Hello again,
Checking my stock of parts I find I have (2) 2" X 6' X 0.120" Thick wall
aluminum; (2 or more) 1.750" X 6' X 0.120" TW AL; (2) 1.500" X 6' X 0.120" TW
AL. The reason I did not build the antenna with this larger stuff was finding
some kind of insulating boom-to-mount at a reasonable cost. I do also have DXE
fiberglass tubing (1) 2" X 8'; (2) 1.5" X 8'. Then some smaller pieces. Making
the B-to-M clamp will be the biggest problem but there is a place in town I
found I could possibly come up with some aluminum plates to do the job. I would
like to use two plates and make a pivot joint so the dipole can be rotated down
to reach the resonance adjustment slip joint or coil/tap/whatever.
Terry
KI7M
> On August 15, 2018 at 6:50 AM Matt <maflukey@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@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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