< 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.>>
K7GCO
>>
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