Burned Antenna Trap

Discussion in 'Wireless & RF Design' started by Art, Jan 13, 2016.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi Guys :)
    If I can fix this Comet H-422, then I will have a very cheap multi band dipole for HF.
    These are commonly known to blow their 20 meter traps, and that is the case with mine.

    I’m hoping someone might have fixed one of these before, or at least a heads-up on what to expect.
    I understand they are all bandstop filters made of tank circuits connected across the antenna segments.
    Is the capacitor physical or somehow formed with the casing? Or are the traps only inductors?

    The big question is, of all six traps, only one looks like this, but they all measure the same DC resistance.
    Maybe the inductor did blow open and has shorted back to the antenna segment again.

    [​IMG]

    [​IMG]

    Cheers, Art.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I have checked the resonance frequency of traps with a grid-dip meter. I have also used an inductance bridge (which measures at a low frequency, like 1 megHz) to measure just the inductance of the coil inside the trap. I can also measure the R+JX as a function of frequency of the trap using my antenna impedance analyzer.

    Never fixed a burnt-up trap like yours, but have found plenty of oxidized joints and spider nests inside tri-bander traps. I got a weather-beaten (Az) HyGain tri-bander given to me, so I disassembled all 12 traps, cleaned them up, installed new plastic parts, and the antenna works as good as new.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
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  4. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Yes it is expensive new, and funny it needs waterproofing from the get go, or it becomes a mess after rain.

    I do have an old-school GDO, I suppose I could measure resonance of the bad and good looking traps.
    This is assuming the trap is actually an LC circuit. Maybe it’s a capacitor that made that mess.
     
  5. PRFGADGET

    Active Member

    Aug 8, 2011
    50
    7
    From what little you've stated, I will assume that you acquired this "Second Hand" with the burned trap.
    From the picture, what I see is an "ARC" or "FLASH-OVER" burn (would be interesting to know how the antenna was mounted when this happened and the conditions of the event) , this sort of damage is usually the result of a lightening strike or a good deal of power (above 300 watts) being in close proximity to a ground path (or the antenna itself became the path).
    As to the "FIX", the quick and dirty is to get a new trap from Comet of course, but if you can salvage what you have by cleaning and re-working insulators be sure to use a conductive grease (No-Lox or something similar ) at all physical connection point's and pay close attention to spacing .

    The H-422 is famous for being "QUIRKY" in general and especially on 20 meters , I would suggest that you "GOOGLE" this model for more information on fixes.
    Please keep us posted on any fix as others may need help with this sort of repair.

    73 and good luck !
     
  6. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi :)
    Yes it was cheap secondhand. No dishonesty, the seller told me about the trap.
    I presume it was non functional at least on some desired band because it had been
    temporarily replaced with a G5RV junior that you wouldn’t be using in place of a good one of these.

    The mast was made of water pipe, and it was mounted between 1-2 meters from the
    roof of a two storey house in it’s optional V configuration as pictured in my original post
    (it can also be configured horizontal if you can get it higher).
    That would put this damage reasonably high up in the air, even if it was a metal roof.

    From what I saw of the operator’s practice with the G5RV... installation was rather unfortunate.
    When I went to buy it, the bottom two sections were being used to support each element of the G5RV,
    and there was also an electrical wire tied across the V to prevent it being pulled apart by the G5RV
    which would be tensioned enough to hold it up.
    Everything was kind of DC insulated by wire insulation, but yeah... :D

    I can’t really see any possibility for similar mistakes being made with this one though.

    Even if I can get a new trap, I’ll still pull this apart and take a look,
    and if I can repair it, make a video for YouTube since the Comet is notorious,
    but I can’t find any video or info looking inside one of the traps.
     
  7. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Interestingly, the same seller sold a friend of mine a non-working ATU.
    Also no dishonesty, it was sold non-working... but it could have been attached to this antenna,
    and I will see the guts of that ATU on his bench tomorrow.

    Also, I was sold this baby dirt cheap because of a problem, it has ATU built in, and could not tune up 40 meter band.
    If a transceiver has a problem with 40 meter in Australia, it’s worth a lot less because
    our Foundation licence can’t use 20, so 40 is the main dx band.
    Now whether it was connected to the rest of this setup I don’t know (he has other HF, and this shouldn’t need an ATU),
    but the problem he showed me at his station does not exist at my station, and I tested all functionality to be ok! :D

    [​IMG]
     
    BR-549 likes this.
  8. PRFGADGET

    Active Member

    Aug 8, 2011
    50
    7
    "COMET" is not great about providing detailed information on their products.
    From what little experience I've had with similar antennas from them this "SHOULD" be a simple "LC" trap with the coil wound on an insulating material (of some sort) , with the "can" of the trap providing the capacitance effect in relation to the coil.
    Given what you have described, "Lightening" is a good possibility or some other "Close Encounter" with a high voltage source with the antenna and the tuner taking the brunt of the event , the radio was very lucky to have survived as well as it did.
    Sorry that I can't be of more help , GOOD LUCK!
    73, John
     
  9. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Well here’s the update... pfft :roll eyes: :D

    I do need a new endcap, or to form one from PVC reducers or something.

    EDIT,,,
    I see now one end of the surrounding sleeve is directly riveted to the tube closest to the feed point, and the tube on the other side is insulated from all but the coil connection, so to complete a series LC circuit for a bandstop filter, a capacitor could be formed between the sleeve and one of the tube.... with the inductor wound inside it! only on the end the damage happened, there is potential between the inner & outer aluminium.


    [​IMG]
     
    Last edited: Jan 16, 2016
  10. PRFGADGET

    Active Member

    Aug 8, 2011
    50
    7
    From what I can see, I would check the rivet's are still tight , check for continuity (preferably with a "MEGGER"), applying "No-Lox" or some other conductive anti-oxidant grease as I went , put the can back in place , and put it on a "Test stand" and use an "antenna analyzer" to check for resonance and match (or at least a known good swr meter).
    Fab or buy a new end cap , seal everything up good and "fire for effect" !
     
  11. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    A little bit of progress.... Now become a moderate project to make an antenna mast that allows
    a single person to raise and lower it in a limited space :D

     
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