[TowerTalk] Johnson Match Box

[K7GCO@aol.com]

In a message dated 10/6/00 7:29:10 PM Pacific Daylight Time, n4kg@juno.com 
writes:For one who wants to determine the (real Z) impedance
 range of the Johnson Match Box can do so with a 
 simple experiment.
 
 Connect non-inductive load resistors of various values 
 (say 10, 15, 22, 27, 33, and 2200, 2700, 3300, 3900,
 4700 ohms) across the balanced output terminals
 and tune to 50 ohms at the input using any Antenna
 Analyzer.  
 
 de  Tom  N4KG
  >>
Right on Tom--good suggestion.  You are an "Old Pro" with RF.  It's that 
simple and use reactance's also.  When I tested them for Johnson some 50 or 
so years ago, I got it to match as low as 30 ohms balanced.  Also try some 
antennas also like Bobtails, 1/2 wave verticals and a center fed dipole fed 
with a 1/4 wave of 450 ohm ladder line available today (.89 Vel. Fac.) but 
not then unless you made it.  Also try other than the 2 turns on the link 
either side up to 5 turns for maximum output.  Always check for heating with 
new antennas with maximum power.  Use a heavy clip for convenience on the 
link.  On the 275W MB make sure the 50 ohm tap is 2 turns rather than 5 as 
the factory made some.  Since the tank coil taps are set, this will alter the 
Z range of matching. It's easier to use the tuner for link tap modifications 
with the front half of the case off.  It doesn't have to be shielded although 
it's desirable.  One might add a front sheet of Plexiglas for the shafts, 
switches and fingers.

I've made MB's with plug in coils that I could tap anywhere and also ones 
with 2 variable inductors on a common shaft with a mechanical differential 
gear inbetween so make them rotate the right way.  I added a dual link to 
these coils for proper coupling and matching.  I wish I could show pictures 
of this construction.  I plan a Tuner Handbook with all the variations and 
ways to build any tuner inexpensively. You can match just about anything with 
proper instructions.  I just saw one for $800.  I plan another How Not to Use 
Tuners and devote the 1st 10 chapters to who can't get it right.  There are 2 
other mods that extends it's range even more that I will describe in the 
book. The MB may be a turn or 2 short on 80M but I think a toroid bar or 
donut could be added to the end of the tank that could extend it's range but 
I haven't tried it yet. 

The 275 AM model will take 600W SSB on 10M and perhaps more on the lower 
frequencies.  Add another PL 259 coax jack to the other balanced output for 
100 ohm balanced coax which is a great system.  Banana plugs will plug into 
these nicely.  Record the calibrated settings.  I'd dissemble any MB you 
might buy and make sure every part if good.  They are of 50 years old.

I'm starting the "K7GCO Match Box Protective and Preservation Society" for 
advancing the MB State of the Art and counteracting debunkers.  I will later 
accept "Registration" by all owners for making a mailing list to keep 
everyone up to date with new applications and as I do, make my own parts like 
the dual differential.  I also plan a design with 2 screw driver motors for 
remote control.  More on this later.  The MB and (open wire line) is picking 
up speed--look out.  They can still be found in the Flea Markets for very 
good prices.  Inspect them on the inside if they will let you as some have 
been abused.  K7GCO  

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