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Glen-Martin 4.5 foot Mini-Tower Suggestions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NE2X, Sep 29, 2011.

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  1. NE2X

    NE2X Ham Member QRZ Page

    All...

    I always enjoy the responses, thus my situation and any idea are welcome..

    Have a Glen-Martin 4.5 Mini-Roof Tower. All Assembled and on the ground. Have a Yaesu G-450A Rotor but need to decide what size thrust bearing to buy...

    Theory is to mount Moseley 2 element TA-32 18 inches (1/1/2 feet) above the tower top. Then a 3 or 4 element 6 meter beam 5 feet above the TA', then a Cushcraft A-148-20T near the top of the mast, but a 3 foot piece of PVC will be slipped over the 1/1/4 mast and this will stop any vertical elemental interaction..

    So, GM 4.5 tower, then 10 foot 1 1/2 inch mast tapered to a 10 foot 1 1/4 mast. The 1 1/4 will be in the 1 1/2 mast by about 2 feet, so total mast length is 18 feet with about 3 feet into the router and coming out.

    Is this a reasonable scenario? I want the 2 meter cross-pol on the very top..And , can I buy another A148-20T and phase these 2 antennas?

    I was also looking at some mini-beams, but the TA-32 is the longest I can use here..

    Thanks for reading

    NE2X
     
  2. KF6ABU

    KF6ABU Ham Member QRZ Page

    That will surpass the maximum of 6sq ft wind based on Glen Martins website...

    I have a TH5-DX on about 15ft of mast on my RT-424. I have the mast guyed at 2 elevations with thrust bearings. My mast is over 2 inches.

    Your set up may work, but I wouldnt feel too comfortable with it.
     
  3. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    PVC slipped over the mast to prevent vertical interaction?? HUH?
     
  4. KF6ABU

    KF6ABU Ham Member QRZ Page

    my guess was he ment he would extend the mast with PVC pipe and mount the antenna on the pvc extension portion.
     
  5. KF6ABU

    KF6ABU Ham Member QRZ Page

    Also remember you cannot just use lagbolts to your roof's plywood. It has to be connected to braced rafters/joices.
     
  6. NE2X

    NE2X Ham Member QRZ Page

    Yes, as for PVC, the Cushcraft is mounted on the top so the elements have no interaction...

    As for the lag bolts, custom 4 x 8 deck wood painted the color of the roof, 3/8ths threaded rod ( 16 inches) fed through the roof to another 4 x 8 And braced against 3 rafters Mine are 24 inches apart..
     
  7. N0AZZ

    N0AZZ Ham Member QRZ Page

    If it were mine I would use a mast pipe that was intended for the job and not jury rigged in multi-stages and a factory thrust bearing.
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I read this over and I'd recommend assembling all of it on the ground first, before installing it on the roof.

    That way you can see if it's all working the way you think it will. Even 3' of PVC tubing may not be strong enough for the A148-20T...PVC looks strong and feels strong until you put an antenna on it. When you tighten down the U-bolts to mount the antenna, it will compress the PVC and may never get "tight enough" to prevent undesired rotation.

    I also wouldn't "taper" the mast 1-1/2" to 1-1/4" at all. That's a weak point, and 1-1/4" mast unless it's very special isn't very strong, either.

    I have a G-M RT-832 (8' tall RT on 32" leg spacing) and use a 2" mast from rotator up to "everything." It's strong. It will be eleven years this coming November 1st and nothing has ever budged a millimeter since the day it was installed. I've snapped my fair share of 1-1/4" masts (just wind, blowing the load) and won't do that again.:eek:
     
  9. AH6RR

    AH6RR Ham Member QRZ Page

    I have a GM RT-424 also with lots of bracing under the rafters. I have a 10' 2 inch Schedule 80 alum. mast with a SteppIR 3 element with the 30/40M loop 2 feet above the tower and a 7 element Gulf-Alpha VHF at the top. I use a Yaesu GS-065 thrust bearing and it has been up for over 5 years now with no problems at all.
    I would not use a mast over 10 feet on this tower without some serious guying both to the tower and mast. Why do you not add the 6M elements to the boom of the HF beam some retuning might be needed but it will work just fine and use a separate feed line for it then put the 2 meter antenna at the top of the 10' mast. Oh and using the PVC will do no good because the metal will still be there it will just be covered in plastic. My 2 meter beam is mounted vertical and I have no problems with the metal mast whatsoever.

    Good Luck
    Roland AH6RR
     
  10. NE2X

    NE2X Ham Member QRZ Page

    Roland,
    I like your input and it has me implementing your idea....1 thing. The PVC is a "sleeve" and slides over (not all the way) to the mast...The 2 Meter x-pol is on this. And yes, I have used PVC with masts before and I see no issues with this.

    Total mast length is 12 feet and will be aluminum now...9 feet above the thrust bearing...
     
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