I have heard the Johnson Matchbox described as a modified Z match, which is
not quite right. Having just obtained one, I thought I might describe the
circuit, which appears to be very similar to the link coupler offered by
Annecke in Germany.
The circuit is a straightforward link-coupled circuit. The input with the
relay and associated circuitry includes taps for a 50-ohm transmitter
connection and a 300-ohm receiver connection, since receivers continued to
used balanced input strips long after transmitters had gone to shielding,
pi networks, and 50-ohm outputs.
The secondary coil is tapped at reasonable positions for 80/40/20/15/10
meters, shorting out the unused turns toward the outer ends. Across the
outer limits of the coil is a split stator capacitor, center grounded,
which is used to set the tank at resonance. The required value of
capacitance will vary somewhat as the reactance and resistance at the
antenna terminals is varied.
The terminals are not connected directly to the outer limits of the tank.
Each side of ground passes to a butterfly capacitor. The center of each
butterfly goes to the antenna terminal. A butterfly capacitor is a split
stator variable arranged so that as capacitance on one side goes up, it
decreases on the other. The antenna terminal on each side of ground is
thus set at a certain reactance from ground and certain series reactance
from the tank, while the reactance across the entire capacitor remains
roughly constant. This arrangement forms a voltage divider. It also forms
a means of compensating for reactance at the antenna terminal of the
tuner, allowing it to match a wide range of R+/-jX combinations that might
be present at the antenna terminals and still present the requisite high
impedance to the tank circuit ends.
The design goes back to AM days, so the 275 watt rating is likely not only
conservative, but conservative to the power of the carrier plus side bands
of a 100% modulated AM signal. Capacitors appear to be spaced for at least
3 kV or better (according to some old capacitor listing in the 1952 ARRL
HB advertising section). The KW matchbox uses the same circuit with beefed
up components (7 kV capacitors) and only some slight connection changes.
Perhaps the only thing I would do to improve the design is not mechanically
feasible: to have a rotary coil with contra-rotating sections to permit
the full span of taps in order to eliminate the switch. Goal is to make
available the most efficient coil settings for every possible set of R+/-jX
presented, but I have no idea of how to make that work mechanically and
preserve the center link and main coil center efficiency.
I thought those who have never seen a Matchbox might be interested in the
basic design. Love those elegant Johnson panel colors.
I have one question others might be able to answer for me. My unit has a
U-channel directional coupler contact cemented to the bottom of the
chamber, while the paperwork calls for the external directional coupler
(which I also have). I am wondering if the SWR directional coupler I have
is someone's personal modification or something the factory did in some
runs. The panel meter is original. The relay and receiver terminal strip
has been removed.
I hope the information is useful (and reasonably accurate), and that the
question has an answer (just to satisfy my curiosity).
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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