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[TowerTalk] Re: Joihnson Matchbox Modifications

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Subject: [TowerTalk] Re: Joihnson Matchbox Modifications
From: (
Date: Wed, 10 Nov 1999 04:26:32 EST

    < In fact, I am slowly working on modifying a battered matchbox here to 
remove the 50/300 tap business and replace it with a switched tap and series 
variable input side--the only improvement I could make to that design.  Cebik
   I've made that modification also.  Sometimes there will be a load the 
MatchBox can't match.  By changing the coax link tap to 2.5, 3, 3.5, 4 or 5 
turns it sometimes solves the problem.  Back when the MB first came out there 
was a run of the 100 W models where they got the 50 and 300 ohm taps reversed 
that I found.  I had the KW model also from the factory and found the KW 
model would match loads the 100 W model wouldn't.  I then added resistors 
only for a load and it matched 30-3000 ohms with the right tap.  
        There is another mod I make when it still can't match.  It solved the 
problem one time on 40M when the MB tuned to 1:1 but there was arcing in the 
variable capacitor plates at 1 KW.  The antenna didn't seem to receive quite 
as well as another antenna.  I noticed that the dual differential variable 
was at full capacity on the low or ground side.  That indicated to me it was 
matching a low R load.  I added more capacity to see if I could obtain an 
amount that would require I decrease the variable capacity value.  It still 
gave 1:1 with the variable at full capacity and the arcing continued.  The 
received signal kept getting weaker and weaker with each step of doing this.  
Very strange.  I then did a trick I did before to match certain feedline 
loads that didn't match normally.  Not only did the arcing go away, the 
received signal was higher than the other antenna now, the dual differential 
peaked at a certain value other than full on the bottom side and I had to 
reduce the inductance in the tank coil about 30% and 1:1 SWR.  Less 
inductance lowered the voltage access the tank and variable capacitor 
therefore no arcing across the plates.  Signal strength was great on the air 
also.  Just getting 1:1 doesn't assure everything is right in the MB but does 
most of the time.  I use RF ammeters in the output and always use the 
configuration that gives the maximum output.  What trick did I do? I thought 
you would never ask.  I reversed the connections on the dual differential.  
Draw out the 2 circuits.  The capacity divider affect is altered in a 
beneficial manner.  I'm not sure exactly why but as long as it works I'm not 
going to worry about it.  There another quirk of the tuner affecting the SWR 
and unbalancing the RF current value in balanced 100 ohm coax going to the 
antenna I'll tell you about some time that I can't figure out.  I just know 
how to cure the problem.  
        In regard to a series capacitor in the link to ground, it's not 
needed most of the time with the 2 procedures described.  Sometimes it helps.
        I modified a 100 W MB for 6M one time.  I made the link input balanced
 instead of grounding one link wire to the chassis as is normally done.  I 
had the case off didn't want any stray RF going off the chassis.  I used a 
half wave of coax connected to give me a 200 ohm balanced feedline and 
connected it directly to the link of the right number of turns which I 
determined.  It worked like a charm with no TVI on my or others TV's with 100 
W.  I have RF ammeters in the link circuit also and with 1:1 SWR I can 
calculate RF power in the link circuit.>>


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