[TowerTalk] Question on mast height above tower

[RShirbroun at newportlabs.com]

Thanks Steve.  According to the calculator the guys at Texas Towers use, the mast (2" diameter, 0.25" wall) should easily handle it.  My concern is just if the ratio of 13' above and 2' below the top plate is excessive and would potentially put too much stress on the tower itself.

73,
Randy, ND0C



-----Original Message-----
From: Steve Jones [mailto:n6sj at earthlink.net]
Sent: Wednesday, August 21, 2013 10:13 AM
To: Shirbroun, Randy AH/US; towertalk at contesting.com
Subject: RE: [TowerTalk] Question on mast height above tower

Randy-

If you can find a copy of Dave Leeson's "Physical Design of Yagi Antennas"
(out of print) he explains how to calculate all the moments created by your combination of wind speed, boom/mast lengths and material types.  Then you select the proper material, length and wall thickness to withstand those forces.  That stack sounds like a big load on the base of the 2" mast, but whether it will hold depends on the yield strength of the chrome moly mast you use.

73,
Steve
N6SJ


-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of RShirbroun at newportlabs.com
Sent: Wednesday, August 21, 2013 7:51 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Question on mast height above tower

I didn't get any comments or suggestions to my previous post, so I'll try again.
If this is a dumb question, feel free to tell me!

I have just erected a new HDBX 40 in place of my 30 year old HBX 48, sacrificing 8' of height to gain some strength and load capacity (and peace of mind!).  The stock rotator plate allows 2' of mast below the top plate and I will be using the heavy duty Yaesu thrust bearing (along with the Yaesu heavy
duty 2800 rotator plus the absorber plate).   I have added steel angle
braces
to reinforce the rotator plate.

I'm looking at using a 15' chrome/steel 2" mast (in place of my previous 9'
mast), so 13' of the mast would extend above the tower.  The mast would support a TX38 tribander just above the top plate, a 6 m beam half-way up, and a 40m rotatable dipole near the top, 12' above the top tower plate and the thrust bearing.  The mast, of course, is very heavy, weighing around 75lbs.  The tribander weighs 40lbs and with 5 ft2 surface area; the 6 m.
beam weighs 10lbs with 1.5 ft2; the dipole weighs 10lbs and is 0.5 ft2.

Is this too much mast for this tower?   (BTW - I'm aware the boom length
exceeds the 10' maximum for this tower, but the HBX 48 handled a similar tribander for 30 years, with occasional severe ice-loading, without any
problems.)

Thanks and 73,
Randy, ND0C



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