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## [TowerTalk] Sleeving Masts

 To: [TowerTalk] Sleeving Masts n4kg@juno.com (n4kg@juno.com) Mon, 16 Jul 2001 16:07:43 -0600
 ```Bob, The bottom line is that INNER Sleeving reduces the strength of the mast by a LOT, assuming the same yield strength material for the sleeve as the outer mast. The maximum allowable bending moment for a circular tube includes the factor (D^4 - d^4) where D is the outside diameter, d is the inside diameter, and ^4 means raised to the 4th power. Consider a 2 inch solid OD mast. D^4 = 16 Then a 1.5 inch solid tube D^4 = 5.0625 And a 1.0 inch solid tube D^4 = 1.0 >From this we see that a 2 inch solid tube is 16 times stronger than a 1 inch solid tube in terms of bending moment. A 1.5 inch solid tube is 5.0625 times as strong as a 1 inch solid tube. A 2.0 inch solid tube is 3.16 times as strong as a 1.5 inch solid tube. A 2 inch OD mast with 1/4 inch wall thickness can be thought of as a 2 inch solid tube with a 1.5 inch solid tube removed. D^4-d^4 = 10.9375 A 1.5 inch OD mast with 1/4 inch wall thickness can be thought of as a 1.5 inch solid tube with a 1.0 inch solid tube removed. D^4 - d^4 = 4.0625 >From this we see that a 2 inch OD, 1/4 inch wall tube is 10.9375 / 4.0625 = 2.6923 stronger than a 1.5 inch OD, 1/4 inch wall in terms of bending moment. Going to a SOLID 1.5 inch inner sleeve still results in 10.9375 / 5.0625 = 2.16. Not much benefit for a whole lot more weight! There is one factor in your favor if you *still* want to use an inner sleeve and that is that the bending moment for the smaller tube only acts over the length from the junction to the load, NOT the full length of the mast above the tower. You will still need to address how to hold the inner sleeve in position across the junction, unless you intend to add an inner sleeve that runs all the way down to the rotor. de Tom N4KG On Fri, 13 Jul 2001 14:29:33 -0400 "Bob Thacker" writes: > > Hello all, > > Next project is to increase the antenna spacing on the crank up > tower's > mast. Currently a TH11 is 6" above the tower top and a 402BA old > HyGain 40 > meter monobander is about 10' above it. Currently, there is some > slight > degradation on the TH11's 15 and 17 meter bands resulting in a > slightly high > (3.0 on 17 and 2.2 on 15...not too bad) VSWR. > > The new antenna project calls for a Force 12 EF-30/40 to replace the > HyGain > 402BA. However, after discussions with Force 12 and this reflector, > most > think that 12' spacing would lessen interaction between the > antennas. So the > question is, without purchasing a new mast, how does one elongate > the > current one? I have considered both internal and external sleeving > methods > measuring about 3' for an 18" overlap. To accomplish the extra 2 to > 3' in > mast length, the extension would be the same size as the current > one. The > internal sleeving method appeals most since the antenna can be > placed on the > mast at a lower level then slid up the mast into the 12' position. > The > current mast is 2" OD and .250" wall thickness. Any one tried this > approach? > > Thanks, > > Bob, K3GT > ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/tagj. List Sponsor: Are you thinking about installing a tower this summer? Call us for information on our fabulous Trylon Titan self-supporting towers - up to 96-feet for less than \$2000! at 888-833-3104 www.ChampionRadio.com ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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