What is the recommended height of a bracketed mast?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN4LCJ, Jun 13, 2021.

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  1. KN4LCJ

    KN4LCJ Ham Member QRZ Page

    Hi folks.. new member here..

    I am putting up a new antenna (Comet GP-6) above the gable end of my house. The apex of the gable (where I will install 2 or 3 brackets) is approximately 15' above the ground. I will be using top rail fencing for the mast (1 3/8" diameter). These come in 10' lengths and I have 3 of them. The Comet is 10' long and weighs 3.8 lbs.

    My plan is to dig a hole 1.5' deep to drop the base of the mast into. This will give me approximately 3 1/2' of the 2nd rail above the brackets. I would like to have at least 10' above the brackets, so I can cut down one of the rails 3 1/2', then the bottom of the 3rd rail would be at the bracket. But of course if I can squeeze another foot or two that would be nice, if it would be safe. I will not be using guy wires. Coax cable will be LMR-400.

    So my question is would it be safe to go approximately 13' above the brackets, or just 10'?

    Thanks,
    KN4LCJ
     
  2. AK5B

    AK5B Ham Member QRZ Page

    I believe you are asking for trouble there without any guying above the brackets---what would be wrong with some "invisible" 1mm Kevlar guying? Wouldn't need much to accomplish that, but what if a big storm kicks up gusty winds and you only have the brackets to rely on?

    http://www.spiderbeam.us/product_info.php?info=p241_100m roll Kevlar guy line (1mm).html

    This stuff is unbelievably tough, even though it's so thin. I can now see why bullet-proof vests are made from this!

    73,

    Jeff
     
    WB5YUZ likes this.
  3. KN4LCJ

    KN4LCJ Ham Member QRZ Page

    Thanks Jeff. Unfortunately I don't see anywhere I could attach the guys. Its a simple ranch style house (1 story) and the antenna will be on one of the gable ends.
     
  4. AK5B

    AK5B Ham Member QRZ Page

    Is the house made of wood? Screw eyes in two places and the third could be a tall T-post, stake or screw-in anchor in the ground.

    If not, there's probably many other ways to do so...

    73,

    Jeff
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I’d be very careful with gable brackets and 1-3/8” mast material, and also don’t know what placing the bottom of the mast 3-1/2’ into the ground will do other than cause that part of the mast to rust away over time.
     
    WZ7U likes this.
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Neither is safe or even likely to survive the first gust of wind. All masts should be guyed at the very top, even stronger ones like push-up poles. The strength of all such structures comes mostly from the guys! This is especially true of masts made of home-improvement store grade fence top rail.

    The only exception is those flag pole masts, and they will not support external antennas; the mast itself, or conductive material embedded in the mast, is the radiating element in those cases.

    In fact, having built one 30' fence top rail mast, which is properly guyed, including at the top, I'll never build another. They are just too flimsy. 20-25' is about the limit for me, and again, that's only if they are properly guyed.

    I know you've already put in some time planning this, and I hate to be the bearer of bad news, but
    unless you can guy this mast at the top, you should abandon this plan.
     
    K0UO and AK5B like this.
  7. W4HWD

    W4HWD Premium Subscriber QRZ Page

    You better drop some concrete in that hole with your mast. The base will move around and wallow out that hole if you don't; this I know from experience.
     
    KJ7WRZ and K0UO like this.
  8. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    The joints on fence top rail are not very tight. They have a good bit of play that allows them to wobble.

    You can stiffen the joints with a piece of angle Aluminum and a few hose clamps.
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you go to a commercial fencing company (not HD), you can buy the galvanized fence top rail in 21 ft sections. If the GP-6 is clamped to the top of a 21 footer, which is stood-up in one piece and then clamped to a substantial gable bracket, I have done that without guys, and it survived for years...
     
    WE4E, W2TH, AK5B and 1 other person like this.
  10. WS4K

    WS4K Ham Member QRZ Page

    Spend some money and buy a Rohn push up telescoping mast
     
    WB5THT likes this.

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