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[TowerTalk] Baluns/Johnson KW Matchbox

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Subject: [TowerTalk] Baluns/Johnson KW Matchbox
From: (Jim Reid)
Date: Mon, 14 Apr 1997 09:30:06 -1000
Aloha again from Kauai,

Thank you to the many of you who responded with a wealth of
info and insight into my questions about current baluns.
The summry statement might be that the prime purpose of
the balun is to significantly decrease the unwanted
currents flowing back down the outer conductor of a
coax fed system.  As far as keeping the currents equal
to both output terminals,  only if the balun sees an
exact resistive load; with reactance,  no longer equal
currents, necessarily. To operate properly,  any balun
voltage or current, and give the expected impedance
transfer ratio,  the load MUST be pure resistive, otherwise,
you never know what the transfer might become.

The best answer to coupling power to a balanced antenna
is to use balanced feeders,  twin line,  and a balanced tuner.

Which brings me to the Johnson KW Matchbox.  I am buying one!!
Found two on the newsgroup this weekend.
Seemed as if I ought to find one somewhere after reading the many
responses to my current balun questions.  Anyway,  found two,
offered on both,  missed one,  got there first on the other.

Now,  what have I bought??

It has both balanced and unbalance output terminals;  is rated
as fit for 1 kW input transmitters;  has two variable caps, and
a tapped inductor;  and is said to be link coupled.  Does it
have a band switch?  Will it tune 160?  Why two tuning caps;
I suppose because the link itself is not adjustable? Will it
handle 1.5kW output,  CW?  The internal componets are evidently
pretty massive as the unit evidently weighs around 50 or so
pounds;  steel case?

In the last few editions of the ARRL Antenna Book,  there have been
repeated illustrations of the same balanced tuners;  one with a 
variable cap in series with the primary  of a fixed inductor 
transformer with a second split stator cap across the entire
secondary of the tuner trasformer.  The balanced output is
taken from taps on the secondary coil.  This same circuit
is shown and discussed in Antenna Book editions going back
to at least 1956.

Another tuner in the ARRL book is called a link-coupled
matching network.  Both primary and secondary coils are
tapped; the taps are switched depending upon band of
operation,  and again there is a tuning cap in series
with the primary coil,  and another across the entire
secondary coil,  which again is fix tapped for the 
two balanced output lines.

Is the Johnson Matchbox circuit one of these two,  or something

Thanks again for your previous answers,  and does anyone have
any answers for these new questions about the Johnson KW
Matchbox Tuner?

Mahao and 73,  Jim,  KH7M

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