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

maflukey at gmail.com maflukey at gmail.com
Wed Aug 15 17:22:24 EDT 2018

Thanks John - I didn’t read the full thread and missed the original point.  Thanks for pointing that out.

Hi again Terry - Regarding building an electrically isolated boom to mast clamp...  I have used blocks of UHMW Polyethylene bars from McMaster Carr and machined my own standoffs out of them.    It’s a UV stable material that is suitable for outdoor use and machines easily in an ordinary drill press.   It does not have enough tensile strength to serve alone as the joining member between the two halves of your dipole, but it is strong enough to make electrically insulated standoffs (two or three per each side of the dipole) which are then attached to a section of aluminum plate or tubing (builder's choice) which is selected with sufficient strength to join the dipole together and is subsequently mounted to the mast.  

As far as the configuration of the standoffs, you might try using the 1.5" x 3" bar and cut it to 4" lengths.   You could machine a 2" hole through one axis and then drill a small hole intersecting the 2" hole in a perpendicular axis which would be sized for a machine screw to pass through the block and element to hold the element in place with the block.    Next, drill separate through holes at the four corners of the third axis for mounting the standoff to the plate (or tube).   These, of course, would not intersect with the element.    At 4" long each, you could get three of these per foot of material so about 2' should have it covered.

Hopefully I better addressed your actual question this time, and again, good luck on your project.


-----Original Message-----
From: terry burge <ki7m at comcast.net> 
Sent: Wednesday, August 15, 2018 3:06 PM
To: towertalk at contesting.com; Matt <maflukey at gmail.com>
Subject: Re: [TowerTalk] FW: supporting a 40 meter rotatable dipole with trusses

Sorry, should have said Boom-To-Mast …
> 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
> > 
> > _______________________________________________
> > 
> > 
> > 
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

More information about the TowerTalk mailing list