An aluminum mast to stack a 2el 40M yagi above another yagi? I personally
think its a bad idea. You would have to use 3/8 wall (at least) aluminum
and then weight savings would be just few tens of pounds. Dead weight is
not the issue, its bending moments transfered to the mast from the spacing
of the antennas. For good performance put the 40M a minimum of 10 feet
above the tribander. Now do the clacs for bending moments. Stick with the
4130 alloy steel (chrome moly) at least .25 wall. Do suggest a Rohn 45G
tower instead of the 25G though (if within your budget); you will not regret
it in the least..
>From experience here: 87ft of 45G, 5el 20M (52ft boom) at 88ft and Cushcraft
2el 40M at 100ftm (boom truss overhead at 102ft). Thats 12 feet of spacing
and all antennas (and mast) are happy campers. Been up 12 years NADA
problems. Mast is 21ft 9in of 4130 chrome moly (.25 wall) with 6ft 9in
inside tower. Rotor at 77ft, 2 thrust bearings support the mast and
antennas. I have seen 95mph winds here with torrential rain. I worried
about the trees, NOT the antennas/tower. Often get 50-60 mph monsoon winds.
Aluminum yagi antennas and amplifier chassis. Steel towers and masts. Use
correct alloys for each application.
Thats my take on it from experience.
73 de Arne N7KA
New Mexico
----- Original Message -----
From: "Mahlon Haunschild" <mahlonhaunschild@cox.net>
Cc: <TOWERTALK@contesting.com>
Sent: Tuesday, January 20, 2004 8:41 PM
Subject: Re: [TowerTalk] Weight of mast material
> I don't know about the mast per se, but your overall plan concerns me.
>
> Assuming that the 40m antenna you are talking about is the Cushcraft
> XM240, a little math shows:
>
> Weight Wind area (sq ft)
> mast 90 2 (est.)
> C31XR 82 10.7
> XM240 55 5.5
>
> totals 227 18.2
>
> Rohn 25G is only rated for 8 sq ft at 70 ft. & 110 mph per their
> drawings (you are in 105 mph land, per EIA/TIA 222, so you need to
> design for that). Also, 227 lbs rotating weight is a LOT for the 25G
> structure to withstand. How will you control the torque caused by the
> antenna stopping & windmilling?
>
> If you're married to the idea of a 70 ft 25G tower, one thing you should
> do is get an aluminum alloy mast. Cuts down the weight dramatically.
> Another thing to do would be to not use a 40m beam.
>
> regards,
>
> Mahlon - K4OQ
>
>
> Julio Peralta wrote:
>
> > Thanks to all who replied RE: the weight of Chromolly. I think it would
be safe to figure it a about 90#.
> >
> > Our radio club here in Tampa is considering using this mast to support a
Force 12 C-31XR and a shorty 40, maybe the Cushcraft, on a Rohn 25 tower.
> >
> > Does anyone see a problem with the weight of the mast material in this
application?
> >
> > The tower is planed for 70' and guyed at 3 points.
> >
> > Julio, W4HY
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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