John,
Regardless of it being a Code violation, attaching things (especially antennas) to a brick and mortar chimney, is just plain stupid.
Having said that, quite a few houses over here in NZ wear the Low band VHF antenna (think 5 elements and each element over 3 ft long), as well as the mast to hold it up in the air, usually 1"-1 1/2" galvanised steel.
Often these things are never guyed at all.
But, the torsional forces on the bracket and the strapping are huge in high winds, not to mention the actual wind resistance of the antenna itself.
Chimneys of this sort were NEVER built with the idea of having something attached to them in mind, after all it is a chimney, first and last.
Whenever I see a chimney mounted antenna (of whatever type) it just makes me think that the guy that installed it, just didn't care about his work.
I mean, I've worked on these antenna's, the soot and rubbish that comes out of the chimney, makes things even worse.
There is always a better option.
BTW John, You'd never see a Ham attach any antenna worthy of the name to a chimney, it just isn't done.
