Hi Terry,
If you are going with light elements, as I believe I read in your posts,
then truss cables can be used to help stabilize the elements both in the
vertical and horizontal planes. There is a small diameter Phillystran
available that works very well for this duty. Truss cables are typically
supported from both ends of a horizontal bar mounted above and parallel to
the boom at each element. The opposing mechanical loads on the bar from
both sides of the element help cancel out the bending stresses on the bar.
Most designs I have seen use a bar length of 24" or more mounted about 6" to
12" above the boom. The cables are attached to the elements about half way
out from the boom to the element tips.
As I'm sure you are aware, popular shortening options include capacitance
hats, linear loading, and base coil loading - in typical order of decreasing
efficiency. I believe you will find that about 45' width is a practical
minimum for a shortened element on 40m. One interesting option is that
with linear loading, it's possible to support the elements with metallic
truss wires which do double duty for electrical loading without adding
weight to the ends of the elements, but be sure to use wire that won't yield
(stretch) over time such as Copperweld. This option will require some
short segments of non-metallic (typically fiberglass rod) component in the
element construction at the loading wire attachment points. The loading
wires will also need to be electrically isolated from the boom via the truss
support bar. Element tuning is accomplished by varying the position of a
shorting strap placed between the two support wires back on the boom side of
the arrangement.
The big tradeoff for shortened 40 is the decrease in performance
bandwidth... particularly F/B ratio, which can be rather dramatic if you
are trying to cover both the CW & phone segments with one antenna. IMHO,
the Moxon design provides a much better overall compromise in terms of
performance bandwidth for a shortened 40 antenna. There has been a lot of
discussion on this reflector about this subject in past years so you may
want to look in the reflector archives for ideas.
Hope this info is of help and good luck on your project.
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of terry
burge
Sent: Wednesday, August 15, 2018 1:06 AM
To: towertalk@contesting.com; Grant Saviers <grants2@pacbell.net>; terry
burge <ki7m@comcast.net>
Subject: Re: [TowerTalk] supporting a 40 meter rotatable dipole with trusses
Well, OK you got me redesigning my little project. Don't know yet how I will
change it or just start over again. I do have the 1.50" X 0.120 heavy wall
also 6' long. And I am considering how I could utilize some coils and/or
capacitance hat arrangements to shorten up the antenna. Closer to what I
suppose a shorty forty or something like that. One problem is I just haven't
been up close to a 40 meter beam to get a decent idea what I am really
dealing with. I know when I tried to raise this one above my head probably a
good 1/4 to 1/3 of the outer elements were still on the ground. That before
any kind of serious trussing. Might even go back to the design like in QST
antenna issue a couple years ago with weighted wires hanging off the ends.
But I would prefer something more stable than that.
Terry
KI7M
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