2 Meter Moxon woes

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC0QNB, Sep 28, 2021.

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  1. KC0QNB

    KC0QNB Ham Member QRZ Page

    Over the past few weeks, I have successfully built a six meter horizontal Moxon, and a two-meter horizontal Moxon, those two worked out really well, mainly with an SWR of less than 1.5:1, across each antennas band. Saturday I went to get some pipe for a two-meter vertical Moxon, yesterday I cut the pipe and the wire for the antenna. All of the dimensions are right per the Moxgen program. Today I tested it, and the SWR was over 3:1 across the two meter band. To me, that indicates there is something really off, be it feed line, proximity to metal objects, etc. I replaced the RG58 feeder I originally used with a piece of RG58A since that is all I have connectors and adaptors for. No change. Now I am not saying my coax, wasn't bad to start with, but with two different cables which showed continuity on the shield and the center conductor, and the two were not shorted, I don't think both pieces can be bad, but I can't be sure.

    About a half hour ago, I switched the relative positions of the reflector and DE. Now the DE is fed across the width of the antenna, comes to the antenna on the reflector side, and is fed at 90° to the DE, shield element down.
    Ideas suggestions?
     
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    I also have a vertical Moxon on 2m.
    It's made of wire and is one of those temporary solutions made permanent:)

    2m_vert mox.jpg

    I use a choke at the feed point.
    It consists of two mix 43 ferrite tubes.

    feed point_s.jpg

    Without the choke the coax was sensitive to touch and it was impossible to get a consistent swr reading.
    Now I can't se any influence of the coax.
     
    AK5B and KC0QNB like this.
  3. UT7UX

    UT7UX Ham Member QRZ Page

    Did you measure outside 2m band? I mean you could be wrong with dimensions and accidentally tune the antenna lower or higher due for an example wire's velocity factor. Not necessarily you actually did it, just to be sure.
    You didn't say what the balun type you've used. Not properly built one and especially no any balun at all could cause excessive SWR as well as excessive sensitivity to the cable disposition. Normally cable has to cross reflector's center at 90 degrees and pass further more or less straight to avoid any interaction with reflector until can be bent in whatever direction after few tenths of wavelength.

    My 2m Moxon has B-shaped rectangular frame welded with PVC plumbing pipes. I simply duct taped wires over the frame. Intermediate spacer in the center of the frame is used to wound ugly balun (pretty inefficient at 2m still better than nothing), as feeder support, and as a kind of handle for SAT operations. Sorry, no photos. :rolleyes:
     
    AK5B likes this.
  4. KC0QNB

    KC0QNB Ham Member QRZ Page

    I accounted for the velocity factor of my PVC insulated wire by adding 2% to the 146 MHz frequency, the result was 148.92 MHz, I have no other way to measure except with an SWR meter. I don't own any antenna test equipment. yet.
    I could add 2.5% to the frequency, failing that I am out of wire to use, the end freq would be 149.65mhz.
    I should mention, since I forgot to do so in the original post, I haven't tested the antenna since I moved to the feed point as mentioned, above. I ran out of daylight. I will do that as soon as I have light to work by.
     
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    I just did a test with the Moxon generator and generated an EZnec file.
    Input 145MHz 2mm wire.
    Result: 145.15MHz.
    With insulation: Dielectric constant 3 and thickness 0.5mm added the resonance frequency dropped 3%
    For PVC Diel C is 4 so frequency will drop even more.

    Tell me your wire dimension and insulation thickness and I can run this again.
     
  6. KC0QNB

    KC0QNB Ham Member QRZ Page

    Number 12 stranded with pvc insulation.
     
  7. AK5B

    AK5B Ham Member QRZ Page

    While we wait for SM0GLD's results---and as another fellow cat lover---I must jump in here and thank UT7UX for the link posted on his QRZ page for the ultimate CAT Interface...:D

    73,

    Jeff
     
    UT7UX likes this.
  8. SM0GLD

    SM0GLD Ham Member QRZ Page

    And insulation thickness?
     
  9. SM0GLD

    SM0GLD Ham Member QRZ Page

    I'm eating dinner:D
     
    UT7UX and AK5B like this.
  10. UT7UX

    UT7UX Ham Member QRZ Page

    @AK5B, Jeff, thank you! :)

    @KC0QNB, Ryan, you don't actually need modern vector analyzer (still I'd recommend to have one as it is extremely convenient) to plot a SWR graph; plain SWR meter could be pretty enough. The graph shouldn't be extremely precise, just pick few points at 144.0, 144.5, and so on up to 146.0. The intention is to see regularity, not exact values themselves. Good graph is expected to be gently U-shaped coming with its lowest part as close to 1:1 as possible. Slightly miscalculated antenna will show the graph to be inclined towards the frequency the antenna has been actually built for. That is what I meant when I asked you for out of the band figures.

    IMO it could sound pretty strange for Americans due time difference. :D
     
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