[TowerTalk] FW: supporting a 40 meter rotatable dipole with trusses

[Jim Miller]

Have you not considered the 40m "moxon" that K3LR is using? Look on his
website for info.

73

jim ab3cv

On Wed, Aug 15, 2018 at 9:50 AM Matt <maflukey at gmail.com> wrote:

> 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 at contesting.com] On Behalf Of
> terry
> burge
> Sent: Wednesday, August 15, 2018 1:06 AM
> To: towertalk at contesting.com; Grant Saviers <grants2 at pacbell.net>; terry
> burge <ki7m at 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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