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Author Topic: Question about chimney mounts  (Read 9684 times)
W84IT
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« on: May 22, 2010, 04:10:59 PM »

I ordered a Diamond x3200a tri-band antenna today, and now I'm having second thoughts about how I plan to mount it.  I had planned to order a set or Ronard heavy duty chimney mounts, but after taking a good look at my chimney, I'm starting to wonder if it's a bit too "delicate".  It's not a question of bad mortar, it's a question of cross-section...  I'll include a link to a photo, all opinions are of course welcome.

Silas
AB3KD

http://i187.photobucket.com/albums/x161/Katanova/Amateur%20Radio/ChimneyPic1.jpg
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WX7G
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« Reply #1 on: May 22, 2010, 04:34:08 PM »

I'm guessing that your chimney is 2 x 4 feet. That gives a bending moment of 200 lb-ft at the base of a chimney in a 60 mph wind. The Diamond antenna gives a bending moment of 70 lb-ft at the base of the chimney.
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W84IT
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« Reply #2 on: May 22, 2010, 05:21:43 PM »

I'm guessing that your chimney is 2 x 4 feet. That gives a bending moment of 200 lb-ft at the base of a chimney in a 60 mph wind. The Diamond antenna gives a bending moment of 70 lb-ft at the base of the chimney.

Many thanks!  If I had thought about it a few more minutes, I might have realized that I could just walk up in to the attic and measure the thing.  Unfortunately, it far smaller than your estimate, only 17.25 x 13 inches.
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W3LK
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« Reply #3 on: May 22, 2010, 06:36:15 PM »

This is a BAD idea. Chimneys are designed to carry smoke and exhaust products away from a furnace or fire place. They are designed to support only their own weight. They are NOT designed to support antennas or withstand the lateral and torsional forces imposed by masts and antennas. Add to that the corrosive effects of flue gasses on aluminum and you will not only weaken your chimney, but destroy your antenna within a couple of years, maybe less, depending upon how active your chimney is.

Yea, I know, there have been TV antennas fastened to chimneys for years, and virtually every one of them has cracked mortar joints and are in a weakened condition. BTW, damage caused by a failed chimney that's been supporting antennas likely will not be covered by your property insurance or your liability insurance in the event the chimney falls on someone. It may, but I wouldn't bet on it. Looking at your photo, that chimney is WAY too small to use as an antenna support, even if it is in excellent condition, which from the photo, I doubt.

There are so many better ways to support antennas.
« Last Edit: May 22, 2010, 06:39:17 PM by Lon Kinley » Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K7UNZ
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« Reply #4 on: May 23, 2010, 05:06:20 AM »

Aside from what W3LK mentioned, there is also the fact that the chimney spews a lot of soot, and other nasty stuff, which can make short work of the antenna.

Jim/k7unz
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WX7G
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« Reply #5 on: May 23, 2010, 05:50:14 AM »

That is a small chimney and the antenna will load will exceed the wind load of the chimney.
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W84IT
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« Reply #6 on: May 23, 2010, 07:02:01 AM »

This particular chimney no longer vents anything, the furnace was replaced with a high efficiency model many years ago.  The point about exhaust gases remains valid though, for others who may be thinking about using a chimney mount, and reading this thread.

I think I have conformation here, that this chimney is just too small  As much as I hate putting lag bolts through a shingled roof, I'm going to go ahead and use a tripod.

73's,
Silas
AB3KD
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N5YPJ
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« Reply #7 on: May 23, 2010, 11:44:31 AM »

This is a BAD idea. Chimneys are designed to carry smoke and exhaust products away from a furnace or fire place. They are designed to support only their own weight. They are NOT designed to support antennas or withstand the lateral and torsional forces imposed by masts and antennas. Add to that the corrosive effects of flue gasses on aluminum and you will not only weaken your chimney, but destroy your antenna within a couple of years, maybe less, depending upon how active your chimney is.

Yea, I know, there have been TV antennas fastened to chimneys for years, and virtually every one of them has cracked mortar joints and are in a weakened condition. BTW, damage caused by a failed chimney that's been supporting antennas likely will not be covered by your property insurance or your liability insurance in the event the chimney falls on someone. It may, but I wouldn't bet on it. Looking at your photo, that chimney is WAY too small to use as an antenna support, even if it is in excellent condition, which from the photo, I doubt.

There are so many better ways to support antennas.
DITTO. Look around there are better alternatives to the chimney.
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N7ZM
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« Reply #8 on: June 04, 2010, 03:24:22 PM »

Silas,
       I have used the Ronard Heavy Duty Chimeny mount now for over 3 years. I compared them with Radio Shacks mounts and ended up buying the Ronard. Is is built like a brick and is real heavy duty, no cheezy parts. It started out holding my DXCC antenna, and when I got my tower up, the DXCC went to tower and now it supports my Diamond X-500 2/440 19 foot antenna. Great mount and I highly recommend it!
73 Ron N7ZM
N7ZM@att.net
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KD6KWZ
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« Reply #9 on: June 05, 2010, 04:12:03 AM »

Wow, this depends. I had a small VHF scanner antenna, and a big Starduster 5/8 wavelength ground plane CB antenna attached to the 2 story chimney of our old house. The antennas & chimney made it though the Loma Prieta earthquake & several large storms without coming down. We never used that chimney for fires, so smoke & heat were not an issue. The chimney was about 4 feet by 4 feet, so it was hefty.

That said, the mortar on some chimneys can be rather weak. Yes, that is a small chimney there, so that would make load much more critical.

Quote
As much as I hate putting lag bolts through a shingled roof, I'm going to go ahead and use a tripod.

Make sure the lag bolts "bite" into the boards below the shingles! Caulk around the base of the lag bolts after installation to keep water out of your attic.
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AA4PB
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« Reply #10 on: June 05, 2010, 07:10:08 AM »

You really should make sure that the lag bolts go into roof rafters or 2x6 bracing that you have added in the attic and attached to rafters. The plywood on the roof really isn't suitable to accept and hold lag bolts.

I coat the bottom of the tripod feet with roofing tar before setting it into position. The tar then squeezes out around the bolt hole and edge of the foot when you tighten it down. Then smooth over the top with a little more tar after it is in place.
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W84IT
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« Reply #11 on: June 05, 2010, 09:38:42 AM »

You really should make sure that the lag bolts go into roof rafters or 2x6 bracing that you have added in the attic and attached to rafters. The plywood on the roof really isn't suitable to accept and hold lag bolts.

I coat the bottom of the tripod feet with roofing tar before setting it into position. The tar then squeezes out around the bolt hole and edge of the foot when you tighten it down. Then smooth over the top with a little more tar after it is in place.


I agree.  I ordered a Ronard 2009 tripod from Solid Signal, and it came in today.  It comes with tar pads for the feet, and it is designed so that all the feet line up with rafters, provided those rafters are on standard 16" centers.  I'll use a stud finder to make sure I have the feet lined up with the rafters.

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K2QPN
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« Reply #12 on: June 06, 2010, 08:29:08 AM »

I would advise you not to use a chimney mount for your antenna. I had an 8 foot long 2M and 70cm vertical mounted atop a 5 foot mast on my chimney. I used a normal chimney antenna mount.  My neighbor informed me that the chimney had fallen over. Sure enough, the top foot on the chimney was laying on the roof. It broke between the antenna mount straps. Fortunately the chimney guy was able to patch things up. No more chimney mount antennas for me.

73, Bob K2QPN
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