Input Resistor in MFJ Auto-Tuners

Discussion in 'Radio Circuits, Repair & Performance' started by KC9SWV, Jul 19, 2017.

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  1. KC9SWV

    KC9SWV Ham Member QRZ Page


    I have a question about something I'm seeing in MFJ auto-tuners that I'm not seeing elsewhere. I'm working on designing my own antenna tuner to add some crazy features for my specific use-case that no one offers, so I started looking at schematics and reading manuals for research. I noticed something looking at the MFJ schematics that I don't see on others and I'm wondering what the purpose is.

    On the schematic for the MFJ-993 and 998 tuners, there is a 25 ohm resistor in the input, just after the SWR bridge that can be relay switched to be in series or parallel with the input signal path. What's the point of this resistor? It appears to be in most MFJ tuners, but is absent from the AT-11 tuner and the K3NG tuner. LDG doesn't appear to share schematics and I no longer own one, so I can't check theirs.

    Any thoughts on what this might be for?
    Does it simply introduce enough loss to allow a terrible match to still look okay to a transceiver?
    Are they switching it into the circuit during tuning to protect the transceiver or keep it from going into SWR power reduction?

    If it was one of the last two, wouldn't it be better placed on the input side of the SWR bridge?

    Thanks and 73,

    Attached Files:

    AF6LJ likes this.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    "I'm wondering what the purpose is."

    Dummy Load.
    Last edited: Jul 19, 2017
  3. KC9SWV

    KC9SWV Ham Member QRZ Page

    Okay, but why? It's not rated for the power rating of the tuner, it's 25 ohms instead of 50, and with the relay switching arrangement, it's always affecting the circuit by either providing parallel or series resistance.
    AF6LJ likes this.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    My guess is that they are switched in to Test/Cal using a known load.
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    As to their intention, that I cannot address.
    The twenty five ohm resistor does limit current, and this is a good thing since during tune up some combinations of L and C can be series resonant on the operating frequency.
    That would present a dead short to the tuner and the transmitter. This resistor would limit the possibility of damage.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    That's what I think too. Then as a tuning solution is neared, it's switched out for the final tune.
    AF6LJ likes this.
  7. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    You might want to call their technical service people or send an email and ask nicely. I'll bet they will tell you (if not, nothing lost). They are probably the most open ham equipment supplier on the planet along with Elecraft.
    AF6LJ likes this.
  8. KC9SWV

    KC9SWV Ham Member QRZ Page

    Current limiting does make sense. I hadn't thought about a series's probably much easier to use a current-limiting resistor than to count frequency and calculate all the resonant points and avoid them in software.

    I'll do that, too. You're right...they're pretty open. Just thought I'd try to get a quick answer here, too.

    Thanks everyone for the thoughts and discussion!
  9. G4COE

    G4COE Ham Member QRZ Page

    I would say the resistors are on the OUTPUT of the unit since it feeds the antenna end not the other way round as described in the title.

    Would they not be protecting the SWR components under certain load conditions?

  10. AF6LJ

    AF6LJ Ham Member QRZ Page


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