[TowerTalk] Rohn 25G

Bill Coleman aa4lr at arrl.net
Tue Feb 28 22:46:37 EST 2006

On Feb 27, 2006, at 9:40 AM, kb0fhp at comcast.net wrote:

> I have purchased several sections of Rohn 25G - 3 straight  
> sections, and the 8' top section with rotor plate. I want to attach  
> this to the side of my two-story house, at two locations (top  
> header board, and the bottom board on the second story).  This is  
> about 10' and 20' up from grade.

While you might be able to get away with lag bolts at the 10 foot  
level into the rim joist, you are going to need carriage bolts  
through the header into the brackets.

> What additional things do I need to purchase besides the 10' piece  
> that goes in the concrete; the house brackets; and a thrust bearing  
> for the mast - what else is required?

You don't need any section that goes into the concrete. Just put the  
bottom Rohn 25 section into the concrete. This would give you about  
33 feet and change above ground (with almost 5 feet in the ground)  
The top of the tower would be 13 feet above the bracket. You could  
add another section and be at 43 feet, but that's a lot of tower  
above the bracket.

If you do use a 6' base section, then you would be at 39 feet, 19  
feet above the top bracket -- about as far as I would recommend.

> Antennas planned are Hy-Gain TH3-MkIII, CC 505S, CC 215 and a M2  
> 432-9wl antenna....

That's four antennas - you're going to need a lot of mast for that!  
This will increase your overturning moment. Structurally, you're  
better off with the largest antennas at the bottom of the mast.  
Electrically, the TH3 would want to be the highest, and the 505S the  
furthest away from that with the other two in the middle.

I would run some calculations to make sure your bracketed  
installation will have enough strength to hold all these antennas at  
the height you desire at the maximum wind strength in your county.  
You also might calculate the forces on the top bracket.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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