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RE: Placement of Mounting U-Bolts on M2 Antennas



Dave - N8KXA wrote:

----------
> Currently, I have the U-Bolt assemblies on my M2 2mc14 and 436cp30
> mounted in the recommended place which is about the middle of the
> boom.  In making plans to put up a tower in the next few months, I
> realized that there was no way these antennas were gonna go on it.

> I decided to move them over to the chimney over the next few weeks
> in preperation for the tower.  Then I realized that due to the height of
> my chimney, I'm probably going to have to mount these on a less
> than optimal piece of mast.  I'd like to still be able to clear the
chimney
> by at least 2 feet.  Which means on a 10 Foot piece of 2 Inch mast
> strapped to the chimney, I'm going to have to move the U-Bolt
> assemblies closer to the driven elements by about a Foot and a half.

> How badly is this gonna screw me up?

Should not be an issue with performance of the antennas.  Unless the weight
is reasonable balanced it is harder on the elevation rotor so what I would
do is after getting the antennas mounted with coax, etc, add a
counterbalance if required to the back end of one or both antennas.  Other
than that it should not be a problem.

I'm more concerned about mounting a large antenna array on a chimney.
Assuming it's a "normal" brick chimney around the flue itself, they are not
built to withstand large lateral forces.  Remember that an El/AZ array
generates quite a bit of rotational and lateral forces upon the mast and
then the mounting of that mast.  Having watched my antennas moving, I would
not even consider mounting them on my chimney where the mast clamps can only
be about 18 inches apart.  The other thing to remember about a brick chimney
is that under current building codes (at around here) the space between the
outer brick, and the flue is supposed to be filled with concrete and have
re-bar in it.  If you have an older house, it may not have that.  My parents
found that out when their 1948 chimney was damaged in an earthquake.  There
was nothing between the flue and the brick except air (and you can't tell
with breaking it open).  BTW, had it been build to current code, it would
have survived the earthquake as well.

BTW Dave, a QSL card is headed your way for our QSO while you were out here
in California last month.

73
-------------------------
Jim Walls - K6CCC
k6ccc@amsat.org
http://home.earthlink.net/~k6ccc/
Ofc:  626-302-8515    FAX:  626-302-9999
AMSAT member 32537 / WSWSS member 395

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