[TowerTalk] supporting a 40 meter rotatable dipole with trusses

Wed Aug 15 03:41:04 EDT 2018

Hi Terry,

The full size elements of my 40m yagi start with 2 3/8" tubing 0,1" wall
thickness.
Using larger tubing is far more effective then more wall thickness.

I would go for a short dipole with loading coils + capacity heads with the
sizes of your tubing.

73
Peter

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of terry
burge
Sent: Mittwoch, 15. August 2018 08:06
To: towertalk at contesting.com; Grant Saviers; terry burge
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
> On August 14, 2018 at 9:54 PM Grant Saviers <grants2 at pacbell.net> wrote:
> 
> 
> I second the "too small" opinion.
> 
> IMO spend the $40 for Yagi Mech from DX Engineering and design for the 
> wind velocity expected.  Almost certainly some tube diameters will need 
> single and perhaps double internal sleeving at spots.  From my Y-M 
> modeling experience, the center probably needs to be around 0.120" wall 
> by 1.75" od for 6'+ each side of the boom.  It is also a bit difficult 
> to design full size 20 and 40m 80+ mph beams with only the 6' lengths 
> from DX Eng, although I love their prices.  A little work with YagiMech 
> will illuminate why popular commercial 40m yagis are linear or coil or 
> Moxon loaded, and why full size 3L ones weigh 300# and cost $5k.  Think 
> what 1/3 of that might look like. My 3L 40m came from an great Oregon 
> ham builder and weighs 350# and is very experienced with wind and ice.
> 
> Unless you really upscale sizes, a truss will be needed for full size 
> 40m elements and while dacron is relatively low stretch, Phillystran is 
> the standard for element trusses, essentially no stretch.
> 
> I don't use hose clamps for elements over 1/2" diameter.  Crossed 
> machine screws keep elements snug in 2 planes with short overlaps, are 
> strong, elements won't vibrate, and using nylocks they don't loosen.
> 
> Then the more guys used = more wind resistance and worse the more ice 
> loading.
> 
> I think your dimensions will be easy to tune.  NEC not needed. 
> Mechanically you have the biggest challenge, IMO.
> 
> 124' is a long way to climb to unscramble aluminum spaghetti.
> 
> Grant KZ1W
> 
> 
> On 8/14/2018 18:45 PM, john at kk9a.com wrote:
> > In my opinion, this is very small tubing for a full sized 40m element.
My
> > homebrew 20m beams use 1 1/4 diameter tubing tapered to 1/2 tips.  I
suggest
> > that you start by looking at the element taper schedules in the ARRL
antenna
> > handbook.
> >
> > John KK9A
> >
> >
> > To:	towertalk at contesting.com
> > From:	terry burge <ki7m at comcast.net>
> > Date:	Tue, 14 Aug 2018 13:11:18 -0700 (PDT)
> >
> > Hi Jim and group,
> >
> > Hell of a way to wake up in the morning. Anyway, here are the
dimensions.
> >
> > DX Engineering aluminum tubing all 0.058" wall thickness
> > 1.250" X 6'
> > 1.125" X 6'
> > 1.00" X 6'    Spit on the end allowing adjustment with 1/2 - 1 1/4" SS
Hose
> > clamp
> > 0.875" X 6'
> > 0.750" X 3'
> > 0.625" X 3'
> > 0.500" X 6'
> >
> > Truss 1" X 1" X 0.125" X 3' aluminum angle 'iron' from Lowes. Holes
drilled
> > to
> > also add a muffler clamp on the vertical truss if necessary to the mast.
> >
> > Center 'boom to mast' insulating clamp from SSS Solutions for 1.25"
element.
> > A
> > little small but I figure it will work for my purposes with the
trussing.
> >
> > Comtek 1:1 Current Balun at feed point.
> >
> > Because of Jim's thoughts about horizontal wind load I will also add
another
> >
> > horizontal truss like I did with my quad array of something like 6' long
> > aluminum angle 'iron' with 3/16" Dacron truss lines. Hope this won't
cause
> > too
> > much difficulty reaching the adjustment positions at the 1" to 0.875"
slip
> > joint when rotating the vertical. Will do initial resonance dipping in
on
> > test
> > support lower to the ground to get the SWR near where I need it around
7150
> > Khz. Not sure how much of a difference this will be at 124' versus my
Comtek
> >
> > 4-Square ground mounted. Interesting experiment but since I have always
felt
> >
> > lacking on 40 meters with 'knowing' I could work more stations in
contest on
> > 40
> > meters with a better antenna system.
> >
> > I have not modeled this in Nec2 or whatever. Window 10 raises it's ugly
head
> >
> > preventing that from operating. I know there are ways to deal with it
but
> > I'm
> > not even sure the ARRL Antenna Book disk will run on my windows. And
> > programming computers is not one of my great accomplishments to say the
> > least!
> >
> > I have found when I build something like this I learn more in the
building
> > and
> > seeing just how big and how much of a load things will be. Might say I
try
> > to
> > go by feel and try to 'over built' to handle stresses like wind load.
Not
> > always of course but I do think this will work here in Oregon.
> >
> >
> > Terry
> > KI7M
> >
> > _______________________________________________
> >
> >
> >
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> >
> 
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