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Author Topic: Freestanding Rohn 25G  (Read 5404 times)
KK5NW
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« on: September 30, 2009, 05:12:39 PM »

I am contemplating freestanding 40 ft of Rohn 25G, with 5'- 6' in the ground, atop which would sit a Mosely 32N Jr. (two-element-beam, with a 6' boom). I do not plan to place this antenna on a tall-mast out of the tower-top. This antenna poses minimal wind-load and the tower would be mounted between two, houses.  A recently installed a metal-roof on my house and do not want to bracket the tower at the peak of the roof, as this would only provide approx 20' of clearance between the tri-bander and my roof. I appreciate your input and personal experience.
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WB2WIK
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« Reply #1 on: September 30, 2009, 05:26:25 PM »

I wouldn't do it.

If you need a building permit (as most places will to install a permanent tower) and the engineer who reviews the application looks at the Rohn 25G specifications and your installation, he'd likely deny it.

Also, I don't think you'd want to climb 40' of unguyed/unbracketed 25G.  It will feel fine for the lower 20 feet and feel scary above that...
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K3GM
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« Reply #2 on: September 30, 2009, 05:54:05 PM »

Please don't attempt this!

You could sink 10' into the ground, and it won't matter.  It's the amount of unguyed 25G above the ground you are concerned with.  If you're that close to other structures that they lie within a radial fall zone, you risk putting yourself in serious personal, perhaps even criminal liability from a structure installed contrary to the manufacturer's recommendations.  Other major concerns would be how to climb the unguyed tower to install and maintain the antenna(s).

I've seen 100' installations sitting on a base poured with a couple of 80# sacks of concrete, and guyed to nearby tree trunks.  The major difference being if it fell, it was on the owner's land, and entirely out of range from other structures, or personnel.

The best thing to do is to go to the Rohn site and investigate for yourself.  The tallest unguyed amount of 25G is rated at supporting only a couple of square feet of antenna with no ice load.
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K9KJM
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« Reply #3 on: September 30, 2009, 09:52:42 PM »

Your plans for a base are much too large.  A Rohn base is 3 feet square (One cubic yard of concrete)

As already pointed out, Going deeper or larger will not make the tower any "stronger"

Rohn specs say 40 feet of 25G "freestanding" above the last support. This depends on how much windload your antenna will be.

Out here in the real world, Rohn 25 towers stand at 50 feet freestanding for decades with a large TV antenna (only) at the top. (That includes the 3 feet in the concrete, So above the concrete would be an actual 46 feet.) (9 foot top section.)

At 60 feet freestanding, A Rohn 25 will require constant maintenance to keep the joints tight. They DO move in the wind, But I have never seen one come down.

At 70 feet freestanding, A Rohn 25 WILL COME DOWN in a high wind.  All these examples are with a large TV antenna only on the tower.

I suggest a set of guy wires for your tower. With only a single set, It will be nice and solid.
(3/16" EHS galvanized guy wire)
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AA4PB
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« Reply #4 on: October 01, 2009, 05:49:28 AM »

Rohn specs say 40 feet of 25G "freestanding" above the last support.
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What Rohn specs say that?  My Rohn manual specifies (for small antennas) 20 foot above a house bracket for 25G. A good assumption would be that it also implies only 20 feet of free standing 25G.

I sure wouldn't climb a 40-foot free standing 25G. You may suddenly find you have a "foldover" version :-)

My advise would be to get a Rohn manual and follow there recommendations.
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N4OGW
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« Reply #5 on: October 01, 2009, 06:55:37 AM »

Rohn specs say 40 feet of 25G "freestanding" above the last support.
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What Rohn specs say that? My Rohn manual specifies (for small antennas) 20 foot above a house bracket for 25G. A good assumption would be that it also implies only 20 feet of free standing 25G.
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It's in the specs (barely): Page 25G-1 in my Rohn book, "Self-supporting towers- Allowable antenna areas", DWG. NO. A871266R1. Lists 40 ft max for R25, 70 MPH wind speed, no ice. Says it will support 1.5 sq ft of antenna at that height. It also assumes one 5/8" transmission line per tower face. With 1/2" ice it drops to 0 sq ft. Not allowed for 80 MPH or higher.

Base listed on drawing B870725, 4 ft deep by 4 ft square, 2.4 yards concrete with rebar.

Still, adding guys or a bracket will make it a LOT stronger. Or going to 45 or 55G.

Tor
N4OGW
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AI7RR
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« Reply #6 on: October 01, 2009, 04:01:41 PM »

I put in a 40' freestanding 45G and it's wobbly at the top. Using 25G, you can build it, but I sure wouldn't climb it.

73,,Roger
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W8JI
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« Reply #7 on: October 02, 2009, 05:59:28 PM »

1.5 sq ft of antenna is a medium-large TV antenna and rotor, not a tribander. They call for a 4x4x4 base, serious overkill in solid soils.

Most small tribanders are over 5 sq ft wind area. The rotor might add another 1/2 or so.

Don't count your chickens on that 5/8th feedline. While they allow for one 5/8th feedline on each face, that isn't a whole lot of wind loading on the tower. The feedline runs down the tower, so only the very top of the feeder adds any modest bending in wind.

I would not be scared to climb that tower on a fairly calm day (if it is really Rohn 25, and not 20). I would not want to be near it on a windy day with a medium sized tribander on the top.
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W8JI
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« Reply #8 on: October 02, 2009, 06:01:26 PM »

I think it would be OK with a 2 element short boom tribander, but not in a bad wind over 60 MPH. :-)
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