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Author Topic: Roof Mounted Antenna  (Read 1033 times)

Posts: 21

« on: August 06, 2001, 10:27:28 AM »

Fellow Hams: Just moved to a new QTH and have a problem.  No CCRs, but due to position of power lines, only feasible antenna location is on the roof.  Portion of the roof where I can mount antenna is flat and metal covered.  Due to location, I need to get antenna as high up as I possible can.  Have looked at the Glen Martin roof "tetrapod" towers.  Any other voices of experience here?  BTW, am considering a Force 12 Series 40 antenna 10-20 plus 40.  Any ideas of help is greatly appreciated.  Glenn, N4ZW

Posts: 21764

« Reply #1 on: August 06, 2001, 10:45:26 AM »

The G-M roof towers are strong, well designed, well built and lightweight.  They are a good choice, and using the thrust bearing (a must!) will support a long mast, even with the F12 beam -- the only weakness will be the mast itself.  I'd recommend as a minimum using 2" aluminum hollowbar with 1/4" wall thickness -- readily available from aluminum suppliers.  If you can't find that, opt for high carbon steel mast from Texas Towers, like their 17' lengths having .18" wall thickness ($190).  If you start out with the G-M RT-1832, which is 18' tall, and add a 17' mast, your antenna can be installed 31' above your metal roof, which is just about high enough to let it play properly on 20m.

The G-M is as strong as its installation, so creating a very strong roof attachment is obviously critical.  It comes with an installation guide showing recommended procedures for roof attachment, and the ideas presented there are good.

If you live in a high wind zone, you can guy the G-M roof tower; my experience here in a moderate wind zone is that guying was unnecessary (we don't get hurricanes).

73 & good luck!

Steve WB2WIK/6

Posts: 550


« Reply #2 on: August 06, 2001, 11:07:03 AM »

If you are not ready to go all the way with the expence of a beam and tower yet, you might consider that flat metal roof as a launching pad for low angle DXing. Even if it is not perfectly flat, having a raised ground plane like that is a rare opportunity you might not want to pass up so quickly. A 1/4 wave vertical might do a nice job while you look into a rotatable antenna...

Posts: 3585

« Reply #3 on: August 06, 2001, 12:26:44 PM »

  The GM towers are very strong and will do a fine job for you. The weak link is the mount to the roof.

  You don't say, but if you have access to the bottom of the roof (inside) it's a very good idea to put in a bridge the same size as the rafters, cut to fit snugly between the rafters at the mounting point to bolt the base of the tower to. Use sixteen penny nails through the rafters into the bridges, and metal clips at all four corners of the bridge.

  It's also a good idea to spread the extra load out by installing interior 2X6 cross braces across the bridge pieces and several rafters each way from the mounting point. Use metal "hurricane clips" everywhere one piece of wood butts into another.
  Also take a good look at the way the rafters are attached at each end. I have seen far too many rafters held with one sixteen penny nail at each end. And that's not nearly stout enough. It's not usually too hard to install "hurricane clips" at each end of the rafters to keep everything in place when the wind blows.

  73  Pete Allen  AC5E

Posts: 1435

« Reply #4 on: August 08, 2001, 06:59:44 PM »

You might have better luck on 40 with a 4 square of some kind.  That should lighten up the wind load on your roof.

It gives me the creeps to think of an 18 foot cage bolted to my roof with 10 feet of mast above it and a simple tribander on it.  But, I would trust Glen Martin Engineering, and I do trust Steve's knowledge.

I don't ever want to be in the position of telling my insurance agent that I need roof repair, cause my roof tower blew over in high wind.

Have FUN
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