Alternative materials for masts , or any material leave you playing the
odds. The stronger the materials the more the odds are in your favor.
An antenna of only 5 sq ft of wind area on a 145' mast become the
equivalent of a 75 sq ft antenna. Far, far beyond the ability of water
pipe, sked 40, or even 80. Water pipe is the same material as rigid
conduit which is easily bent with hand tools although hydraulic benders
are used on larger sizes.
EMT come in 10' lengths. I've never seen longer.
For rotators mounted down in the tower, torque resonance is a real
thing. The longer the mast, the lower the frequency. Strength is not a
direct indication of spring
Many materials have been used, but often the success is in spite of
inferior materials and a lot of luck.
I'm surprised EMT would be rated near those specs as it's designed to be
easily bent with hand tools. Larger sizes are available in 90, or sweep
45s and 90s as the would collapse if bent.
Just because something non standard worked for someone does not mean it
wioll work for you.
73
Roger (K8RI)
On 3/15/2016 Tuesday 9:40 PM, Grant Saviers wrote:
Gerald,
Could you point me to the 35kpsi spec? I couldn't find a steel spec
on the Wheatland EMT manufacturer site, or that matter on any other
site I looked at that google turned up re conduit specs.
Here is a section from a Q&A by the Steel Tube Institute re the
controlling ANSI 80.3 standard for electrical conduit
"Rigid conduit is intended for electrical applications, not structural
ones. Its requirements are governed by UL and ANSI specifications, not
by ASTM. The main issue here is that, whereas dimensions may be
similar between the two, rigid conduit is not intended for, is not
designed for, and is not tested for any strength or structural
requirements."
So while the yield number and wall thickness in the mast calculator
turn out ok, YMMV. And likely with the thin wall it will deform with
boom and rotator clamps.
Standard schedule 40 water pipe would be a much better choice for
several reasons:
1. the grade steel is specified
2. it is specified to withstand an internal pressure, which is a
control on the seam weld quality
3. it is allowable in structural applications, although used mostly in
larger diameters.
4. hot dip galvanized inside and out
Caveat emptor. Unless it falls down, and your insurance asks about
the engineering and the plaintiffs attorney finds the statement to the
effect "I don't care if it falls down."
Grant KZ1W
On 3/15/2016 12:35 PM, TexasRF--- via TowerTalk wrote:
Google says 1.5" EMT is actually 1.75" OD and .065" wall thickness and
yield strength is 35,000 psi.
Putting these numbers in a mast analysis program and using a 7 sq
ft wind
load 24" above top of tower, failure happens at a wind speed of 84 mph.
Lowering to 12" above tower top, failure is at 119 mph.
So, it could be useful in a modest installation.
73,
Gerald K5GW
In a message dated 3/15/2016 2:01:59 P.M. Central Daylight Time,
infomet@embarqmail.com writes:
Let’s not call names.
An EMT mast is great, as long as you don’t get greedy and make it
too tall
outside the tower.
Mine is about 2’ above the tower top and I think it will survive any
wind
likely in my area. And I really won’t cry if the $150 beam is damaged!
It’s several years old and shows no sign of deterioration. It cost
almost
nothing and weighs next to nothing.
So I have a used tower, used tribander, used rotator, EMT mast, but new
coax.
I also have a HB amp made from used components, with an easy KW+ output.
It’s a rare pileup I can’t get through.
I am VERY happy, but may change to an LPDA so I can get the WARC bands,
someday.
I also use junk tubing from a kid’s swingset for Field Day. It’s worked
fine for a decade or so.
Successful systems can be designed around most any components, as
long as
one doesn’t try to do too much!
I’d far rather see a ham put up 30-40 feet of used tower and a used
tribander than envy those with more elaborate, expensive setups!
A few more hours on the air will make up for a few dB of antenna
performance and there will be money left for beer.
Wilson
W4BOH
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73
Roger (K8RI)
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