How high should beam be above tower?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4JNS, Feb 3, 2012.

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  1. K4JNS

    K4JNS Ham Member QRZ Page

    I have a triband beam and this is me first beam and tower install. My question is how high should the beam be above the top plate??? Thanks
     
  2. KA0JON

    KA0JON Ham Member QRZ Page

    I put my Tennadyne T-8 Logperiodic 1ft above the top of the Rohn 25 9ft Tapered top section, that way
    I had a place on the vertical pipe in the rotor to make a "rotor loop" in the coax.
     
  3. NM7G

    NM7G Ham Member QRZ Page

    You need to think mechanical, since obviously you want it as high as possible from the RF perspective. However, the specs of the thrust bearing, strength of your mast material, plus wind loading for the overall structure (yagi, mast, coax, tower, and any guys) all drive the decision. Will the triband be the only antenna on the tower?

    Since we know nothing about your tower, the quality of base it's fastened to, the size of the yagi, and wind profile for your QTH, I won't offer a guess. Did you have an engineer provide calculations for a construction permit?

    73, Gary
     
  4. K4JNS

    K4JNS Ham Member QRZ Page

    Yes the triband will be only antenna. Its 30ft of 25g plus the top cap. Wall mount at 16-18ft and 3x3x3 concrete base. 8ft of water pipe is what i was going to use for the mast but once the antenna was bolted on I realized I can not lift it and the antenna the four foot needed to bolt the rotor in. Was thinking about trimming 3ft off it, which would give me 12-16 inches out the top.
     
  5. K9STH

    K9STH Ham Member QRZ Page

    JNS:

    Water pipe is a VERY poor choice for a mast. Not only is it heavy, it is also very weak in terms of the force needed to bend the pipe.

    Now the actual answer to your question is "high enough so that the tower doesn't interfere with the rotation of the antenna. With most antennas this is a matter of inches. Any higher is generally better. But, for an absolute answer, just high enough to rotate.

    Glen, K9STH
     
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Why would you trim it? If there's more mast than you need sticking out the top, just leave it there; might be handy for a second antenna someday.

    That kind of mast and antenna would normally not be heavy for me. But when masts become 16'-24' long (as some of my installations are) and loaded with big antennas, then that definitely becomes too heavy; in that case what I do is use a scissors jack to lift the mast from its bottom until it's above the rotator, then align it and drop it into the rotator using the same jack. This works fine, and is the procedure used by a large number of professional antenna installers.

    I've used that technique to lift and drop 24' of 2" chromolloy mast (where the mast alone weighs 160 lbs) with five HF beams on it (adds about another 200 lbs) on towers. I can't lift an assembly like that one inch, alone. But the jack can.
     
  7. N0AZZ

    N0AZZ Ham Member QRZ Page

    I use nothing but antenna mast that's meant for the job the one I always buy is the double walled 1/2 way up for the extra strength it provides. My multi-band a 10-40m Mosley Pro-67 C3, 133# is 1' above thrust bering next up is a M2 24' 6m yagi, then a M2 M25WL 2m 17 ele yagi 33 1/2 ' boom at 70' a 2/432 17' vertical GP. I have a lot of weight and wind load on my 55' US Tower and their 20' mast.
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I agree with Glen that water pipe isn't a great choice for this.

    My masts are usually thickwall high strength extruded aluminum alloy; 2" O.D., 1.5" I.D. It's quite light and very strong. Available in 12' lengths at most aluminum yards.
     
  9. K9STH

    K9STH Ham Member QRZ Page

    The mast on my main tower is 2-inch diameter, 0.095-inch thick seamless steel tubing. Come July, it will have been "up" for 40-years! Still going strong.

    The mast on my "short" tower is actually a vertical support from a DB Products model DB-408 antenna. It is double-walled aluminum tubing. I don't know the exact alloy, but, considering that DB Products have survived for decades hundreds of feet above ground on towers all around the world, I "suspect" that it is pretty good! :p

    http://k9sth.com/uploads/Antennas_At_K9STH.pdf

    By the way, the trees in the photos of the short tower and 40-meter vertical are now gone. The tree by the short tower died and the one by the vertical started leaning. Therefore, bye-bye trees!

    Glen, K9STH
     
  10. K4JNS

    K4JNS Ham Member QRZ Page

    OK water pipe gone replaced with aluminum alloy. I'm glad I remember my friend that does CNC work he had 6ft scrap laying in the shop floor. :) so much lighter. I lost two foot but it's still 2.5ft from the plate so I'm happy with it. Thank you all for the response and information.
     
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