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Moding a Johnson KW Match box for 160m

Discussion in 'Amplitude Modulation' started by WA4JK, Aug 26, 2018.

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  1. WA4JK

    WA4JK Ham Member QRZ Page

    ive read until I’m blue in the face on this subject, but I wanted your input.
    I want to use my matchbox for the only tuner on my OWL fed diapole. But I want to build a 160m inverted Vee.
    So I’m thinking two 400uf 15kv caps and just hard connect them to the ground connection and spade lug them and incert them when I want to go to 160m.
    What ideas or input other then a resonant inverted L
    Which I don’t have the ability to install y’all enough on the L to do justice.
     
  2. K5UJ

    K5UJ Ham Member QRZ Page

    I think this is one of those situations where you have to experiment. First, I'd get a KW matchbox if what you have is the 275 watt model. Bill WC3K had an article on using the Matchbox on 160 in the March 2001 issue of Electric Radio. The mod. is to put 200 pF parallel with the coil. He used four 50 pF 15 KV doorknobs paralleled with each other. One end bonded to one end of the coil (or somewhere nearby), other end of the caps bonded to the opposite end of the coil. I would solder one end and use a coil clip on the other end to take it out of the circuit for the higher bands. You will probably have to run the MB without the cover no matter what you do. That isn't a problem for me because I run all of that kind of gear exposed anyway, but not everyone is me. It's easier if you have the model that doesn't have the vswr meter. I'd get the antenna put up, then start messing around with various value of capacitance across the coil and see what happens, using a vswr analyzer or one or two watts from some other source. Another thing to try is tapping the link at different points. It's around 5 turns but only two are used for transmitting. Varying the tap point on the link may help also. But WC3K wrote that the extra 200 pF across the coil got him on 1885 with a 180 foot long dipole. I think I'd try finding a single section 7 KV or more 200 pF air variable or large broadcast mica. The doorknobs are okay if they can handle 2 or 3 amps at 1000 kc. One thing about this idea is that the Matchbox may not handle 1 KW input on 160 like it can on 75.

    There are probably better ideas out there though. I have not gotten into anything about homebrewing a link coupled tuner with a bigger coil because my impression is that you want to use the EFJ Matchbox.
     
  3. WZ5Q

    WZ5Q Ham Member QRZ Page

    Howdy Jerry,
    I sent you an email containing some modification information I have collected for doing just that.
    One is from Bowie Bill - WC3K which Rob mentioned above, and the other is from Johnson.

    I never got around to implementing either one as I decided to build a homebrew LCT instead.

    In the email is also included some pictures of the Bowie Bill mod my friend Eddie, W5NVI did with his.
    His antenna was a 160m Doublet fed with 122' of 450 ohm Window Line.
    He used 220pf of capacitance and a vacuum relay to switch it.
    Hope this helps.
     
  4. WA4JK

    WA4JK Ham Member QRZ Page

    Thank you for the reply’s guys. I’ll look at the emails. It’s what I have so it is “ use what you have”
    I’m gathering parts for a link coupled tuner, but it will take time on that.
     
  5. WA4JK

    WA4JK Ham Member QRZ Page

    This is an aged old question that I’m sure has been hashed about. I figured out I could put up a inverted L but the vertical section would only be 40-43ft tall. Is it worth it if I can’t get up 70ft. And there will be no radials because I have to cross over the goat pasture and they would be a no-no.
     
  6. W2WDX

    W2WDX Subscriber QRZ Page

    Personally, making something that might work by modding something that doesn't is a bass-ackwards approach. I'd build the tuna WZ5Q has as his avatar.

    It's the coil that needs to be made larger in the Matchbox as well as more capacitance; otherwise you're limited with the type of reactance you can tune out.
     
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Few hams can get a wire that high. Yes, it will work. If it gets you on the air it is worth it, right?

    Working only against a ground rod will give poor performance. Even a single radial of whatever length is significantly better than earth ground.
     
  8. K5UJ

    K5UJ Ham Member QRZ Page

    Hi Jerry, my inverted L is only 50 feet high. Your problem won't be height--it will be not having a ground system. Surely you have at least some room for radials? A lot of radials, over 100 but only 30 feet around the feed point would be better than zero.
     
  9. WA4JK

    WA4JK Ham Member QRZ Page

    i know I can tie the ground side to the new climb goat fence which is grounded to two six foot ground rods and would extend out 2/3’s the length at unknown angles
     
  10. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    I used a inverted L of the non-resonant type, typically referred to as a 3/8ths wavelength. It actually was
    no more than 40ft up and wasn't even really exactly vertical. It even looked more like a sloppy letter J.
    Looking at it from above. It fit in a 30x110 ft area. Even the flat top wasn't flat it kinda went wavy too, up and down on the existing pines available for moral support. Fed it with 75ohm hard-line as with 200pf
    in series brought it to a measured impedance of nearly 80ohms. I used donated 100pf 5kv ceramic doorknobs to share the current... Overall length was about 156ft or so for coverage of the lower 70khz
    of 160. Also donated and scrounged was 32,000ft of bell wire which was split in half for two pizza pie
    Shaped radial fields which I turn was buried 4-6", 600 radials in each slice. These were ruffly radials
    Just being 30-40ft each in length. The radiator was 10ga THHN. The bell wire was like 22 or 20 ga insulated, tediously un-twisted pair. In testing, it was found adequate to survive the forces of foot
    traffic from above when buried in Sandy loam.

    I used two inch to three inch ceramic repurposed electrical pole insulators to keep it dressed away
    from anything that might catch fire.

    No tuners needed, and reliable day in and day out performance. Bonus it played on 80/75m with
    A tuner in the Shack. And was likewise used to the same feat on 40m. I simply shorted out the
    200pf on 80/75,40m usage and suffered the slight losses...


    KA2RRK
     

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