Gremlins and strange radio faults that can drive a person crazy!

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK4FUQ, Jan 17, 2018.

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  1. VK4FUQ

    VK4FUQ Ham Member QRZ Page

    G'day all, I love Ham Radio and I love electronics as a hobby, but has anyone ever had a weird and/or intermittent radio system fault that seems impossible to solve? These kinds of faults always seem the weirdest and hardest to solve!

    My IC-718 has long had a strange 20 m 'no transmit' fault that makes no real sense except that using a 4:1 'hybrid balun' (with balanced line) is generally successful. Using any other type of balun often causes the issue to appear! I have no idea why? It is totally bizarre. Any ideas? Regards, Felix vk4fuq.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The obvious questions would be:
    - What's the antenna system?
    - Does this ever happen when transmitting into a dummy load?
    - What does the SWR look like with the 4:1 vs other baluns?

    Sounds like either a mismatch problem or common mode on the feed line causing trouble back at the rig.
     
    K6CLS and VK4FUQ like this.
  3. VK4FUQ

    VK4FUQ Ham Member QRZ Page

    G'day mate, yes I have long suspected common mode issues but I can't really prove it one way or the other! Certainly when I use my 4:1 hybrid balun the issue almost never occurs. This type of balun is reputed to have better than the usual degree of common mode current impedance which again might explain why this issue is rare when it is being used. In any case if the basic issue is common mode currents I am at a total loss to explain why! I have run all your suggested tests previously, with inconclusive results. Regards, Felix vk4fuq.
     
  4. W1VT

    W1VT Ham Member QRZ Page

    It could be that other 4:1 baluns lack the common mode rejection you need and that 1:1 baluns won't bring the impedance close enough at the tuner for a match to be found. The hybrid may be the only one of your collection that can do both.

    Zack W1VT
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A clamp on RF current meter can quickly prove if you have excessive common mode with one balun configuration and not another: http://www.miketooley.info/g8ckt/simple_rf_clip_meter.pdf or: http://www.instructables.com/id/Clamp-on-RF-Current-Meter/ or: https://www.mfjenterprises.com/Product.php?productid=MFJ-854

    Or clamp on some RF chokes around your coax feed line between the balun under test and your rig to see if symptoms change with the chokes in place. A few of these could work: https://www.amazon.com/RF-Choke-315...qid=1516204092&sr=8-2&keywords=mix+31+ferrite or better yet, one of the big 1" clamp on chokes with five or six turns of coax wrapped through the core: http://www.kf7p.com/KF7P/Ferrite_chokes.html

    If adding a choke like that to your feedline eliminates the symptoms then the culprit is almost certainly common mode currents.

    So what antenna system are you running? You mentioned balanced feedline, but is that feeding a reasonably balanced antenna like a loop or a doublet or perhaps something unbalanced like a zepp or something else?
     
  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Does the problem go away with reduced power?
     
  7. VK4FUQ

    VK4FUQ Ham Member QRZ Page

    G'day all, reduced power? No change, the issue still exists. The issue occurs with two separate (and different 20 m antennas), a ground plane vertical and a full wave loop, but only on 20 m, all other (lower in frequency) HF bands appear to be unaffected. Recent testing with the how brew 4:1 hybrid balun has been 100 percent successful (so far). Common mode issues increasingly appear likely however I cannot understand why. Regards, Felix vk4fuq.
     

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