper) that should be out no more than
1/2 way to the element tip.
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] overhead truss for 80M rotary dipole
From: Chuck Gerarden <cgerarden@atomix.com>
Reply-to: Chuck Gerarden <cgerarden@atomix.com>
Date: Wed, 09 Jul 2014 19:50:18 -0500
I have had several 80 meter rotary dipoles over the years and they
have failed in the same manner due to very high windsthey get bent but
never actually break. I wonder if the placement of the overhead
element truss may cause this effect as thewind blows. The truss is
pulling up on the element but as the wind blows harder, the truss is
actually pulling on the element
at an angle other than "up" due the the wind deforming the element.
The harder the wind blows, the truss pulls the element harder into the
mast.
I am thinking the truss is too far out on the element and maybe it
should be moved in closer. This changes the "pivot point"as the wind
is hitting the element and the outer element area is moving more and
the inner area is more stable.
Is there a formula or does anyone have empirical knowledge on where
the best place on an element or boom the truss should attach? Each
element is 41' long for a total length of 82'. The antenna is center
coll loaded for resonance and fed with a25 ohm balun.
I of course could have 1 overhead truss and a side truss to resist
horizontal forces, but I would prefer to keep it simple ifthe
engineering allows it. This entire problem may be the aluminum tubing
is not big enough or thick enough to beginwith. The elements start at
2 1/2 inches and taper to 1/2 inch.
My latest solution is to use tapered 40' fiberglass poles as the
elements with a wires inside. With big antennas I have often had
better results after a wind storm since they return to their original
position.
Anyone out there have some good engineering advice on building 80
meter rotary dipoles?
ThanksChuckW0DLE
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