[TowerTalk] Question on mast height above tower

[Doug Scribner]

Randy,

I think that I would be more concerned with the tower holding up vs the 
mast.

Rohn rates the HDBX-40 for 18 sq ft @ 70 mph wind speed. This assumes 2 
things:

1) The load is no more than 3 feet above the top
2) The boom length does not exceed 10 feet

You've exceeded those 2 specs already. In addition, your County is listed @ 
85 mph in the County Windspeed Data so the 18 sq ft is no longer valid.

I would seriously re-think that install...

Doug - K1ZO

----- Original Message ----- 
From: <RShirbroun at newportlabs.com>
To: <n6sj at earthlink.net>; <towertalk at contesting.com>
Sent: Wednesday, August 21, 2013 11:32 AM
Subject: Re: [TowerTalk] Question on mast height above tower


> 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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