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

[terry burge]

Sorry, should have said Boom-To-Mast …
t
> On August 15, 2018 at 12:59 PM terry burge <ki7m at comcast.net> wrote:
> 
> 
> 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 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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