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TH7DX trap repair?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by GW4FRX, Nov 29, 2021.

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  1. GW4FRX

    GW4FRX Ham Member QRZ Page

    I have a TH7DX which has given great service since installation in 1997. Following a recent storm the 14MHz VSWR is higher than usual and the 21 and 28MHz VSWRs are something over 3:1. A defective trap seems to be the most likely cause and there's some material about repair on the internet but I can't find anything resembling a step-by-step guide to dismantling one. I'd be very grateful if someone can point me to a reference or has some first-hand knowledge.

    I've a feeling I might need some spares (such as the PK-TH6-PS plastic parts kit) but there doesn't seem to be a UK source and I can't find any reference to TH7 parts on the MFJ web site. Anyone know better?

    Any help much appreciated.

    73 John
  2. W9GB

    W9GB Ham Member QRZ Page

    Hy-Gain TH7DX Super Thunderbird
    Manual (MFJ web site)
    Contact Tom Stone, AA5MT , Manager for HyGain / Cushcraft antennas at MFJ.
    Earlier QRZ Tech Forums (Antennas) on this Subject
    September 2020
    Hy-Gain traps for the Tri-Band yagi antennas are basically similar, no matter the model,
    be it an ancient TH6; a relatively recent TH11,; a TH3, an Explorer 14 and so on.
    Hy-Gain tri-band yagis have 4 traps,
    • 10 meters: there is a different version for Parasitic Elements on some yagis
    • 15 meters, for Directors and Reflectors
    • 15 meters, for the Driven Element
    The TH7 being 7 elements adds 20-15 reflector, 10-20 director.

    They are mostly distinguishable without the part numbers printed on the outer body, which is a good thing since the labels fade with age. The 10 meter traps are shorter than the 15 meter traps, the 10 meter and 15 meter parasitic element trap coils are wound with aluminum wire, and the 15 meter driven element trap coils are wound with copper wire.
    Hy-Gain used steel or lightly galvinized fasteners from 1950s to the 1980s.
    Stainless steel hardware and clamps were first used in the 1990s.
    Refurbishing Trap Antennas (Hy-Gain)
    by Lee, ZL2AL
    November 4, 2014

    Typical Hy-Gain trap resonance freq. are:
    13.6 MHz (20m), 20.6 MHz (15m) and 27.4 MHz (10m).

    Hy-Gain Trap Repair
    by Ron, VE3GN
    October 31, 2019
    Last edited: Nov 30, 2021
    WA7ARK likes this.
  3. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    "...14MHz VSWR is higher than usual and the 21 and 28MHz VSWRs are something over 3:1."

    This points to a defective driven element 10 meter trap.
  4. GW4FRX

    GW4FRX Ham Member QRZ Page

    That's exceedingly helpful -- very many thanks. For some reason my searching didn't throw up any of those links.

    73 John
  5. GW4FRX

    GW4FRX Ham Member QRZ Page

    Thank you very much for that. I think the time has come to refurbish all the traps but that'll give me a good starting point.

    73 John
  6. W9GB

    W9GB Ham Member QRZ Page

    Attached is the 1983 version of the Hy-Gain TH7DX Manual, when Telex owned them.
    NOTE Parts List : There are only four (4) type of Traps used.
    ** IF you need to order parts from Hy-Gain/MFJ … compare to Current Production.
    After MFJ purchase, some Antenna models were modified from original design. **

    Photograph and Mark Trap Orientation (3M colored Electric Tape, #33 or #88)
    BEFORE Disassembly. Too many post-disassembly confessions (I did not do the, Now What?).

    Attached Files:

  7. GW4FRX

    GW4FRX Ham Member QRZ Page

    Thank you kindly -- much appreciated.

    73 John
  8. M0TNX

    M0TNX Platinum Subscriber Platinum Subscriber QRZ Page


    Please may I suggest you look up Lee, G0DBE? He's around the corner from me in Bootle, North Liverpool and has a plethora of information on TH7/TH11 - Possibly with some spares.

    He's tricky to contact via mail, but if you'd like me to approach on your behalf, I'm more than happy to do so.

  9. GW4FRX

    GW4FRX Ham Member QRZ Page

    Hi Kev and many thanks for that. If you have a minute, please pass on my email address (gw4frx at hotmail dot com) and ask him if he'd kind enough to drop me a line.

    73 John

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