What size mast should I use

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD7VEA, Jan 24, 2021.

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  1. KD7VEA

    KD7VEA Ham Member QRZ Page

    I am about to put my tower back up with some new antennas, and I’m wondering what a good mast option is. I would love to run 2 inch 1/4 wall aluminum, but that’s getting pretty pricey, and I think it’s over kill. I have a 30 foot Rohn 25 tower that I now have a flat plate on top with the Yaesu 065 thrust bearing. I will have my Cushcraft A4s (weighing 37 pounds) about 1 foot above the thrust plate and then a 3 element 6 meter, 14 element 2 meter, 5 element 220, 18 element 440, and a 55 element looper on 23CM all spaced about 12-18 inches apart, so I will have about 6-10 feet of mast above the tower/thrust bearing. What is a good option without breaking the bank? I ran this exact set up years ago on a piece of 2” conduit, but I was running a small cushcraft MA5B instead of the A4s. I realize that’s not a great idea, so I’m wanting to improve on that.
    Last edited: Jan 24, 2021
  2. AK5B

    AK5B Ham Member QRZ Page

    X-length of Rohn 45 with a concrete base and proper guying for that "farm in the sky" ought to do it. Follow the manufacturer's instructions to the letter to be safe, whatever you do.


    WA7F likes this.
  3. KD7VEA

    KD7VEA Ham Member QRZ Page

    I am already using a 30 foot Rohn 25 tower with a concrete pad and guys. I am asking about the rotating mast at the top of the tower.
  4. AK5B

    AK5B Ham Member QRZ Page

    OK, sorry for my misunderstanding. WB2WIK is bound to know the answer; betcha he'll show up tomorrow or thereabouts...


  5. WA7F

    WA7F Premium Subscriber QRZ Page

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Conduit is designed to be bent and formed. I wouldn't use that.

    The 2" aluminum hollowbar (extruded tubing) with .25" wall thickness is a very good choice and comes from the mills in 24' lengths but most of the resellers will cut it to 12' for you, making it easy to transport and also half the weight (although it's not heavy).

    The big advantage, though, is really just "easy to work with because it weighs less than steel, but is still strong enough for the job." It is more expensive.

    Rohn sells suitable masts of galvanized steel. The 2" OD is a good starting point since the A4S will clamp to that and be very happy with it. The UHF antennas don't need a 2" mast, but check their clamping systems to make sure they open up to at least 2" or you'll have to do some modifications.
    AK5B likes this.
  7. N7WR

    N7WR Premium Subscriber QRZ Page

    Calculate the wind load of all of those antennas and I'm sure you will conclude that any aluminum mast will fail. You are looking at a HD molysteel mast if you want to insure the survival of those antennas. I had a 100 foot Rohn 45 double guyed tower a few years ago with a TX2 rotator and a Force 12 C 321 XR yagi. I used a heavy wall molysteel mast and it lasted for years in heavy winds, ice loading, etc. Along came a rogue 103 MPH burst of wind and the mast failed. Most likely there was a flaw in the mast because it shouldn't have failed. Any lesser quality mast would have failed in much lighter winds. Aluminum masts are NOT your friend except with very, very light loads.
  8. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    Using the DX Engineering mast load estimator (thanks WA7F), and accounting for 35,000 psi aluminum rather than 100,000 psi steel, 2.0" x 0.25" wall aluminum (35,000 psi) is good at least 70 mph. I say at least because the estimator speed only reports a pass or a fail. I can obtain a better estimate (it will give a higher wind speed) by adding the correct amount of wind loading along the length of the mast. And, there is the good old paper-and-pencil method that I can do but am too lazy right now to tackle. That will give an exact wind velocity at which the mast will yield.


    OP says, "I will have my Cushcraft A4s (weighing 37 pounds) about 1 foot above the thrust plate and then a 3 element 6 meter, 14 element 2 meter, 5 element 220, 18 element 440, and a 55 element looper on 23CM all spaced about 12-18 inches apart, so I will have about 6-10 feet of mast above the tower/thrust bearing." I used 2' spacing and the mast is 9 ft.

    No ice
    10 ft mast, 2" OD, 0.25" wall, 6161-T6 aluminum
    Cushcraft A4s, 5.5 sq ft at 1 ft
    3 element 6 meter, 1.8 sq ft at 3 ft
    14 element 2 meter, 1.65 sq ft at 5 ft
    18 element 440, 0.7 sq ft at 7 ft
    55 element looper on 23CM, 1 sq ft at 9 ft (my estimate)
    This is equivalent to 3.7 sq ft at 9 ft.

    The mast load estimator is for 100,000 psi tubing. 35,000 psi tubing has 35% the yield strength and can be accounted for in the estimator with a 0.35^0.5 wind factor. A 120 mph rating for 100,000 psi material becomes 71 mph for 35,000 psi material.
    Last edited: Jan 24, 2021
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Good data.

    I've used 2" OD aluminum hollowbar (this is "construction grade" tubing, not used as conduit or anything else, and it's seamless, extruded tubing) all over the place on lots of towers over 30+ years and never, ever had one fail with way heavier loading than what the OP describes.

    At the K2XR contest site up on a very windy hilltop owned by K2OWR in NW NJ, we had this installed on an LM-470 70' telescoping tower:

    Hy-Gain 205BA 20m monobander
    Hy-Gain 155BA 15m monobander
    Hy-Gain 105BA 10m monobander
    KLM 7 element 6m beam on 28' boom

    Spacing between bays >6 feet. Mast length 24 feet (about 3' used up inside tower to reach the T2X rotator).

    That's a whole big bunch of wind loading. Mast was 24' of 2" aluminum hollowbar, .500" wall thickness (1.00" ID).

    Winds up there hit 75-80 mph routinely (windy hilltop with completely open exposure to the NW where most of the winds came from).

    It was up 15 years exactly as described and nothing ever failed.

    Same tubing was used on his other 70' telescoping tower, an HDX72 by U.S. Tower, and that one supported a 3L 40m beam and 5L 20m beam. That one never failed, either.

    This was a small M/M DX contest station assembled over a period of years with most tower work done by WA2VUN, who was a Tri-Ex dealer, then a U.S. Tower (and a Force-12) dealer in NJ for a long time; Mike's also a professional welder and his main business was welding big stuff including underwater pipelines. He trusted the aluminum just fine and used it almost everywhere.

    I wouldn't sweat the small stuff.:p
    Last edited: Jan 24, 2021
  10. KD7VEA

    KD7VEA Ham Member QRZ Page

    I think I may have found a solution, and it’s within my price range, give me your thoughts to make sure I’m not over looking something. I am using a Yaesu 065 thrust bearing at the top of a 30 foot Rohn tower. I will have 6 antennas(7 once I get on 900mhz). At about 1 foot above the thrust bearing will be my Cushcraft a4s requiring a 2 inch mast, everything above the A4S will be 6/2/1.25/70cm/23CM each weighing a few pounds and the 2 biggest of those will the the 55 element 23CM looper and a 14 element 15.5 foot boom 2 meter. All of the VHf and up antennas use a 1.5 inch mast, so here is my thought. I will get a 3 foot section of 2” 1/4 wall aluminum tube.this will clamp into the thrust bearing and handle the A4S. I will run a piece of 1 1/2” 1/4 wall aluminum tube up the center of the 2”, and all of the VHf antennas will mount to the 1 1/2” tube. I will get 20 feet of the 1 1/2” tube so that I can mount the rotator down at the roof height so maintenance can be easy. I can either run a bolt through the 2 pieces to lock them together(not sure, that might weaken the mast). Or I can have the 2 pieces welded together to make 1 solid mast with a 2” 3 foot long jump in the middle. What are your thoughts? Am I overlooking anything?

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