[TowerTalk] mast for two big antennas

[Mike Ryan]

Jorge, I have no idea of what the availability of this type of mast would be 
to you.  I have been using it here in Florida (and also at my station in 
Virginia when I lived outside the D.C. area in the '90s). This is 4130 
chrome molly steel tubing. The word tubing is the primary descriptor I 
think, in that there is no seam such as you might find when using some kind 
of pipe. I bought a half dozen or so 24 ft lengths from the Dillsburg 
Aeroplane Works up in Pennsylvania back in the late '80s when I was putting 
together a station in northern Virginia.  I had them galvanized before using 
them. These were 125,000 psi tubes very similar to the materials used to 
build racing car safety cages as I recall.  I don't recall if the units I 
bought were 1/4" wall or 3/8" wall but they are heavy to be sure at these 
lengths. But as an antenna mast they worked great for me. I stacked a HyGain 
205BA over a HyGain Discoverer III ( that's a 5 element 20mtr monobander 
over a 3 ele 40mtr monobander) and this mast held up very well.  I am using 
a shortened length of one of these same masts now here in Florida on my 70ft 
crank-up. -73, Mike

-----Original Message----- 
From: David Aslin
Sent: Thursday, December 27, 2012 12:46 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] mast for two big antennas

Jorge,

Depends how big your 'big yagis' are, what wind speed you'd like the
mast to survive.  You have to do some calculations.

For your existing mast, the tricky part can be to identify what material
the mast is made of.  Makes a huge difference to the mast's ability to
survive in strong winds, so it is important/essential that you know what
the mast is made of.

Here I have an Optibeam OB4030 and an Ultrabeam 4-el (like Steppir 4-el,
but stronger).  That's a total of about 20 sq ft of antennas.

When I did the calculations, it showed that for a 15ft mast and antennas
separated by 10ft, 2inch diameter 0.25in wall S355 steel  would fail at
a wind speed of 63mph.  Too low for my risk tolerance.

To get 100mph wind survivability, I had to go to 2.5in diameter S355
with 0.25 in wall.  (I used S355 because that's what I could get with a
minimum strength certificate.  It is similar to, but slightly lower spec
than DOM1026 - I can't get steel to the US DOM 1026 spec in UK).

If you have even medium sized yagis, I suspect your 0.125 inch mast will
fail at quite a low wind speed.  Not surprised you see it bending!



Some resources for learning more:

http://www.arraysolutions.com/images/KT9OMpartI.pdf   Part 2 is also on
the Array Solutions site.

K7LXC's MARC mast calculator program from Champion Radio Products.

Dave Leeson's (W6NL now, W6QHS when he wrote the book) 'Physical Design
of Yagi Antennas' has excellent coverage of materials and mast
calculations.



Hope that helps,

Dave G3WGN WJ6O



-----Original Message-----

From: Jorge Diez - CX6VM [mailto:cx6vm.jorge at gmail.com]

Sent: Thursday, December 27, 2012 14:13

To: towertalk at contesting.com

Subject: [TowerTalk] mast for two big antennas



Hello





Wich type of mast do you use when you have 2 big yagi?s on the same
mast, maybe 10 ft of separation?





Mine 0.125 inch wall mast bend a little with strong winds





73,



Jorge



CX6VM/CW5W



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