Cushcraft A6270-13S Question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD9VV, Sep 14, 2017.

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  1. KD9VV

    KD9VV Ham Member QRZ Page

    This 3 band antenna is quite nose heavy (boom to mast) when mounted according to the instructions.

    I'm wondering if the design engineers assumed amateurs would be using conductive mast above the rotor?
    I'm going to use a 3 foot section of PVC.

    ..or are they concerned the coax would be too close to the driven elements if balanced on the CG?
     
  2. W1VT

    W1VT Ham Member QRZ Page

    Yes, the designers assumed a conductive mast and didn't want to have the vertical conductive mast running though 2M vertically polarized Yagi.
    Instead, the 2M and 440 Yagis are rear mount, so the effect of the mast is very small. This also allows the coax to exit behind the rear mount Yagis, where the coax outer shield has much less effect as well.

    Zack W1VT
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't use PVC as an "antenna mast" other than very temporarily.

    Why not use aluminum or galvanized steel tubing? It's a lot stronger.
     
    K9ASE likes this.
  4. KD9VV

    KD9VV Ham Member QRZ Page

    As my OP indicated, I want to balance the antenna. The current suggested location makes the antenna nose heavy.
    I may use a stay to brace it.

    Nothing wrong with using a 2 foot section of 1.25" PVC..it's quite strong.

    @W1VT: Ya, I am a little worried about interaction of the coax shield if I bring it forward of the driven element.
    I may test both ways.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With a stronger support mast (like galvanized steel), "balancing" is no issue: You tighten the clamps very hard and the antenna will be level, even supported by just one end.

    The problems I've had with PVC mast (other than for very temporary use) is you cannot tighten the boom clamps or even rotator clamps "fully" without compressing and damaging the PVC, which was never intended to be clamped that way.

    Inserting a snug-fit and very straight solid wooden rod can make it possible to fully clamp it, tightly enough so things won't be damaged or rotate unintentionally. I find a 1-3/8" closet pole is a snug fit into 1-1/4" "nominal" Schedule 40 PVC pipe which has an actual I.D. of 1.380" (and an O.D. of 1.660") but to make it fit the pole must be very straight and some lubricant may be required. But this does make PVC "strong" enough to clamp to -- otherwise, it really isn't.
     
  6. KD9VV

    KD9VV Ham Member QRZ Page

    Interesting and thanks concerning compression of the PVC pipe..
    The PVC is 1.25" and will be 20" in length.
    I can see it compressing but I will have to test that before raising this antenna.

    Even though I can do as you mention, steel piece of pipe and crank it down hard, the rotor is light duty; so I have a concern about the force vectors not being vertical, but now askew, forcing the rotor to work harder.

    I will be trying both ways and will post a short video.

    Thanks for all the advice from everyone; it is all useful.
     
  7. KD9VV

    KD9VV Ham Member QRZ Page

    UPDATE:

    The antenna is up, balanced and supported with 2 feet 1.25" PVC pipe above the rotor.
    SWR is all good, no issues. No apparent coupling between the coax shield and driven element; feed poing Z is right where it should be.

    The way they designed this antenna being so nose heavy is ridiculous IMO.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've used the same antenna before, although it was 3 years ago and may have been slightly different (MFJ/CC changes stuff), and it mounted horizontally with no boom sag or tilt at all when clamped to a 1-3/4" galvanized steel mast. That requires very tight clamping which I think would bite into and deform PVC.
     
  9. KD9VV

    KD9VV Ham Member QRZ Page

    I'll try to post a few pics soon.
    So far so good, no slippage, seems tight etc...I'll know more after a year; right? LOL
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Fer sure!

    A small downtilt on such a beam wouldn't really matter. It might actually help if you're on a mountaintop.
     

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