[TowerTalk] Johnson Matchbox forward

[Tom Rauch]

Here's what Walt asked me to forward!

To Tower Talk re Johnson Matchbox Tuners:

Legend has it, and thus it is generally believed 
that the
dual-differential capacitor at the output of the 
Johnson Matchbox is an
impedance divider, because it is believed that 
the individual sections on
each side of neutral are voltage dividers. I'll 
now explain why this is a
misconception.

Observe that the four separate variable 
capacitors of the
dual-differential capacitor turn simultaneously 
because they are all
mounted on a single shaft. Note also that the 
two inner capacitors form a
series capacitance connected between the output 
terminals of he tuner,
which places this capacitance directly in 
parallel across the input of the
balanced transmission line. Aside from balancing 
the transmission line
with respect to ground, the only other effect of 
this capacitance shunting
the input to the feed line is to place a 
capacitive load across the line
input that decreases the input impedance of the 
line. Consequently, as
this capacitance changes during the matching 
procedure the effective
line-input impedance also changes. Thus, this 
capacitive line loading
plays no direct part in the matching process, 
and as we will see later, it
actually reduces the impedance-matching rang

Now observe that the two outer capacitors of the 
dual-differential
capacitor are connected in series, respectively, 
between the ends of the
tank inductance and the balanced output 
terminals of the tuner. Ignoring
the inductive coupling between the input loop 
inductance and the tank
inductance for the moment, observe that the tank 
inductance and the series
capacitors comprise two L networks, forming a 
dual-balanced L network.
Therefore, in conjunction with the mutual 
inductance in the coupling
between the unbalanced inductive link and the 
balanced tank inductance,
the balanced L network performs the impedance 
matching and balun
functions. 

Then what is the purpose of the capacitors that 
shunt the input of the
feed line? As long as these capacitors turn 
simultaneously with the series
capacitors of the L networks they serve no 
useful purpose, and are in
fact, detrimental to the range of impedances the 
tuner can match. The
reason is that as the series capacitors are 
varied to achieve the
impedance match at the input of the transmission 
line, the effective input
impedance at the input of the line is changing 
simultaneously, because the
shunt loading capacitance is also changing. 

The matching range of the tuner will be 
increased somewhat if the shunt
capacitors are disconnected, thus allowing the 
full range of the series
capacitors in the dual L network to function 
into a constant line-input
impedance. As an experiment, with the Matchbox 
in the original condition,
tune for a match into a balanced line. Now 
disconnect the shunt
capacitors. With a slight readjustment of the 
main tuning capacitor the
match will be reestablished, but now with the 
input impedance of the line
unchanged by the shunt capacitors while the main 
capacitors are adjusted.

However, if the shunt capacitors were on a 
separate split-stator
capacitor, permitting adjustment independently 
of the main capacitor, the
impedance matching range could be extended 
dramatically when the line
input impedance is higher than what the tuner 
can match in its present
form. This is because, as stated above, the 
input impedance of the line is
reduced by capacitive shunt loading. 

I have thus shown that the dual-differential 
capacitor in the Johnson
Matchbox is not an impedance divider.

Walt Maxwell, W2DU




73, Tom W8JI
w8ji@contesting.com

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