[TowerTalk] Johnson Match Box Tuner Modifcations

[K7GCO@aol.com]

In the use of the Johnson Match Box or any other tuner I want to emphasize 
that just obtaining 1:1 SWR does not mean maximum transfer of energy 100% of 
the time.  The MB tank circuit taps and the 2 turn link tap are compromises 
that most of the time are optimum.  The "K7GCO Rule Of Tuner Adjustments" is 
to have output measuring devices like Plug In RF ammeters or shunted light 
bulbs.  I have Philemore knife switches across the RF ammeters terminals to 
increase the range of a low current meter like when on another band in the 
area of Hi-Current or provide a path if no meter is plugged in--very handy. 
One can then see with a change of the link tap from 2 to say 3, 4 or 5 turns 
gives more output.  Or if a series variable Xc in the link to ground or a 
change in tank circuit inductance is beneficial in bandwidth, more output is 
obtained or just to get 1:1 SWR.  I seldom use the series Xc in the link as 
it's seldom needed and it's another knob to turn and maximize--2 is enough.  
Tuners without the differential always need the series Xc in the link 
contrary to what ARRL articles have shown--for about 70 years.  No one there 
ever properly tested what they printed despite being told of it several 
times.  

I have installed banana plugs with a 6/32 screw stub on the posts of the RF 
ammeters.  Most RF ammeter posts are the same spacing. I make adapters if the 
post spacing needs modification.  A #34 drill is used in the post and tapped 
for 6/32.  I use a larger drill than normal for 6/32 as brass is tricky to 
tap and one can break a tap off easy--lubricate it.  Or if I used a balanced 
100 ohm coax (series connected 50 ohm coaxes--add another SO-239), I have a 
SWR bridge in each coax.  A tuner doesn't change the SWR on the feed line 
after the MB so it was just for output measuring--I thought.  I have found a 
tuner condition in 3 different tuners that actually affects it.  Using both 
RF ammeters and SWR bridges in each leg, I can get a tuning condition with 
1:1 input SWR where, using 100 ohm coax:

1. Balanced SWR occurs in each bridge (sometimes 1:1 if antenna load is 100 
ohms and RF ammeters read the same value and associated with P=I squared R.
2  Unbalanced SWR above 1:1 and equal RF ammeter currents or
3. Balanced SWR readings and unbalanced RF ammeter currents.

The last 2 are hard to believe unless you see them and I have demonstrated 
this concept to several.  To obtain condition #1 I would change the turns in 
the link or inductance of the tank circuit and retune to 1:1 input SWR.  If 
there are disbelievers with some money to bet, come around.  Others have left 
money here.

I have another tuner called a Balanced L that I really like and all moving 
part are on 1 shaft for a real quick tune up and it will do the same into 
balanced coax loads.  I have several versions of "1 Knob Antenna tuners."  I 
can get 2 or 3 conditions of input 1:1 SWR but ONLY 1 is correct.  The SWR 
bridges and RF ammeters tells me which input 1:1 SWR is correct.  I wouldn't 
know this unless I had both the SWR bridges and RF ammeters.  I also run 
similar tests with open wire line.  Sometimes unbalanced currents occurred 
that I thought was due to surrounding objects.

I had a L0-Z 40M load of open wire line that caused arcing in the tank Xc in 
a KW home made version of the MB with 1:1 input SWR and 1 KW input.  
Comparison also with other antennas gave a slight clue that maximum output 
and input on receive may not be obtained--the desired goal.  I reversed the 
connections on the dual differential and retuned it to 1:1 SWR.  It required 
less tank circuit inductance and different settings of the variable Xc's.  
The ARCING STOPPED and the OUTPUT INCREASED 1.5 dB calculated from the RF 
ammeter change.  About 300W of the 1 KW was not going up the feedline and was 
dancing around in the tuner.  I never ran a heat test as it arced and was all 
I needed to know.  Had I been running 100W I'd never had the arcing clue.  So 
one more test to use when running low power is to reverse the differential Xc 
connections and retune for maximum output.  To do this quickly I added a big 
dual knife switch.  It presents to the tank circuit a different capacitance 
divider configuration that it might like better.  Hardware stores have good 
ones for about $6.  

I have several home made MB versions with plug in coils and one with 2 rotary 
inductors with a mechanical differential inbetween them all on one shaft so 
they turn in the right directions.  This allows me to initially maximize the 
right settings, I record the dial settings of the Xc's and have a counter 
dial on the variable inductors.  You can't easily change the tank tap 
connections in the Johnson MB and have to live with whatever they are set at 
and are close very often.  I want maximum output, assure it with my 
modifications.and don't need any heat tests after maximizing the output.  
What you gain in dB's to and from the antenna over coax can be lost in not 
using the optimum settings of the tuner.  Having the right settings means for 
example I can run a 30L1-600W into the 275W AM rating MB on 10M with no 
arcing and very minimum heating--more on the lower bands. I also ran 1500W 
into a 500W 10 M tank coil in a home made MB and it ran cold.  

Switch in a MFJ analyzer in the external receive antenna circuit (in some 
receivers) for quick tune up of the tuners.  Connect analyzer to a fixed 12V 
power supply  

K7GCO

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