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Author Topic: Guy or not guyed  (Read 669 times)
K3JVB
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« on: December 01, 2007, 07:02:14 PM »

Hello all

Need some opinions. I am setting up a few omni loops for vhf. These will not be on my mast with my hf and vhf beams. I want to do a vent pipe mount.

The vent pipe is standard issue 4 inch diameter steel.
I was wondering if I can get away witout guy wires??
From the pipe to the top , the mast will be about 15 feet. It will hold a par of stacked 6 meter loops 12 feet apart, and also a pair of two meter loops.

Do you think the wind loading will be too much , without guys..?? I(f I have to, I can do away with one of the 6 meter loops, and keep the overall lenght 11 feet.
The mast is 1.75 inch alum.
Thanks
JohnB
k3jvb
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W3LK
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« Reply #1 on: December 01, 2007, 07:57:42 PM »

I think you are putting entirely too much strain on the vent pipe. A 15' moment arm on a typical 4" x 18" vent pipe is asking for trouble, even with the minimal load of a couple of loops. It was designed to vent sewer gas, not support antennas.

If you must use the vent pipe to clamp the mast to, I would strongly recommend that you guy the mast at about the 10 or 12 foot level.

73,

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K3JVB
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« Reply #2 on: December 01, 2007, 08:10:44 PM »

Thanks Lon

Yes, that was what was on my mind. I may keep it at a ten foot level, and scratch one of the 6 meter loops.
As it will be mostly used for local nets, the extra little bit of gain may not play that big of a role.

JohnB
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N3BIF
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« Reply #3 on: December 01, 2007, 11:02:43 PM »

 I put a 5 element 2m Yagi on a 6 foot mast on my vent pipe and while it seemed OK at first I discovered that it caused creaking and vibration in the vent pipe which caused quite a commotion on the interior of the house. By the time I  got back up there ( few days) I found the it had worked all the shingles loose and the hardware that had secured the vent flange to the roof surface. Fortunately I was able to patch things up with a bit of tar and some new nails and no internal damage was noted.
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K9KJM
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« Reply #4 on: December 01, 2007, 11:48:30 PM »

All modern day houses are PVC plastic for a vent pipe. That type will not support much downward pressure weight (Or side pressure either!)
Older homes used CAST iron pipe for vents, NOT steel. They also had lead/oakum joints that will support very little side pressure (But should support a pretty fair amount of down pressure) So a few steel "clothesline" type guy wires should support your mast attached to a cast iron vent pipe.
(I would not count on that old cast iron/lead joint pipe for a good ground, Do run some heavy wire to ground it also)
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KB9CRY
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« Reply #5 on: December 02, 2007, 06:06:01 AM »

I wouldn't mount anything to a vent pipe.  
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KE4DRN
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« Reply #6 on: December 03, 2007, 05:31:35 PM »

hi,

many homes on west coast use ABS for dwv pipe.

I agree, don't use the vent pipe.

73 james
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K1CJS
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« Reply #7 on: December 21, 2007, 05:35:42 AM »

You will be much, much better off using a roof tripod AND guy wires.  Even if the vent pipe will support your setup, it is NOT made for or meant to do such a thing.  If the roof tripod is not feasible, buy a couple of extra mast sections and go to the ground level.  Three feet of 3/4 inch iron pipe pushed into the ground with a bolt put through it to support the bottom of the mast will stop it from walking and from sinking into the ground.
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AA4PB
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« Reply #8 on: December 21, 2007, 05:23:33 PM »

I wouln't use a vent pipe. They are not intended to support anything and are often not fastened very securely to the structure. Any slight movement of the vent pipe may result in breaking the seal between the pipe and the roof, resulting in leaks. You'd be much better off to use a tripod designed for the purpose.
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