Byron,
Inside or outside sleeving of round tubing will make the section moment
that of the combined wall thicknesses. This is covered in Leeson's
"Physical Design of Yagi Antennas." Outside sleeving is much more
effective since the moment of inertia increases geometrically with the
outside diameter for a given wall thickness. If you internally sleeve
your 2" mast with a 0.125" wall tube the moment increases 20%. If you
sleeve the od with a 0.125" wall tube the moment almost doubles.
So if you want to stick with aluminum, then get some 2.25" od 0.120"
wall tube for an od sleeve. Usually tubing is exact for od, so to
sleeve the id needs to be increased, which may or may not be true for
the mast you have. Since some of the "HD" steel masts are sleeved for
only the lower 10' where the stress is the highest, that is also an
alternative.
Here is another online calculator that is handy
http://civilengineer.webinfolist.com/str/micalc.htm
Grant KZ1W
On 8/26/2014 6:23 AM, Byron Tatum wrote:
Hello-
For years I have used the 2" OD, 6061-T6, 1/4" wall x 24' long aluminum
masts to stack VHF and UHF antennas. I have seen these takeyour a bend a time or two over
the decades.
I wanted to ask:
1. If I was to switch to 2" OD steel, as a mast to extend about 15'-18'
out the tower top for stacking VHF/UHF long boom yagis, what would be a suitable
grade of steel, the finish and wall thickness?
2. I have heard of inserting a smaller aluminum tube inside the 2" aluminum mast, in
the area where the mast exits the tower top and on above a bit. I wanted to ask if someone
here has experience with inserting an aluminum tube inside the 2" OD mast? I would assume
it would be 1-1/2" OD, of similar material and length and what wall thickness?
I guess reasons I have stayed with the aluminum masts is that they are
lightweight and easy to handle, plus no rusting down in this Gulf Coast area.
Thanks, Byron WA5THJ
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