[TowerTalk] supporting a 40 meter rotatable dipole with trusses
grants2 at pacbell.net
Wed Aug 15 00:54:35 EDT 2018
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
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.
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
> 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
> 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
> 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.
> 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
> 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
> 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
> 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
> 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
> I have found when I build something like this I learn more in the building
> seeing just how big and how much of a load things will be. Might say I try
> 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.
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