[TowerTalk] Question on mast height above tower

[RShirbroun at newportlabs.com]

Thanks for the numbers Gerald - I appreciate the calculations.  Obviously I should have gone through more of a detailed analysis before I ordered the mast for this specific tower.  (The 15' 2" 0.25" wall mast is from you folks at Texas Towers.  The shipping was more than the mast, but that was my choice!)

I'll be going with a shorter, narrower wall mast (that I got from you guys also, about 5 years ago) and skip the 40m dipole component.  (The height above ground was going to be pretty marginal to be very effective with it anyway. - Bummer!)

Now I just need to find somebody that wants to buy a really rugged 15' mast!

Thanks again, and 73,
Randy, ND0C

From: TexasRF at aol.com [mailto:TexasRF at aol.com]
Sent: Wednesday, August 21, 2013 11:41 AM
To: Shirbroun, Randy AH/US; towertalk at contesting.com
Subject: Re: [TowerTalk] Question on mast height above tower

Randy, a quick look at your proposed system shows that at 90mph, the total force applied to the top section is 666 foot pounds. 195 fp for the 40m ant, 315 fp for the 6m beam and 150 fp for the Triband beam.

If that force was from a single large antenna it would be the equivalent of 22 sq ft mounted one foot above the tower top.

This is with NO safety factor and does not include loading for the 2" mast or feedlines.

This is more than the tower rating and you will have to decide if the risk is acceptable. The bottom line is that the system would fail with a wind speed somewhat less than 90mph, maybe 70mph or so.

The mast proposed is an overkill. My computer program shows that a 2" OD mast made with 1026 DOM, .125" wall will fail at 130 mph. This material is available at Texas Tower of course!

Hope this helps.

73,
Gerald K5GW
CEO Texas Towers

In a message dated 8/21/2013 8:25:38 A.M. Pacific Daylight Time, RShirbroun at newportlabs.com<mailto:RShirbroun at newportlabs.com> writes:
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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