8 inch through-wall so259 adapter causes high SWR in 70CM band

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN6OAK, May 8, 2021.

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  1. KN6OAK

    KN6OAK Ham Member QRZ Page

    Interesting problem. Antenna direct SWR @ 446.000 50 watts = 1.2, Radio to dummy load - same results, so I know my connectors are good to go. When plugged into the through-wall adapter, swr spikes to well over 3.0 @ 446.000 @ 50 watts. 10 watts it is back to normal. 2 Meter does not show this problem at 50 watts.

    My guess is the through wall adapter may be grounding to the drywall or moisture barrier on back side of stucco so I wrapped electrical tape around the total length and the same bad results.

    Has anyone had this particular problem.?
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Coax shields are grounded all the time. It makes difference in SWR.

    -Measurements at different powers should not yield SWR variations or spikes in feedlines in good condition. Intermittent shorts, kinks or migrating center conductors may cause this. Has the cable been 'abused"?

    -Are you using a quality barrel or a cheap hamfest/internet no-name device?

    -Are you using the exact same cables and connectors?

    -Have you checked coax & connectors for continuity & shorts end to end? Flex all along the length and flex the connectors.
     
  3. K6LPM

    K6LPM Ham Member QRZ Page

    My guess is that lengthy barrel connector has some sort of lower quality dielectric material that just isn't up to handling the Higher Freq. RF. It is likely breaking down and causing a major Impedance bump.
    Pretty much want to avoid PL259/SO239 "UHF" style connectors above 2 meters.
    Real UHF enthusiasts will use N connectors if they must. A continuous length of LMR 400 is better than a feedline with breaks and splices for sure!
     
    WA8FOZ and W2TH like this.
  4. W9IQ

    W9IQ Ham Member QRZ Page

    Have you tried repeating the measurements with the 8 inch barrel connector installed but removed from the wall?

    - Glenn W9IQ
     
  5. KN6OAK

    KN6OAK Ham Member QRZ Page

    Coax shields are grounded all the time. It makes difference in SWR. = Correct, but when connected to an un-insulated 8 inch barrel, the ground is exposed hence wrapping it in electrical tape to form a thin insulation barrier.
    Has the cable been 'abused"? = Brand new Belden 8214
    Are you using a quality barrel or a cheap hamfest/internet no-name device? = That would be correct, no name barrel.
    Have you checked coax & connectors for continuity & shorts end to end? Yes Flex all along the length and flex the connectors. = All cable and components have been verified to be free of shorts or defects.

    All testing without the barrel show perfect SWR, so it's the barrel. I'll see if Amphenol has a barrel. My question is weather the wall is causing a ground short, thus high swr reading.
     
  6. KN6OAK

    KN6OAK Ham Member QRZ Page

    Had I been able to do an attic install, a direct feed would have been used.
     
  7. KN6OAK

    KN6OAK Ham Member QRZ Page

    That is my next test... I didn't even think of doing that, Thanks.
     
  8. KN6OAK

    KN6OAK Ham Member QRZ Page

    Test complete. Barrel connector removed from wall tested at 3.0 and when removed tested at 1.3. So cheep garbage barrel connector seems to be verified as the problem.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've been doing this a long time and have used the 70cm band since 1967 and have never in my life seen an 8" long barrel (feedthrough) connector made by anybody that works on 70cm.

    The good ones (very high quality, American-made) only go to about 2" long and the big-time RF connector manufacturers stop there.

    To feed through a wall where an 8" length is required, for use on 70cm I make a feedthrough assembly of two type N bulkheads with 7" of high quality 50 Ohm cable and two type N male connectors, thus "homebrewing" an 8" feedthrough. Works perfectly, no change in VSWR, handles all the power I can generate (700W or so) without any heating, etc. Same approach also works well on 902 and 1296 MHz, and of course all frequencies below 432 MHz.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    As stated above this one, "all" 8 inch long "barrel" feedthroughs are likely "cheap garbage," as the serious RF connector manufacturers don't make anything like that.:)
     

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