Tuner Puzzle - Balanced Line MFJ 974HB Tuner and Cobra UltraLite Senior Doublet

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4PSA, Mar 15, 2016.

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  1. W4PSA

    W4PSA Ham Member QRZ Page

    I am a newer operator looking for advice on what may be a matching problem between my 140' doublet and a new MFJ 974HB balanced line tuner. Any suggestions are greatly appreciated!

    I purchased a an MFJ 974HB balanced line Johnson type tuner and am unable to get it to operate at all (both SWR and power meter needles only move slightly when attempting to tune). I have been using a Deluxe Versa Tuner 949E and can get it to work when I swap out antenna tuners. I have followed the instruction manual's adjustment procedures and am unable to get the tuner to show any more than 5 watts power output across the 2 bands I tried, 40 and 80 meters. I verified the power and SWR readings with an external meter. I have opened the case and checked solder connections inside the tuner for continuity and all appears to be OK.

    Station Equipment:
    *Kenwood TS440S AT Radio with internal tuner switched to Pass Thru mode with 3’ RG58 coax jumper to radio
    *Cobra UltraLite Senior multi-band doublet antenna (160 – 10 meters, 140’) suspended 45’ above ground with 81’ of 450 ohm ladder line feed to the balanced posts on tuner. The product brochure says that the Cobra normally works like a 3.5 Mhz doublet but also has linear-loaded responses on 2.8 Mhz and 160 meters.

    (matching instructions from MFJ tuner manual are attached to this thread as a PDF file)

    Station and antenna grounding to ground rods in soil 10’ away

    Tuner Settings:
    *Coax Wire Switch Set to balanced line
    Tried all antenna, inductor and transmitter settings per preset table in manual
    All tuner settings tried with less than 100 watts output power from radio


    Russ Adams, W4PSA

    Attached Files:

  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    With manual tuners, you typically pre-tune by listening to the receive signal, and adjusting for maximum. This gets you pretty close to where you need to be for transmit tuning. If you aren't hearing this, there may be an issue.
  3. AF7ON

    AF7ON Ham Member QRZ Page

    Yours is virtually identical to the set-up I have, except that I am using some home-made open wire line for the first 50 feet or so to a home-brew wire doublet. My tuning process is to switch the inductance level to maximise received noise on the band in use, then follow with the antenna/transmitter capacitance adjustment. I can usually get to a fairly decent match before transmitting. You may have to do a couple of iterations to find the best match - I try to use the lowest value of inductance possible on each band. I also have a Johnson Matchbox, but find the MFJ works just as well and has wider matching capability.

    If it won't match at all, then it's likely that something is wrong - double check the wiring and connections and verify your coax input goes to the correct place (we have all used the wrong connector at some time in our ham career!)

  4. W1VT

    W1VT Ham Member QRZ Page

    When it doesn't work it is often a problem with a bad cable.
  5. WB2JIX

    WB2JIX Ham Member QRZ Page

    A couple of items.
    1. I am pretty sure the Cobra needs a length of coax for matching? Could be wrong though. I sell a lot of antennas to people fed-up with the Cobra.
    2. Most balanced tuners require a jumper to be in place, on the rear of the tuner. I've worked with LOTS of hams that forget that little piece. Balanced won't tune without it.

    Good Luck!
  6. AF7ON

    AF7ON Ham Member QRZ Page

    The doublet should match just fine if you run the ladder line straight to the tuner without any coax.

    The MFJ 974 (if it's like mine) doesn't need any extra jumpers to operate with balanced line. Brian is right, though, there are some others that do.

    Check all connections, including any coax jumper cables.


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