MFJ-948 switching HF and V/UHF antenna issue

Discussion in 'General Technical Questions and Answers' started by KJ7VAB, Oct 27, 2021.

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  1. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    When I plug in both the HF EFHW and a vertical 2m/70cm dipole, the SWR for both go crazy.
    It's an HF tuner, but I still thought it would work as an antenna switch.

    Does this seem expected or is there some other issue going on?
     
  2. W1VT

    W1VT Ham Member QRZ Page

    Looking at an online photo of the insides, the antenna switch adds perhaps half a foot of hookup wire connecting the two coax connectors inside an aluminum box.
    This is pretty significant at 10 meters and above. The effect of this switch should be negligible on 80 meters.
     
  3. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    If I unplug the 2m coax, it all goes back to normal.
    I haven't actually tested with 2 HF antennas. So maybe there's something coupling internally?
     
  4. AA5MT

    AA5MT Ham Member QRZ Page

    If you are using ant1 and ant2 plugs, there should be adequate isolation. Be aware that this is not a vhf device. If you run a ladder line and ant1 at the same time, they are connected in parallel inside.

    Tom
     
  5. K8JD

    K8JD Ham Member QRZ Page

    The 948 Is not suited to VHF/UHF operation at all.
    I have one and would never try it to switch anything but 2 HF antennas. There are some long runs of wire between componants in the tuner and some are getting close to a half wavelength on UHF.
    Get a good quality (expensive) Coax switch to switch your V/UHF antennas and use the 948 only for HF antennas ! Read the freq specs of a Coax switch before you buy one !
     
    N2UHC likes this.
  6. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    After playing around with this extensively, I'm starting to think the connectors on the unit are bad.

    If I plug my antenna directly into my nanoVNA, it looks correct. Jiggling and slightly bending the cable near the connector doesn't change anything on the nano. Even adding all of my patch cables to the end of the antenna feed and flexing/ bending each one doesn't change the reading on the nano.
    However, if I plug only the HF antenna feed into the 948 and connect the "Transmitter" out/in into the Nano and move the cables around, things jump around significantly.
    All of the solders are solid but I can feel a little bit of movement in the center. I can't feel that in any of the chassis connectors I recently purchased.

    T
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Find which connector is bad and if the solder is all perfect, just replace the connector.

    If the solder is not perfect, re-solder.

    This is not an unusual problem.
     
    KJ7VAB likes this.
  8. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    Yeah, that's what I'm leaning toward. Maybe start with the one labeled "Transmitter" and see if the one labeled "Dummy Load" is still solid.

    Just heat the existing solder and suck it out with the sucker? Drill out the rivets...
    I'm new to all of this stuff.
     
  9. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Couple things you can check before resoldering things:

    Rivets are known to be a problem with these tuners; they can become slightly loose and then the secure connection of the shell to the ground of the tuner goes bad. Drill as least one out and secure it with a nut and bolt and lockwashers.

    Check the center connector of the coax connector sockets. When the center part of the plug is inserted, it should make a good friction fit. Sometimes the leaves of the socket get spread apart and then the center connection will always be loose.

    73.
     
  10. KJ7VAB

    KJ7VAB Ham Member QRZ Page


    Thanks. The rivets look really solid, but I'll try that before commit completely. The centers of several jiggle slightly inside of the insulator. Not enough to break the connection completely, but maybe enough to change the Ohms?
    Some of my coax ends are extremely tight where my solder made them on the thick side. Had to file them down a bit to get them to even go in.
     

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