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ROHN Allowable vs. Effective table help?

Discussion in 'Amateur Radio Antennas' started by CB590, Jan 9, 2019.

  1. CB590

    CB590 W9WDX Member

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    What's the difference? Confused here.



    Rohn 25G 30'. Table says 1.7, other says 7.9 sq ft. Both say 90mph.. I'm looking at Gizmotchy with 5.7 sq ft

    Screenshot_20190109-094102.png

    Screenshot_20190109-094117.png
     

  2. undertaker

    undertaker Undertaker

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    25G will be fine...
     
  3. CB590

    CB590 W9WDX Member

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    So chart 2 is the one to use. The 'Effective Projected ??'
     
  4. undertaker

    undertaker Undertaker

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    Yes sir................
     
  5. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    What is your question in particular? From what the charts tell me at 90 mph for three seconds at 30 feet your maximum Square footage should be no more than 1.7 SqFt. in chart number one.
    Chart #1 is a Designer's safety margin to over design the installation.
    Chart # 2 is a actual performance guide. It is what you can truly can expect to be the maximum.
    Commercial Liability for a defective design is very expensive if it falls down on the public. Liability is one of the considerations for an engineer for maximum safety and longevity of the project.
     
    #5 Tallman, Jan 9, 2019
    Last edited: Jan 9, 2019
    Slowmover and Shadetree Mechanic like this.
  6. CB590

    CB590 W9WDX Member

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    I'm trying to figure which chart to use based on my 5.7 sq ft. Proposed beam..

    I'm thinking ROHN 45 at 30' and 80 mph then based on what your saying.

    Though undertaker is saying the other chart and good with Rohn 25..
     
  7. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    I probably should not got into this thread, I Diesel design everything.
    (Humor) I designed an aircraft could taxi like crazy, turn on a dime, zero to 500 mph in less than four seconds, but was too heavy to fly. If I spent money on a tower, rotator, and a beam antenna I want it to stay up there forever. I personally go with the 45G at 40 feet.
    But remember I over do design.
     
  8. CB590

    CB590 W9WDX Member

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    I wanted to know which chart I should go by.. but I get it.. the 45G would be better for longevity. The issue would be the $300-350 price increase over the 25G.. with the $250-350 I could buy the tilt base plate though..
     
  9. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    I would use the second chart for the 45G. I look at it this way, The antenna cost good money, the rotator cost good money, the coaxial cost money. If the 45G survives 2 or 3 storms that the 25 might fall down it has paid for itself. JMHO.
     
  10. CB590

    CB590 W9WDX Member

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    I do like the 6.5 sq ft 110mph rating on the 45G @ 30'..
     
  11. CB590

    CB590 W9WDX Member

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    3star has the 45G for $904 and $170 freight.. not bad.. the 25G is $530 and $170 shipping.. $370 difference.. hmm
     
    Tallman likes this.
  12. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    What is the cost of the rotator, antenna, and coax?
    Is this is the one you are buying? $150.00 from ebay
    [​IMG]
     
    #12 Tallman, Jan 10, 2019
    Last edited: Jan 10, 2019
  13. CB590

    CB590 W9WDX Member

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    No the 10/11 m G45 version is like $360.. not the 6m one..
     
  14. Blackcat630

    Blackcat630 Sr. Member

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    I would also recommend 45..if financially possible. Having climbed both, there is definitely a noticeable difference in stability when you're on them. I have 48ft of 45g and with 2 guys working at the top it moves less than 1 guy on top 40-50 ft of 25.

    One other thing to keep in mind with regard to cost. The 45 requires a larger base. So you're looking at spending more on concrete.
     
    #14 Blackcat630, Jan 10, 2019
    Last edited: Jan 10, 2019
  15. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    I agree. I usually stay away from these too except for recommending proper guying lengths and heights and maybe what type of base of put in. Personally I myself will NEVER use a guyed tower of any sort but that is just me. I have several antennas to mount on the same structure and getting them up and down a tower with guys is a royal PITA. With my free standing tower it is a piece of cake with no guys in the way. Raising a multi-element HF yagi with guys is just too hard unless you have the ca$h to spring for a crane. I had a crane truck install my 64 foot Trylon Titan in one piece and that was expen$ive enough at $150/hr with a two hr. minimum. I also tend to over-engineer things. All my antennas come with the standard mounting brackets for a 2.0 inch OD mast. I upgraded to a 2.0 inch ID mast which is about 2 3/8 inch OD. Not that much difference but the wall is thicker and it means changing out all the brackets on most of my antennas. I also threw away the 1/4 inch U-bolts that came with them and upgraded to stainless steel 5/16 hardware. If the antenna already has 5/16 hardware I upgrade to 3/8. I just do this out of the box.....why wait for an ice storm to point out what I should have done before. I also reinforce the boom and elements where brackets attach to keep the tubing from deforming and to allow greater clamping pressure. Antenna manufacturers build for the average installation. Here on the east coast where we can get heavy ice conditions and a nor'easter of near hurricane strength together I tend to err on the side of caution or as we like to say on the side of common sense.
     

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