Here is a neat mast calculator:
http://www.math.niu.edu/KARC/mast/
Plugging in some numbers for 1.5" schedule 20 water pipe:
Safety factor f 2
Wind loading factor k .004 lb/ft2/mph2
Maximum stress S 30,000 psi
Outside diameter D 1.9 in
Inside diameter d 1.61 in
Total length of mast 8 ft
Height above support h 4 ft
Antenna 1 height h1 4 ft area a1 6.1 ft2
Antenna 2 height h2 ft area a2 ft2
Antenna 3 height h3 ft area a1 ft2
Survival velocity: 63.0 mph
Failure velocity: 89.1 mph
Weight of mast: 21.79 lbs
---------------------------------------
As indicated above, I wouldn't be confident in using Sch 20 water
pipe. That 30,000 psi yield strength for water pipe is hard to
prove, because water pipe is not specified for structural
purposes. Also, the last time I priced water pipe, I was amazed
to see how expensive it had become.
You are much better off using structural tubing, which has a
specified yield strength. If you go to a metal supplier and ask
for structural tubing, he should be able to quote you some prices
on various strength material. You will probably have to buy a
20-footer though. Another thing you can do is to order some
galvanized steel mast from Texas Towers. They will sell you a 10'
piece for $139, and cut off a foot, which makes it UPS-shippable
(at least the last time I did it, in 1978). Their material is
87,000 psi. Plugging that into the calculator:
---------------------------------------
Safety factor f 2
Wind loading factor k .004 lb/ft2/mph2
Maximum stress S 87,000 psi
Outside diameter D 2 in
Inside diameter d 1.75 in
Total length of mast 8 ft
Height above support h 4 ft
Antenna 1 height h1 4 ft area a1 6.1 ft2
Survival velocity: 107.0 mph
Failure velocity: 151.3 mph
Weight of mast: 20.07 lbs
That will definitely work.
Dave Hachadorian, K6LL
Yuma, AZ
-----Original Message-----
From: Rick Scott
Sent: Wednesday, March 09, 2011 10:51 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Sorry to bring up an oldie - Mast Material
Im really confused. Id like to buy Local, because the mail order
houses have
Really high prices and high shipping
plus they only do 5 or 10ft intervals.
Ive seen all manor material from them from chrome molly to
Aluminum and AES even
has Fiberglass.
I have a Glen Martin 9ft roof mount tower. Its just shy of 4ft
from the rotor
to the top of the tower. Right now I have
a 2 1/2" Fence Post mast (really heavy wall) that I hold an A4S
up with.
A4 is going bad so I decided to get my dream antenna the standard
SteppIR 3
element beam and nothing else will go on the mast. I would
like to get 8ft as that will give me 4ft above the tower, I can
put the antenna
at 2 - 3 feet and be ok.
So I need 2" mast material,
I would dearly like to buy local and we have a few metal shops
but I need to
know exactly what I have.
I searched the forum and man there is a lot of material. K7XLC
used to sell
mast but his site doesnt.
I posted on the local Snohmish County hams and I got a gazzilion
answers.
One guy swears by Home Depot stuff
One says only buy Sched 40pipe nothing else needed
Another says this and that.
Another says 6061-T6 Extruded Tubing
So not really wanting to open a can of worms, whats the best
material to hold
what I plan to put up
for the best cost.
Scotty W7PSK
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