Hi Lee, one possibility is use of aluminum pipe purchased through your
local metal supplier such as EM Jorgensen Co.
I built a couple of full size 3-el 40m beams about 30 years ago using the
stuff and they survived multiple 90+ mph winds. This probably is way
overkill for a Moxon style beam but you asked.
If you start with 3.5"" OD with .3" wall material, the next smaller size of
2.875" OD will slip in no problem. The 2.875 material is available in
various wall thicknesses but for robustness, the one that is .218" thick is a
good choice. The two sizes can be pinned together with three or four 3/8"
diameter bolts.
The stuff comes in 20ft lengths usually so you will end up with some
leftover material dependant on how much you telescope the pieces.
You can ask about different alloy choices. 6063 is commonly used for these
large extrusions. You should be aware that the yield strength of 6063 is
less than 6061-T6 but the larger diameter makes 6063 useful in many
applications.
In any case you likely will want to include a boom support not for strength
but to take out the sag, These booms are heavy!
I can see making the boom longer would allow adding some fore and aft
element trusses to provide added wind survival to your Moxon.
73,
Gerald K5GW
In a message dated 5/25/2015 12:23:02 A.M. Central Daylight Time, L@w0vt.us
writes:
I have been contemplating building a Moxon Yagi 40 meter beam for a long
time and I need to get started with the project. The bottom line is I
need a strong boom 30 foot long. I'll use a boom truss but am thinking
of aluminum around 3 inches in diameter and maybe 1/4 inch wall
thickness. I realize this won't be cheap. Maybe this is not even
strong enough for a 2 element beam. I live near Houston, so I am
thinking maybe an aluminum provider might be able to sell it to me in
one piece. (Trying to figure out how to connect affordable 6 foot
shippable pieces has kept me stumped from starting the project up until
now.)
I'd sure appreciate hearing suggestions on how to get the job done. I'm
not trying to do this on the cheap but the best reasonably affordable
way to do it. I'm thinking the cost of a boom like this might be in the
$300 to $400 range. (I don't mind splices, but where to get large
telescoping tubing? I wrote DX Engineering and they discouraged me from
using their tubing as they said it was not intended for booms even
though I wanted to beef it up with sleeves). Boom diameter is a factor
because cast boom to element clamps don't come in every size. (I have
purchased heavy duty aluminum channel stock for boom to element
connections to be used with aluminum cast U-clamps.)
Any help would be appreciated.
Lee, w0vt
Houston, Texas
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