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Re: Cross boom length?



At 05:04 PM 3/18/2006 -0500, AJ9N@aol.com wrote: 
>
> In a message dated 3/18/2006 3:54:57 PM Central Standard Time,
> al7eb@acsalaska.net writes:
>>
>> http://www.qsl.net/al7eb/sat.htm
>
> Ed, 
> I took a look at your rooftop setup and have a few questions if you don't
> mind.  I have essentially a flat room on most of my house and really would
> hate to put holes into it while trying to hold the tripod down.  It looks
> like you built some frame up and are holding it down with bricks.  I have
> thought of the same thing.  Are you using wood or metal for the frame?  Did
> you have to get a structural engineer involved to determine the roof
> loading?  How did you decide on the number of bricks to put up? 
>  
> Thanks in advance. 
>  
> 73, 
> Charlie Sufana AJ9N 
> One of the ARISS mentors
>

Charlie,

My decision to build a "non-penetrating" antenna mount was prompted at
first by
the desire to get something up fast.  Also, in my mind was that holes in a
"nearly" flat roof = "leaks" inside the roof...also the house is 30-years old
of unknown construction so where would you place the screws?

My first attempt (motivated by speed and cheapness) was made from 2x4 wood and
nailed together.  I weighed it down on the corners with two cement blocks. 
65-mph wind in 2003 proved that was not good enough with the result of a
broken
in half 1296 loop-yagi with back half of the elements flattened and a broken
reflector element and twisted boom bracket on the 432 antenna.  The dish was
saved because the long yagis took the force of the tip-over without striking
the dish (it slipped a little around the crossboom).

So version-II was made from 1-1/2 inch steel angle iron bolted together
(1/4-20
hardware, locking nuts)...and the use of four cement blocks at each corner
with
one extra block in the upper mid span.  So far (3-years) no problems.  My most
challenging time of the year is March-Spring which is windiest.  From late
Oct.
to Apr. the roof is snow covered and the frame is essentially frozen onto the
surface.

I weigh 175-lbs and have not fallen thru the roof.  That is the extent of
engineering structural analysis done.  I live in the borough (similar to
county
in the lower-48) which has no building regulations or permitting.  I can build
whatever I desire.
I am an EE so have some sense of building design (also built my own 2-story
log
house some years ago -- yes its still standing, see my website under misc. for
photos).

Up here house roofs have to built to at least 80-lbs./sq-ft for snow loading
(new guidelines or regs suggest 120-lbs/sq-ft).  But elsewhere the norm is
40-lbs/sq-ft roof design.  Practically, if you can walk on it without any
sagging or punch-thru it will be fine for the antenna support.  The steel
frame
is about 12x14 foot square...this provides a wide foot-print to avoid tipping.

73's,
Ed - KL7UW 
===================================
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
Amsat #3212
Modes: V - U - L - S
=================================== 
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