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Yaesu FC-40 Mis-use

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AC0GV, May 28, 2021.

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  1. AC0GV

    AC0GV Ham Member QRZ Page

    I would like to permanently install a Yaesu FC-40 "tuner" in my fiberglass RV to feed "ham stick" type antennas with coax. Has anyone ever added a SO239 connector to one? What about out side the box?
    TNX ac0gv
  2. W9WQA

    W9WQA Ham Member QRZ Page

    most tuners not made for coax the book
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Remote tuners want to drive a load that is somewhere between ~10 and ~1500 Ohms. A piece of 50 Ohm coax is right at the sweet spot that such tuners tune most easily into! There is no reason why you cannot have a short piece of coax between the tuner and the "load", even if the tuner doesn't have a SO-239 and the coax is connected directly to the screw terminals.

    Some remote tuners, like my MFJ993 and '998 have both screw terminals wired in parallel with a SO-239 on their output side. The user gets to decide where to connect the load at the output of the tuner.

    The tuner is an unbalanced device, meaning that there is a common-connection from the threaded part of the input SO-239 to the threaded part of the output S0-239 (or to the "grounding screw terminal") if so equipped.

    If the load is "balanced", like a dipole or loop, then it is best to put a balun between the output of the unbalanced tuner and the balanced load. If the load is unbalanced, like a monopole, then connect the HV terminal from the tuner to the monopole, and connect the tuner's grounding terminal to the radials under the monopole.
  4. AC0GV

    AC0GV Ham Member QRZ Page

    This one has a grounding screw. I did make a jumper with some rg-58 ending with a UHF female connector.
    I wanted rg-8X but could not find it in my price range. the rg 58 was in my junk box, my price range.
    Will it work? I'll find out when I get a radio.
    Tunner output.jpg
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    What kind of antenna are you going to connect to it? I don't worry about the tuner seeing the coax. However, the coax can act as a bit of an impedance transformer so the tuner will see something different compared to being connected directly to the antenna with almost zero wire length.
  6. W9WQA

    W9WQA Ham Member QRZ Page

    my comments are not from my knowledge or experience but from tuner mfgrs who advise against coax, sgc being one.

    they say depending on the ant there could be thousands of volts to handle.
    so i wonder just what is right with some tuners ??

    and, "A piece of 50 Ohm coax is right at the sweet spot that such tuners tune most easily into'

    is it seeing 50 from the ant?

  7. AC0GV

    AC0GV Ham Member QRZ Page

    On the RV I use ham stick type antennas tuned to the band. I also carry tuned dipoles for use in the trees if I think the campground will allow it. When I ordered the tuner I told them ham sticks and this was the tuner I was sold. The RV is all fiberglass, so I'm sure I will find something that will work.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    No. The remote automatic tuners are all LC networks. They are fine tuning into R>15 Ohms and R<1500 Ohms. At the high impedance end of that range (caused by an anti-resonant antenna), the tuner could be damaged by high voltage. The capacitors/relays have a lower breakdown voltage than a piece of coax. The tuner maker cautions about not operating the tuner there. The shunt capacitance provided by the coax will only serve to lower the peak voltage that the tuner would see if the coax wasn't there.

    In Kenneth's case with his hamsticks (almost, but not quite resonant in their own right), his antennas are more likely to have an R less than 50 Ohms, so the Swr on the coax is low (<3). The coax doesn't care about that Swr, and the tuner doesn't notice the coax...
  9. AC0GV

    AC0GV Ham Member QRZ Page

    I found that yaesu sells a mobile whip for this device, fed with coax. YA-007FG Mobile Fiberglass HF Whip Antenna. They are out of stock.
    I'll find out how well my system work when the radio gets here.

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