Hi Jason,
Before even thinking about the engineering of your idea, I did some quick
checking on the Web.
For about the same money, you could get a US Tower MA40 crankup mast (min.
height is a couple feet more than you say you need, but it IS engineered for
your situation) and a Cushcraft A3S. I dare say the A3S is a design proven
over decades of use, the Hex is relatively new... As well, you get to lay
the mast over for antenna work if you wish.
I'm curious as to how you would reach the Hexbeam mast, the bottom of which
would be nine feet in the air... Perhaps your deck is high enough, I don't
know.
In any case, my way, you would be spending the $849 (plus concrete) on the
tower and $449 on the antenna. Your way, you would be spending $889 on the
antenna and $450 (or more, plus concrete) on the tower.
Logic would suggest that in your case the safer route is to spend the most
on the tower, not on the antenna.
That the minimum height of the MA40 is 21.5 feet I wouldn't worry about.
It's still low enough when retracted that you couldn't see it from the
street, which is all your city really cares about anyway. In a legal sense,
your mileage may vary.
I hesitate to disparage your idea, but it appears you may be making
assumptions about windloading and the transfer of forces that may not be
quite accurate. Five square feet at the top of a 30-foot mast will transmit
one heck of a lot more force to the tower than five square feet at the top
of the tower. Add the five square feet you estimate for the mast and I would
guess you would exceed the ability of the Rohn in very short order. I'm not
an engineer, so I can't calculate exactly the forces at play, but it would
make me very nervous. At the very least, thrust bearing or not, the mast may
exert lateral force on the rotator that the rotator may not be able to
withstand. The sway of the mast would pivot at the thrust bearing.
In a good wind, the tower may stay vertical, but it could tear apart the top
of the tower.
I also worry that torsional forces would amplify in the mast, also exceeding
the rotator's ability.
Don't forget that the people who extend 30 feet of mast out of the top of a
tower do so with a tower and rotator of substantially more strength than 25G
and 450XL. And they typically use more than one thrust bearing so that
lateral forces using the top thrust bearing as a fulcrum are put into the
lower thrust bearing, not the rotator.
Sorry to rain on your parade.
73, kelly,
ve4xt
----- Original Message -----
From: "Jason Hissong" <jhisson1@columbus.rr.com>
To: <TOWERTALK@contesting.com>
Sent: Sunday, October 13, 2002 12:45 AM
Subject: [Towertalk] An idea...oh boy
> Hi gang,
>
> I think I found a good compromise solution for my tower quest. The city
> wants me to go through alot to get the permits and being that I am a
casual
> DXer and contester, I really don't want to spend the $3000.00 to get this
> thing installed (and the trouble of the city). The city says that I can
> install a retractable tower that when retracted must be below my house
> height (19') and when extended, no higher than 35'. Here is my idea:
>
> Get a SB25G base section and a 25AG4 top section (with a thrust bearing
and
> rotor shelf). Basically, a 9 foot tower simply to install the rotor (I
> have a Yaesu 450XL). Get a Hexbeam and their HDX30 telescopic mast.
> Install the HX5Bi on top of the mast (which the HX5Bi is only 19 pounds).
> The rotor would turn the mast (and the beam) from within the 9 foot tower
> section. I could install the "tower" right behind my deck. Everything
> would be accessible for me (I am afraid of heights anyway) and when things
> get really windy, I will lower the mast. I of course would be grounding
> everything like I should.
>
> The Rohn specs state that a free standing 10 foot 25G section can handle
> 19.7 Sq Feet of wind load. The Hexbeam would be 5 ft2 and the mast no
more
> than 5 ft2. The HDX30 is designed for the HX5Bi. I would still make the
> foundation to Rohn specs (2' by 2' by 4' drilled pier type foundation).
> Would I be able to mix this concrete myself (Rohn catalog states that a
> round pier would be about .5 Cu Yards)? Or would ordering it still be the
> best? Also, could I get buy with a smaller foundation (the base section
is
> only 3 feet high)?
>
> Thanks for the input.
>
> Jason Hissong
> N8XE
>
> _______________________________________________
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>
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