RF BURNS FROM FRAME OF MFJ-949D TUNER

Discussion in 'General Technical Questions and Answers' started by K8MOT, Aug 20, 2017.

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  1. K8MOT

    K8MOT Ham Member QRZ Page

    Has anyone had a problem with getting RF burns from the frame of a MFJ-949D Tuner ? it's bonded to the radio and a common ground tie point. The antenna is on a grounded tower. It even does it when properly tuned to below 1.5/1. Weird problem.....

    Jim K8MOT k8mot@sbcglobal.net or here
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The cause of this is usually common mode current, which really has nothing to do with SWR.

    That it can match a load perfectly won't solve this.

    A fantastic RF ground at the frequency you're using would solve it, but that's not just a "ground." It would need to be an effective RF ground, at that particular frequency. If the ground return from chassis to earth has any impedance above nearly zero at the radio frequency you're using, it's not really an RF ground, it's just a ground.

    You didn't mention what the antenna was, but "most" common mode issues (which cause the "hot chassis" effect) can be resolved using ferrite line isolators that introduce a very high impedance to the outermost shield on the coax transmission line at the relevant frequency.
     
    AF6LJ and KK5JY like this.
  3. KK5JY

    KK5JY Ham Member QRZ Page

    You can get common mode feedline current from an antenna even at 1:1 measured SWR.

    Bigger picture, I wonder if you have a solder connection in a coaxial cable or internal to the tuner that has either opened or shorted accidentally?
     
  4. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Are you sure it's the frame? Are you just touching only the frame when getting burned, or are you adjusting a cap? The antenna capacitor can have very high voltage on it, and that extends through the shaft to the brass set screw on the knob. I've got bitten a few times from that when trying to tune something with silly high SWR.

    73,


    Mark.
     
    N2SUB and K2CAJ like this.
  5. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Is it a coax fed antenna or balanced line?
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    How far away from the antenna are you ?
     
  7. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    And what type of antenna do you use?
     
  8. K8MOT

    K8MOT Ham Member QRZ Page

    Thanks for the suggestions, I have rf chokes on the link between the radio and the tuner and also a torrid core choke on the output feedline to the antenna. I will double check the solder connections.
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    If the tuner chassis is "bonded" to the Xcvr chassis and the tuner chassis is RF hot then the Xcvr chassis should also be RF hot.
     
  10. K8MOT

    K8MOT Ham Member QRZ Page

    Found the problem, i exchanged the tuner with another one, no RF burns, Took the old tuner apart, found a coil touching the chassis. Re-positioned that and put the tuner in the closet.
    Thanks for all the suggestions. This problem is burned into my fingertips........

    73 Jim K8MOT
     
    AF4RK, KE0EYJ, N2SUB and 4 others like this.

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