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[TowerTalk] Summary: Johnson KW Matchbox

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Subject: [TowerTalk] Summary: Johnson KW Matchbox
From: seay@Alaska.NET (Del Seay)
Date: Tue, 15 Apr 1997 11:55:28 -0700
We are assuming it is the big one? 
There were two (2) versions of the matchbox. One was originally
sold as a 250 watt version, later changed to 1 KW because of the
primary use of ssb.
The large version was originally called the "KW-MAtchbox", and that
was for 1 kw/ 100% plate mdulation or 3KW PEP.
If that is the one you have - then you're okay. Otherwise, I have
seen the smaller version with the coil melted down unde a good KW
of power.
The large one is about the size of a modern day, 500 watt microwave oven.
Again - that is the one you want - or you'll be stuck with less than max 
allowed transmit power.  de KL7HF

Jim Reid wrote:
> Aloha again from Kauai,
> Thanks to the many owners of the Matchbox's who responded
> with information.
> In summary:
> Now,  what have I bought??
> 1. It has both balanced and unbalance output terminals;  is rated
> as fit for 1 kW input transmitters;
> Correct,  AM modulated transmitters.  Had two responses saying it
> handles both CW and SSB at todays 1.5kW legal limit,  no one has
> seen arcing,  etc.
> 2. Unit has two variable caps, and
> a tapped inductor;  and is said to be link coupled.  Does it
> have a band switch?
> Yes, a five position band selector: 80, 40, 20, 15, and 10.  The
> switch selects equal tap points down from the secondary coil
> ends.  On 80,  the entire secondary inductance is used.  On
> 10, just a few turns either side of coil center.
> Will it tune 160?  No,  apparently not enough inductance
> on the secondary,  and someone noted that there did not
> appear to be enough room in the case to add turns,  nor
> install a bigger coil. Anyone out there been able to modify
> the box to operate 160?
> 3. Why two tuning caps. I suppose because the link itself is
> not adjustable?
> Correct,  it is not.  But,  unlike the balanced tuner circuits
> shown in current editions of the ARRL Antenna Book,  there is no
> capacitor in series with the primary transformer coil.  Both
> multiple section tuning caps are in parallel across the entire
> secndary coil.  C1 is a regular dual section, unit with each
> section connected to one end of the secondary, and the other
> cap side is grounded. These ard gang-tuned, of course, on a single
> shaft.  C2 is the "dual-differential" unit,  having each
> differential pair connected in parallel with one-half of the
> C1 tuning cap. That is,  each differential pair is connected between
> one end of the secondary coil and ground.  This Matchbox circuit
> was published in QST,  May 1995,  in an article about transmatch
> evaluation written by AI1H,  Frank Witt.  The article was
> suggested to me by Steve, N1SR,  and again,  thank you Steve!
> The balanced output from the Matchbox is obtained from  between
> the differential pair tuning capacitors of C2.  Obviously,
> this is a totaly different appearing antenna tuner circuit,
> than any other I have seen before!
> 4. Will it handle 1.5kW output,  CW?
> Yes,  and SSB also.  In fact,  one user commented that it would
> tune more output power that I could probably afford to feed it!
> Wonder about RTTY.
> 5. The internal componets are evidently pretty massive as the unit
> evidently weighs around 50 or so pounds;  steel case?
> Yes,  the unit is housed completely by a steel case,  but
> the weight is probably more like 30 pounds.  No one actually
> disconnected from their set up and weighed the box!  John,
> K1ER sent along the following:
> "To play with the innards you remove the three knobs, take all the sheet
> metal screws out of the big steel box, and the two pieces come apart. two
> sheets with two 90 deg bends. Front, top, bottom lip. Sides, bottom and
> back.
> Big ceramic Cardwell switch, wide spacing on the caps.  If you arc it,
> you can get in to fix it.
> I don't think there's room to add turns to get to 160 m."
> Well,  that is the summary of info so far.  InFrank Witt's
> QST piece,  he does give some of his test data on both the
> low and high power Matchbox's.  The data is presented in
> very much the same way as the antenna tuner review article
> data in the March 1997 article by the ARRL tech staff.  They
> credit Frank and his technique of balun testing and his
> "geometric" test boxes.  These boxes are described
> in Part 1, April, 1995 QST, "Hoe to Evaluate your Antenna
> Tuner", by Frank Witt.
> Some brief notes about the Matchbox's from Witt's '95
> test report:  the low power unit has a bit lower loss
> than the KW box: eg, 11% power loss on 20 meters, while
> the low power box was somewhat less than 10%, this surprised
> Witt.  But as he points out that the tuning adjustment
> of the KW unit is "touchy", perhaps this means quite
> high coil Q,  and thus higher circulating current at
> resonance tuning,  just a guess by me.  He also reports
> that the output balance of the Johnson boxes is the best
> of any of the balanced tuners he has tested.
> So,  to the many who requested the summary data,  enjoy.
> 73,  Jim, KH7M
> On the Garden Island of Kauai
> --
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