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Author Topic: Chimneys and masts  (Read 1092 times)
WB4SPT
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Posts: 145




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« on: October 14, 2012, 05:50:39 PM »

So, my new house is now 2 story and has a 28' tall chimney, brick about 6' by 2' in external dimensions.  I realize that a chimney has structural limitations, and I don't relish the idea of mortar cracks, etc.  My chimney sweep says that these are not built with any steel.  I"m thinking about putting up a hexbeam to about 40'.  I've tossed around several ideas, from a selfsupporting AN wireless tower to a push up pole using the chimney at a couple of places for support.  The overriding design limit is no guy wires.  My last QTH, I used a Rohn 25 with a house bracket into a concrete/steel lintel at 8'.  The tower went to 30'.  That was a CBS home and made a very solid tower install, now I've got a house of sticks.  I'm not thinking I want to bracket a R25G off of a wood house, at any height.  Any comments on using the brick or wood structure?  

Oh, and I'm in a 70mph design loading area, with .5" ice.
« Last Edit: October 14, 2012, 05:54:57 PM by WB4SPT » Logged
W9GB
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Posts: 2623




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« Reply #1 on: October 14, 2012, 07:05:58 PM »

Quote from: WB4SPT
I'm not thinking I want to bracket a R25G off of a wood house, at any height.  Any comments on using the brick or wood structure?  
My father's house had a 50 foot Rohn 25G house bracketed to roof framing above second floor (~ 30 feet above ground, built around WWI) ... for 35 years -- NO issues when properly installed.  
In other words, need to check wooden framing and support at attachment points.
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K2DC
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« Reply #2 on: October 15, 2012, 04:06:54 AM »

I would not advise using the chimney at all.  I once had a Mini-Quad on a short mast attached to a brick chimney with chimnely straps.  It was a steeply pitched roof and relatively shielded from the prevailing winds.  It still nearly brought the chimney down.

73,

Don, K2DC
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K3VAT
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« Reply #3 on: October 15, 2012, 04:41:47 AM »

I would not advise using the chimney at all.  I once had a Mini-Quad on a short mast attached to a brick chimney with chimnely straps.  It was a steeply pitched roof and relatively shielded from the prevailing winds.  It still nearly brought the chimney down.  73, Don, K2DC

I second Don's concerns.  If the chimney is the best location after evaluating all factors then:
1.  consider mounting the yagi/hex beam only a few feet above the rotator and the rotator only a few feet about the top of the chimney.  Your design of 12 feet above the chimney top would increase low-angle radiation for most bands, BUT without guying this structure it would lead to serious problems with wind/ice/vibration.  
2.  consider a high-performance mini-yagi such as the new series of 2-element N6BT models (http://www.n6bt.com) OR the mini-hexbeam from http://www.k5bob.com/
3.  finally, there are several trap dipoles (or make your own as I did) that could give you 5 band operation and only require a mini-rotator.

Mounting any antenna, especially one that may be hard to handle atop a steep pitched roof isn't recommended.  The Rohn tower along side the house may be the better route.

GL, 73, Rich, K3VAT
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N8CMQ
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Posts: 362




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« Reply #4 on: October 15, 2012, 04:22:46 PM »

Chimneys do not have any strength and WILL fail.
Unfortunately, I have experience with such a failure.
A TV antenna strapped to a block chimney in a high wind
damaged my roof and cracked one of the rafters
when it failed.
If you feel you HAVE to use a chimney, have it inspected first.
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