[Top] [All Lists]

[TowerTalk] Summary: Johnson KW Matchbox

To: <>
Subject: [TowerTalk] Summary: Johnson KW Matchbox
From: (Jim Reid)
Date: Tue, 15 Apr 1997 09:40:25 -1000
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 

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

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>