Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Bracketed Rohn 25G  (Read 2306 times)
W5DWH
Member

Posts: 43




Ignore
« on: August 04, 2008, 06:25:28 PM »

If I bracket a 50' Rohn 25G tower at 16' what kind of wind load will the tower be able to handle?

73
Logged
K3GM
Member

Posts: 1801




Ignore
« Reply #1 on: August 05, 2008, 04:19:10 AM »

According to Rohn specs, a 50', 25G, bracketed tower should have 2 bracket points; one at 18' and one at 36'.  That will give you an allowable antenna area of just under 7 sq. ft.
Logged
W3LK
Member

Posts: 5639




Ignore
« Reply #2 on: August 05, 2008, 11:28:41 AM »

34' above the bracket? Probably not much more than the tower itself.

73,

Lon - W3LK
Naugatuck, Connecticut
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K7MH
Member

Posts: 339




Ignore
« Reply #3 on: August 05, 2008, 11:40:02 AM »

I used to have 40 ft of 25G house bracketed at 12 feet.
Uh...it was a little spooky! Had a Cushcraft A4 on it. I was in a very low wind area, low in a valley. It was more wobbly to climb than I care for and when it was somewhat windy it would "creak" inside the house from flexing. The bracket would transfer the noise to the house, kinda like a guitar. I wouldn't do it that way again although for about 20 years I never had a problem with it.
Logged
K0XXX
Member

Posts: 36


WWW

Ignore
« Reply #4 on: August 05, 2008, 07:45:42 PM »

Rohn 25G is surprisingly strong and forgiving. However, it only takes one failure to be sorry. Plus, if they are used tower sections, you don't know what types of stress that they have been subject to in the past, or if there is any internal corrosion. I definitely wouldn't climb it, with it un-guyed above the 16 ft. level. If you decide to do this, make sure that your insurances (liability, health and life) are up to date. ;?) 73!
Logged
KF7CG
Member

Posts: 836




Ignore
« Reply #5 on: August 06, 2008, 05:47:08 AM »

Search for Rohn and find their engineering website. It has all the specifications for Rohn 25. This includes the allowable free-standing wind loads.

With the proper concrete base, Rohn 25 can support at least 3 square feet at a height of 40 feet.

I am doing this from memory as I don't have the time at this point to run down the data and don't have my printed copy in my office.

KF7CG
Logged
WB2WIK
Member

Posts: 20599




Ignore
« Reply #6 on: August 06, 2008, 08:21:28 AM »

No, it's not 3 s.f. at 40'.

25G is rated 1.5 square feet at 40' at 70 mph; 1.4 s.f. at 80 mph; and "0" s.f. at 90 mph.  Many places now require 80 mph or 90 mph ratings, and I've seen places in Florida requiring 110 mph ratings.

Now, even those ratings are based on having the antenna right at the top of the tower (not above the top on an extension mast) and are for antenna designs using round members only (that is, round tubing, rod and elements and no square or flat stock which degrades the rating).

The allowable antenna area sheet for the Rohn "G" tower products is here:

http://www.antenasystems.com/pdfs/rohn/25G/25G-1.pdf

I would not install 50' of 25G bracketed at 16' and then put any antennas on it at all; it wouldn't be safe to climb so a bucket truck or crane would be required for all antenna installations, and that's expensive.

WB2WIK/6
Logged
W5DWH
Member

Posts: 43




Ignore
« Reply #7 on: August 06, 2008, 08:43:54 AM »

According to the Rohn specs a freestanding 35' can handle 3.6 sq ft at 35'. Since my tower would be 35' above the bracket I would assume that I could have 3.6 sq ft as well.

That's not enough for a small beam, rotor and mast though.

If I decreased the height to 45' (30' above the bracket)  the windload goes up to 6.4 sq feet.
That would cover a small beam, rotor and mast.
Logged
WB2WIK
Member

Posts: 20599




Ignore
« Reply #8 on: August 06, 2008, 11:41:33 AM »

It might, and it probably will.

However, remember the 6.4 s.f. at 30' is based on no mast, all round antenna members, no ice, and 70 mph wind.

Check your local zoning/building codes regarding wind ratings for antennas and supports.  In many places, it's still 70 mph; in some, it's 80 or 90 and in some it's 110 or higher (hurricane zones).

I don't like the "feel" of 25G that's not guyed, it gets wobbly after just one extension section.  With a sufficient foundation and bracket, it's probably okay, just feels funny.  Secure, properly placed and tensioned guys eliminate that feeling and the tower becomes solid as a rock.

WB2WIK/6
Logged
K9KJM
Member

Posts: 2415




Ignore
« Reply #9 on: August 06, 2008, 11:27:21 PM »

There are many HUNDREDS of Rohn 25 towers in this area that were installed in the 1970's (Over 30 years ago) With one cubic yard of concrete for a base, FREE STANDING, With very large (Around 2 square feet windload) "TV" directional antennas and rotors. NONE have come down until taken down. There are also a number of 60 foot "free standing" Rohn 25 towers with the same type of antenna installed. At 60 feet the towers do require at least yearly maintenance to keep the sections tight.  

There were a few 70 foot Rohn 25 free standing type towers tried.   A Rohn 25 will COME DOWN in a high wind at 70 feet!  (The tower will buckle and fail, Not the one cubic yard concrete base)

I would be very comfortable with a Rohn 25 50 feet tall with a solid bracket at 16 feet up with around 4 or so square feet of windload.  
And I live in a "high wind" area.

Yes, A rohn 25 DOES sway a bit several sections above its bracket. A very strange feeling until you get used to working on them.  I too much prefer working on a well guyed tower. But the point is, The free standing and bracketed DO WORK.

Rohn rates the 25 to go 40 feet above it's highest supporting bracket.  Or at least used to.

Logged
KI4VEO
Member

Posts: 166




Ignore
« Reply #10 on: August 10, 2008, 09:20:46 PM »

You're a better man than I am, Gunga Din !
Logged
W3LK
Member

Posts: 5639




Ignore
« Reply #11 on: August 11, 2008, 07:12:45 AM »

<< Yes, A rohn 25 DOES sway a bit several sections above its bracket. A very strange feeling until you get used to working on them.>>

I hope you have good life insurance.

73,

Lon - W3LK
Naugatuck, Connecticut
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
WB2WIK
Member

Posts: 20599




Ignore
« Reply #12 on: August 11, 2008, 09:10:05 AM »

>RE: Bracketed Rohn 25G       Reply
by K9KJM on August 6, 2008    Mail this to a friend!
I would be very comfortable with a Rohn 25 50 feet tall with a solid bracket at 16 feet up with around 4 or so square feet of windload.<

::I wouldn't, and I've tried it.  R25G ratings for 35' above a bracket are 3.6 s.f., no ice, all round antenna members.  That doesn't include a climber, just a modest antenna, all round member construction, no ice -- and 70 mph.  Under the same installation conditions at 90 mph, the rating is "zero" (not rated at all).

>Rohn rates the 25 to go 40 feet above it's highest supporting bracket. Or at least used to.<

::Still does, at 1.5 s.f., no ice, all round antenna members, and also no mast and no climber, at 70 mph.  Rating at 80 mph is "zero."

That rocking motion is very distracting for me, I won't climb one that moves.

WB2WIK/6

 
Logged
K9KJM
Member

Posts: 2415




Ignore
« Reply #13 on: August 11, 2008, 10:53:35 PM »

An important item I need to clarify is that what I (And others in this area) Call a Rohn "50 foot tower" is four straight 10 foot sections, And the top 9 foot section. The bottom section is actually below grade 3 feet (In the cubic yard concrete base) So the above grade portion of that tower is only an actual 46 feet.
But then add a 10 foot mast with at least 4 or more feet sticking out of the tower so the antenna IS actually 50 feet above grade..........
Same for the 60 foot towers I described. (I will admit those free standing were no fun to work on at that height)  More foolish than I attempted the "70" foot ones that did fail in high winds.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!