[TowerTalk] How much can a chimney support?

[K7LXC@aol.com]

In a message dated 99-01-10 14:46:06 EST, mlbruss@ucdavis.edu writes:

> I have a brick chimney that is in excellent condition.  It's 2 x 3 ft and
>  protrudes 3 ft above the roof.  The center flu is 1 x 1 ft and has a
>  fireclay liner.  Currently there is a 10 ft mast (1.5" in diameter)
>  attached to the chimney with two stainless steel bands.  Since the chimney
>  is not at the edge of the roof, no mast can extend to the ground, so the
>  only supports for the mast are the steel bands.  On top of the mast is a 6
>  ft vertical antenna.  This setup has worked fine for about 1.5 years.
>  
>  What I would like to do is put a TV-type rotator on top of the 10 ft mast,
>  and then attach a 3 ft mast section to the rotator.  To the 3 ft mast, I
>  want to attach a Yagi (Cushcraft A505S, 12 ft long, 11 lbs, wind load 2.9
>  sq ft) and put the 6 ft vertical on top of the 3 ft mast.
>  
>  Ice is not a problem here, but it can be windy.
>  
>  Can the chimney take it?  Are there any other problems I need to consider?
>  
       First, Yolo County is a 75 MPH windspeed zone. (TIA-222 windspeed info
is available at http://www.championradio.com.)

      I think that your chimney will take the load but the straps probably
won't. What I would suggest is to use a piece of pipe attached to the chimney
and a mast-mounted rotator. I use them all the time in tree mounted yagis. Get
a 3-foot piece of galvanized water pipe and drill 3 mounting holes in it,
leaving enough pipe above the top hole for rotator mounting and clearance
above the top of the chimney. Use a masonry bit and drill through the chimney
in the three places on the mast. Use 1/2 inch all-thread cut long enough to
stick out both sides a couple of inches. Use big flat washers on the flat side
and attach the pipe to the other. With your small antennas, torque on the pipe
is minimal. For anyone contemplating a small HF yagi or similar, welding a
flat plate to the mast will eliminate any system damaging torque. Put a short
mast in the top of the rotator and install your antenna to it. Mount the
antenna just above the top of the rotator - not up the mast - in order to
minimize the bending moment forces. Hook up the cables and away you go. 

       Use safety goggles when drilling the chimney and BE SURE to be safety
roped when you're on the roof. I wouldn't recommend the ten-foot mast you have
previously since it'll create a large bending moment on the chimney mount. 

Cheers,    Steve     K7LXC

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