The 750 psf considers tensile bending stress in a 4' mast section only (no
antenna) assumed yield 35 ksi - probably a little high. Agree that 27 ksi
is a better design limit w/ some safety factor. The calculation also does
not consider any shape factor - just velocity pressure. The point was that
it is not probable that a 4' section of 1.5" schedule 40 by itself would
yield from design wind loads alone, or even with a small vhf antenna.
Regarding longer sections of pipe masts supporting HF antennas I have
three masts - one is 2" x .180" 85 ksi yield alloy steel, one is 2" x .250"
85 ksi yield alloy steel, and the other is a surplus piece of 1.5" SCH 160
galvanized steel pipe. For the same length and loading, the .180" alloy is
~2.4 times stronger than the SCH 160, and the .250" alloy is ~3.0 times
stronger (taking the yield of the pipe @ 30 ksi).
73,
Matt
KM5VI
_____
From: TexasRF@aol.com [mailto:TexasRF@aol.com]
Sent: Saturday, September 15, 2012 7:53 AM
To: w6nws@arrl.net; km5vi@flukey.cc; towertalk@contesting.com
Subject: Re: [TowerTalk] FW: Antenna spacing on mast
Hi All, running this problem through a mast calculator program shows a wind
survival of 93 mph with the following assumptions:
1.9" OD .145" wall water pipe
yield strength of pipe 27ksi
pipe length 48"
antenna load 5 sq ft
Changing the antenna load to 2 sq ft increase wind survival to 143 mph.
73,
Gerald K5GW
In a message dated 9/15/2012 6:32:45 A.M. Central Daylight Time,
lknain@nc.rr.com writes:
Don't forget about the moment generated by the antenna. Wind pressure of 100
mph is about 40 psf IIRC. That means an antenna at 4 feet that has a wind
load of about 182 lbs would deform the pipe at the bottom of the 4 feet
(based on your 750 psf figure and ignoring the wind load on the pipe
itself). At 40 psf that works out to about 4.5 sq ft of antenna. If the
pipe were longer the antenna size would be less for the same effect.
73, Larry W6NWS
-----Original Message-----
From: KM5VI
Sent: Friday, September 14, 2012 5:57 PM
To: towertalk@contesting.com
Subject: [TowerTalk] FW: Antenna spacing on mast
It would take a wind pressure of over 750 psf to bend a 4' section of 2" SCH
40 pipe to the point of permanent deformation. 750 psf is about 30 times as
much pressure as would be generated by a 100 mph wind. Strength-wise it
would be no problem for a vhf antenna at this length. Interference-wise,
the VHF antenna is really too small to affect the HF antenna even at close
wavelength. At VHF, 4 ft is substantially enough wavelength away that the
HF antenna that it should not adversely affect the VHF performance.
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of David
Thompson
Sent: Friday, September 14, 2012 2:21 PM
To: Michael Zeglen; towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna spacing on mast
Please don't use Sch 40 water pipe even for a VHF antenna. Exposing 4 or
more feet increases the chance of it bending in the wind. Water pipe
strength is a flow inside not outside wind strength.
Dave K4JRB
----- Original Message -----
From: "Michael Zeglen" <kl7ai@hotmail.com>
To: <towertalk@contesting.com>
Sent: Friday, September 14, 2012 2:42 PM
Subject: [TowerTalk] Antenna spacing on mast
>
>
>
>
> Want to mount my 2m - 440 small beam antenna above my Pro57B. How far
> shouuld I space it or or does it really matter as long as there is 3 - 4
> feet spacing. I realize the more the better. What do you suggest? What is
> good rule to follow. Sched 40 pipe is NO problem my son is a plumber!
> _______________________________________________
>
>
>
> _______________________________________________
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>
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