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Sigma 4 / Vector 4000

Discussion in 'CB Antennas' started by smalltiredart, Feb 3, 2020.

  1. smalltiredart

    smalltiredart Active Member

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    Been playing with a Vector 4000.

    Built a teflon high powered gamma for it, and direct fed it.

    When mounted on the tower performance is great. Was able to dial in resonant on the analyzer, however when I install my 4 element Yagi 12 feet below the Vector, the match and resonance change, which tells me that it is interacting with the Yagi.



    The antenna does have a feedline choke, and shows no signs of CMC.

    Anyone ever run into this ?

    I know there are a few on here who have ton's of experience with this antenna.

    Any input appreciated.
     

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    Redbeard U812 likes this.

  2. Shockwave

    Shockwave Sr. Member

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    Whenever I have to work around problems like this, I measure the direction and amount the bandwidth is shifted when the second antenna is added. If resonance shifts upwards by 300 Khz. I tune the antenna 300 KHz. lower before the second antenna is added. Then the shift makes the resonance drop right into place where I want it. There will still be some interaction with the antennas but it should not be all that noticeable once they are tuned as suggested.
     
    CB590 and L2 like this.
  3. smalltiredart

    smalltiredart Active Member

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    Good info. Thanks for that Shockwave.
     
  4. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    STD, I assume the beam is horizontal, right?

    What was the match before and after the change?

    Did the beam change too?


    Below are 2 Eznec models of your idea. One with a two element yagi 12' feet below and the NV4K standing alone. There is a very small difference in the match, and the overlays of these two show the patterns are almost identical.

    Before you try and fix the NVK4 match, maybe consider to move the beam around the mast a bit (maybe 1/4 turn,) without moving the Vector, and see if that makes a difference in your match. Might try watching the match on the beam too.
     

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    #4 Marconi, Feb 3, 2020
    Last edited: Feb 3, 2020
    gamegetter likes this.
  5. smalltiredart

    smalltiredart Active Member

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    Yes sir Marconi the beam is horizontal.

    The NVK4 behaves like it is electrically long once the beam is added to the mast pipe.

    I'll study the models.

    Thanks for doing them.
     
  6. smalltiredart

    smalltiredart Active Member

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    If this helps the NVK4 is 51 feet at the feedpoint
    The 4 element yagi is 39 ft at the feed
     
  7. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Shouldn't make much difference except for the gain and angle possibly due to the increase in height.

    How about the 2 questions I asked?
     
    #7 Marconi, Feb 4, 2020
    Last edited: Feb 4, 2020
  8. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    STD, try the search feature on this forum...it can be a real timesaver.

    Below is an old post I found in a WWDX search that talks about an idea using an Imax over a horizontal beam. I think it also talks a little about some points that Shockwave mentions above.

    I think the Imax probably acts better being close to the Yagi, in the link below, than your NV4K will, but you might find something else doing another search...that talks about your idea.

    Below is an example only.
    Imax 2000
     
  9. HomerBB

    HomerBB Sr. Member

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    I used this setup without problems... guess I got lucky.
     

    Attached Files:

    Jay Mojave, smalltiredart and Marconi like this.
  10. smalltiredart

    smalltiredart Active Member

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    I will need to check the Beam on the analyzer.
    I'll get more data.
    Thanks
     
  11. smalltiredart

    smalltiredart Active Member

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    Shockwave, do you use a choke with this antenna as standard practice ? Or only add it if needed ?
     
  12. smalltiredart

    smalltiredart Active Member

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    I may try them closer together like you have yours
     
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  13. Shockwave

    Shockwave Sr. Member

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    I do not use chokes in my installations. I only see Common Mode Current issues when the antenna is not properly matched to the 50 ohm line or it is being operated outside of its bandwidth.

    If you use it over a wide range of frequencies or with an antenna tuner for more coverage, a choke may not be a bad idea. But if you want a choke to work, you have to isolate the mast at the same point you choke off the transmission line. Another thing to consider is the CST modeling indicates any residual CMC directly under the feedpoint is in a constructive phase for up to another 1/4 wave.
     
  14. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Steve, I don't believe I've ever heard this idea on Common Mode Currents explained this way before. This thread is about the S4/Vector4K. Does your idea here work the same with other CB vertical antenna designs, or is this idea unique with the S4/V4K design?
     
    #14 Marconi, Feb 14, 2020 at 10:17 AM
    Last edited: Feb 14, 2020 at 10:24 AM
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  15. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    That is interesting. Now I have to figure out why I need a choke on my 4k.
     

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