2M Yagi, slant mount?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by ND8D, Mar 17, 2021.

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  1. ND8D

    ND8D XML Subscriber QRZ Page

    I am in the process of planting a 50' aluminum tree that is overkill for my hexbeam.

    I got the idea to put a 2M beam up there to have directional gain at my disposa1. Initially I was going to get a 7-10 element and mount it horizontal for 2m SSB. I ended up getting a Cushcraft 4218 XL "Boomer" for an absolute steal of a price from a local ham. It is an 18 element 29' long monster of a beam with 17.2 dBd of gain. I want to use it.

    A friend of mine and I were talking about it and we both had this thought to slant mount it at 45 degress so I can have use of it for both vertical and horizontal polarization modes, taking a ~3dB hit from either use.

    On the surface that seems like a reasonable proposition, could only go wrong if I try to work somebody else with a yagi slanted the other way no? Am I missing something? Just looking for some discussion on the topic.

    As an aside, nowhere have I been able to find an online copy of the Cushcraft 4218XL manual, the OM who sold me this one had the original manual which I have scanned and uploaded as screenshots below for perusal, if somebody comes across this thread and wants the PDF version, please feel free to email me.
     

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  2. WA1YHO

    WA1YHO XML Subscriber QRZ Page

    You have a serious 2M beam! I had the next generation Cushcraft 3219 up in the air for many years. Nice antenna. But it is big. And it has a very sharp pattern. Make sure you have a good rotor and be prepared to use it a lot.

    Mounting the antenna at a 45 edge angle is a common practice for those wanting both polarizations. My interest was weak signal work so I wanted every dB in the horizontal plane. I put a 10' Comet vertical on top of the mast for FM.

    You ask an interesting question and I am not aware of any convention on whether CW or CCW twist is preferred.
     
  3. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page


    Is this all we can get from the proof of purchase tag on a book of zigzag leaves in 2021?

    RRK
     
  4. AI3V

    AI3V Ham Member QRZ Page

    I would just mount it horizontal.

    My thinking: the stations you work who are serious about simplex dx will allmost all be ssb and horizontal.

    And even with a mismatched polarization on fm repeaters I think you will be able to "bring up" one on just about every frequency pair around you.

    When I lived in Pittsburgh I could, with about the same EIRP all things considered.

    Rege
     
  5. WA1YHO

    WA1YHO XML Subscriber QRZ Page

    I had the feeling that I was forgetting something really important when I hit the button on my previous post....

    Cushcraft does not show a graph of the SWR but I suspect that the bandwidth is quite narrow. The design was optimized for low end, weak signal portion of the band. The data sheet just says 144-145 MHz. I don't know what SWR they considered acceptable, 2:1 perhaps, but I suspect the SWR would rise very quickly above 145MHz. And the beam plot would also deteriorate.

    Mount the antenna horizontally and enjoy the SSB performance.
     
  6. ND8D

    ND8D XML Subscriber QRZ Page

    Good point there, I'll likely stick it horizontal and be done. The element spacing is quite wide which I understand narrows up the bandwidth.
     
  7. N9DG

    N9DG Ham Member QRZ Page

    Yepp, this is pretty much the fatal flaw of the 45 degree mounting idea. If everyone does that, then everyone will be throwing away around 20 dB of performance whenever they are pointed towards each other. The problem occurs when both users are slanted the same way and pointed towards each other. Throws away more gain than the even largest yagi's can provide.

    So if you secretly do this, don't tell anyone, and don't popularize it! :D
     
  8. WA9SVD

    WA9SVD Ham Member QRZ Page

    I, too would mount it horizontally. While that is certainly best for CW/SSB operations (especially weak signal work) and the 'crossed polarity" is a major concern, the high gain may still give you acceptable performance with vertically polarized FM operations. What WILL be required (if it works) is you WILL need to rotate the antenna in the direction of the repeater (or simplex FM station) you want to contact, it will NOT be an "omnidirectional" antenna like most vertical antennas. What MAY be a significant factor is how high the SWR rises between the SSB portion and the FM portions of the band. The SWR may rise to the point where your radio is either very unhappy, and/or reduces power because of a high SWR.
    Good luck!
     
  9. WA2CWA

    WA2CWA Ham Member QRZ Page

    Here's a copy:
    http://www.popular-hifi.com/shack/ant_cc18/cushcraft_2m18.pdf
     

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