MFJ-948 switching HF and V/UHF antenna issue

Discussion in 'General Technical Questions and Answers' started by K9ARI, Oct 27, 2021.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: L-Geochron
ad: abrind-2
ad: L-MFJ
  1. K9ARI

    K9ARI Ham Member QRZ Page

  2. N7EKU

    N7EKU Ham Member QRZ Page

    The center can wiggle in the insulator and not be a problem,

    But solder blobs on the connector pins (like you had) can spread the spring leaves of the socket apart to the point where they stop making good contact with the pin. Then they need to be bent back towards the center a bit. Always make sure to remove extra solder from the outside of the pin after soldering (or you can wipe it off with a dry rag while the solder is still wet).

    Is the end fed antenna a tuned system like from MyAntennas, or is it just a wire on a balun? In the latter case, it is highly likely that the shield of the coax is part of the antenna and also anything connected to the shield (like the tuner and anything new connected to the tuner like another antenna). So these changes in SWR would be expected in that case.

    K9ARI likes this.
  3. K9ARI

    K9ARI Ham Member QRZ Page

    hmm, ok. Good to know. Maybe it's not bad connectors.

    I have a 64:1 QRP transformer and I made a 47:1 transformer from scratch (currently using the 47:1). So an EFHW like My Antennas. With a torroid at 25 feet roughly.

    Just tested everything again. Seems like just touching the shield/braid/screw-on-part on the "Coax 2" and "Transmitter" cables while they are connected to the tuner makes things change.
    Maybe this a grounding issue?
    Guess I'll rule that out tomorrow when I get up and going.

    Currently I'm just running the coax into the house from the transformer with 9-13 (cant remember) turns through a 240 31 torroid. (which I tested at ~ - 30 db across the HF range) From there into the house through a hole in the wall.
    I've been running the 818 off of battery so I haven't addressed grounding yet. Trying to test each stage as I go, in case I need to scrap the current setup for something different.
    Didn't think the tuner would need any grounding until I plugged the radio into the house power. Perhaps I do?
  4. K9ARI

    K9ARI Ham Member QRZ Page

    Alight. I rigged up a ground wire and nothing changed.
    But while I had the cover off, I pinched the connectors on the rivets and things stabilized on the nano readout. Seems like the rivets might be the issue.
    Should I drill them out and replace them with screws or drill new holes to add screws to the two opposite (missing) corners?
    Or maybe just solder a wire across all of the connectors and add that to the existing ground?
  5. N7EKU

    N7EKU Ham Member QRZ Page


    Not exactly like a MyAntennas one, because they use a custom-tuned transformer plus some special tuning aids on the wire itself. What kind of SWR's are you getting on each band? The MyAntennas is generally made for use without a tuner:


    What kind of SWR's is your antenna showing before using the tuner? Does the SWR get worse with the tuner when touching the connectors than not using the tuner at all?

    It could be that the tuner is not tuned well too. T type tuners have infinite ways to tune them and some settings may make the system more susceptible to what you are seeing. The ARRL has a free article on how to properly tune T type tuners and is a good read.

    Just trying to think of what could be wrong.

    K9ARI likes this.
  6. K9ARI

    K9ARI Ham Member QRZ Page

    It's pretty similar. I removed about 4' of wire (to manually tune it to 75m), so everything is off. But the curve is solid. I'm not totally sure if all of the dips line up with the 4' section put back. I was going to test all that after I get the tuner working. The original goal was to just use the tuner to swap from 80m to 75m instead of pulling off a section of wire.

  7. K9ARI

    K9ARI Ham Member QRZ Page

    alright well the chicken in me suggested I test before modifying... So I just twisted some wire around the connectors and connected that to the external ground post and it looks promising.

    Seems like the rivets are probably the issue.
  8. N7EKU

    N7EKU Ham Member QRZ Page


    That's why I mentioned it as I had a similar MFJ one that did the same thing. I would probably drill two out that are diagonal from each other on each connector. Then use a machine screw and nut with a star washer on the inside, or even on both sides (under the head and under the nut). Drilling out two would keep the connector stable even if the other two rivets get looser.

    Sounds like a good plan (for 75/80m) and a nice job on the antenna. Just got yourself enrolled in MFJ's repair course haha!

    K9ARI likes this.
  9. K8XG

    K8XG XML Subscriber QRZ Page

    I found the Crimp connectors I bought at Giga parts from a national brand would not fit the female hole properly to fill it up in MFJ products. Any weird movement would screw up the connection. I had to use solder on types with them. Had no issues on LDG tuners or Dywa switches with the crimp connectors; only MFJ SO239s on their products.
    Last edited: Oct 29, 2021
    K9ARI likes this.
  10. N7WR

    N7WR Premium Subscriber QRZ Page

    It is not an antenna switch. It is an HF tuner not suited to have any VHF antenna connected to it
    K9ARI likes this.

Share This Page

ad: Mircules-1