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Author Topic: Wind load versus EPA, non-standard Rohn 25G installation  (Read 1169 times)
W1BTM
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Posts: 6




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« on: March 28, 2013, 06:30:08 AM »

Is Effective Projected Area (EPA) that is listed on Rohn tower specifications the same unit as wind load that is listed on Cushcraft antenna specifications?

Last year I put up 65' of Rohn 25G at my house. Winter brought snow soon after I finished so putting up antennas got put on hold. Spring is near and I'd like to get some idea of the wind rating as I plan antennas.

Most of what I read in preparation said not to do anything "the manufacturer didn't tell you to do." Well, I violated that a bunch. While I have a fair amount of land, yard and field here in the country, it was difficult to find a location where the guy wires weren't going to be likely to be hit by one of our farm tractors, and also weren't in the woods where a tree was likely to fall on them. So the tower installation is somewhere between a guy installation specified by Rohn and the crazy "I used to put up a billion feet of Rohn 25G for CB base stations with my bare hands" stories.

The tower is house bracketed at 20 feet and guyed at 40 and 60 feet. This distances were chosen at smaller values than the usual 30 feet for added safety factor because two of my guys go to large eyebolts added near the peak of the roof. I provided a lot of bracing for the eyebolts. I put 2x6 perpendicular across a few rafters which are 2x8, and added two 4x4 blocks in between the roof sheeting the 2x6 to make solid wood in between the two ends of the eyebolt. I also added vertical and horizontal 2x6 braces between the rafters and the floor joists, and the rafters on both sides of the house. The eyebolt is a 1/2"x12" forged and shouldered galvanized bolt (Grainger 3LVY2) with 1/4"x2" galvanized square washers on both ends (Grainger 5RU26). The third guy anchor is pretty much Rohn spec, with a GAC303 in a 3x4x3 block of concrete in the ground. The tower base is about 3x4x5 feet of concrete mixed with a Harbor Freight mixer, with a little over 5' of a 10' section of tower sitting on a little bit of crushed stone for drainage. It's pretty much right up against the house basement in a lot of nice gravel fill so drainage should be good.

The tower is grounded to the house ground, has a 10' ground sunk next to the base, two 10' grounds about 20' out (I forget) at 45 degrees from house (90 degrees from each other) and another 10' ground next to the on concrete guy anchor. I haven't decided yet if I should do anything about grounding the eyebolts back to the tower structure. The six guy wires are all broken once by insulators.

I've been figuring that the 20.3/15.2/20.2 wind ratings (Exp B Rev G/Exp C Rev G/Rev F) on the 60' Rohn 25G90R060 guyed tower are a safe estimate.

Photo links are mixed in above, but the whole set is here: http://www.flickr.com/photos/btmspox/sets/72157632210343082

I picked up a used IC-718 over the winter and plan on upgrading to General just as soon as the testing opportunity gets here. I've been reading and thinking about what path I should start down on for an HF antenna.

Inverted V from the tower to the roof? House probably isn't long enough and the ground slopes away on one end.

Put a beam on a rotator up top? Decent investment, and have been get more confident about the wind load

A Dipole from the tower out to one of the trees in the backyard is pretty doable.

Bryan
W1BTM
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W5CPT
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Posts: 557




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« Reply #1 on: March 28, 2013, 07:22:25 AM »

I am not a Mechanical Engineer, so all the comments should be taken with that in mind.
After looking at the pictures, I wouldn't hesitate putting up beam on that installation.
I have seen towers with much less guying and bracing hold up large antennas for a long time.
(Mine for instance.)
I have not looked at a Rohn manual in a long time, but there was as much lawyer in the specs as there was engineer. 
Meaning = they put a considerable safety cushion in to avoid litigation.
Many of the failures I have seen were to undersized long masts that was used to get the last few feet of height.
With your tower being a wave length high on 20M and it appears that the ground slopes away from the structure, that installation should do well with only a short piece of thick walled mast. 
DO NOT use plumbing pipe.  As attractive as the price and availability are, you will rue the day you did that, when you have to figure out how to get a antenna down from a bent pipe. (Hint - a crane is involved).

Clint - W5CPT -
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