[TowerTalk] [Bulk] Re: MASTS

Wed Mar 16 14:21:33 EDT 2016

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 at 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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