[TowerTalk] EFJ Matchbox Question

[Jim Reid]

Hi,

Am fooling about with an old kW Matchbox.  I have just
yesterday put up one of W7FG's  "true ladder line"
fed 80 meter doublets.  Uses #16 gage wire,  spaced
about 3 3/8 ",  or about 580 ohms or so line Z.

Have the doublet up as an inverted-V;  apex at about
55 feet,  ends about 15 or 20 feet,  and the angle 
between the legs is around  130 degrees or so,
broadside more or less oriented on toward the US
mainland.  Worked like "gang busters"  last afternoon
on 40 meters,  at least about Hawaii;  fellows on the
other islands said the signal was as strong or perhaps
more so than what they receive from the Force 12
EF-140S dipole.  Closest other island station was about
120 miles away;  more distant down on the Big Island,
about 325 miles or so.  Maybe we are really comparing
NVI performance on 40 in the latter afternoon hours.

Anyway,  my question about the Matchbox,  the "output"
capacitor is a dual differential job.  I find that on both
of my most frequently used 75 and 40 meter frequencies,
for local KH6 land contacts,  that this cap wants to be
set at the highest dial reading,  and then even a bit
more!  Scale goes to 100;  but seems to give the
best match at 108 (estimated).  Assume this just means
the knob has slipped on the shaft or something over the
decades of use.  But at max setting of this control on the
kW Matchbox,  is this dual-differential variable at a
max value of C on one section,  and a min on the other,
or what??  The Matchbox manual indicates more typical
settings,  when coupled to a 600 ohm resistive load would
be a setting of around  32 on 75 meters,  and about 45
on 40 meters.  Of course,  that is to  a pure R load.
Just seems odd that I would wind up with this dual-diff
cap at a max setting (whatever max means using a
dual-differential variable).

Per the original instruction manual,  the "tuning" capacitor
is a dual variable which is used to tune each half of the
secondary coil of the coupling inductor to resonate the
coupling circuit.  The "matching" capacitor is the dual-
differential variable which acts as a voltage divider to
match the antenna load.  The secondary coil of the
coupler is tapped on each side of the balanced output
circuit;  different taps provided for the five bands
80 up to 10 meters.  Note on 30 meters,  the 20 meter
tap position works fine;  the 15 meter taps seem best
for the 17 meter band.

On the modern T-tuners,  the most efficient operation occurs
when either the input or the output variable C is at max
capacitance;  but do not believe that is true at all when
using the Matchbox -- an entirely different circuit.

Anyway,  were the case of the Matchbox not held on with
absolutely dozens of sheet metal screws,  it wouldn't take
too long to remove and have a look inside,  hi;  but as it
is.........too much trouble,  so just thought I would ask in
case someone on the reflector knows how things are operating
inside,  hi.

BTW,  this Matchbox has been modified a bit,  per the notes
on LB's,  W4RNL web site.  There is now a series tuning
variable in the primary or "link"  input circuit;  and each of
the primary coil turns has been tapped.  This allows the
primary circuit to be more quickly and adequately set
to resonance or the needed input Z,  and also provides 
rapid "tweaking" as the two secondary circuit resonating 
and the voltage divider caps couple reactance back into 
the primary "link" circuit when they are  adjusted.

I always set this tuner up using the 259B antenna analyzer
before the rig is connected to this antenna system.  The
Matchbox circuit can develop some extremely high voltages
within if running even low power in to tune it up.  E. F. Johnson,
in the original instructions, caution to NEVER tune up the
Matchbox with live RF.  They requested that an SWR bridge
always be used for tune up (manual,  pg. 4).  Interesting...

Thanks for any info about the dial vs.  capacitor settings
within the old Matchbox.

73,  Jim,  KH7M


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