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[TowerTalk] Use of the Johnson Matchbox

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Subject: [TowerTalk] Use of the Johnson Matchbox
From: (Jim Reid)
Date: Tue, 30 Mar 1999 09:24:07 -1000

>Vic Rosenthal wrote,  in part:
>I forgot to add, for those who don't know, that the Matchboxes are
>link-coupled parallel-tuned, balanced circuits.  The 4 to 1 balun in 
>most modern tuners is very inefficient when the SWR on a balanced 
>line gets above 3 to 1 or so.

>These (modern) tuners are really designed for coax or for balanced 
>lines which are 'flat' (like a folded dipole fed with 300-ohm line).

Aloha Vic,

I have a V-dipole antenna;  it was supposed to be a V-beam,  but
I am not sure it provides any "beamed" gain,  as the 600 foot leg
wires are strung across a rather deep ravine.  The wire terminate
just above the ravine on each end.  Angle between the wires
at the feed point is around 30 degrees,  so it should have been
a V-beam,  hi.

Anyway,  I am presently feeding it with 450 ohm windowed twin
line,  from an Amidon 4:1 balun which is fed from an XMatch
Classic Low Bander Tuner.  This set up allows use on 160,  80,
and 40 meters up to 1500 watts out.

Now,  I also have a modified kW EF Johnson Matchbox.  The primary
or link coil has now been tapped to all 5 turns of the link,  switch
selectable.  Also added variable and fixed capacity has been
added to the series primary circuit.  Of course,  there is not
enough inductance in the secondary to tune up on 160 meters,
and I cannot figure a way to add enough there either,  hi.  BTW,
the kW Matchbox is modified pretty much as described by
LB Cebik at his web site where he discusses balance antenna
tuners,  including the tapped primary link coil,  etc.

Anyway,  have been wondering if I would increase my efficiency
of operation if I used the Johnson on 80 and above,  and only
put in the XMatch and Amidon balun on those occasional QSO
searches on 160?

I also have the new Palstar AT4K tuner.  This unit features the use
of the balun at the input of the tuner!  The capacitors and inductor
are of course off-ground,  so the entire unit can be used in the
"balanced" configuration when balanced output is desired.
Presumably,  this eliminates the balun inefficiency of which
you wrote when a high VSWR exists on the balanced line,  as
it nearly always does,  hi.  The Palstar tuner unfortunately has an
operating nuisance,  however,  as when balanced output is
desired,   the set up on the back of the tuner must be changed.  
That is,  one end of the balanced pair of output standoff insulators 
is tied to ground when using the usual coax fed antenna,  but must 
be lifted off ground when using a balanced transmission line.  

Further,  I am suspicious of
the AT4K efficiency,  as vary little of either of the variable
capacitors is used on any band except 160!  With the XMatch,
one side or the other of the inductor uses an  effective,  fully
meshed capacitor,  that is a set of door knob capacitors is
switched  to either the input or the output side of the inductor;
and the variable is also interchanged.  Thus providing,  per
the XM designer, Paul Schrader,  N4XM,  the highest efficiency
low band tuner available in the amateur market.

Just thought you might be interested in all of this.  I am pondering
ways of actually measuring the efficiencies of these tuners,  but
am not sure how to proceed.  Because a power meter on the output
side of the tuner is not going to be in a 50 ohm Z circuit,  so will
not,  in fact,  give an accurate power reading,  at least the
Bird 43 says it is only accurate when both source and load
Z are 50 ohms.  Always a challenge!!

73,   Jim,  KH7M
On the Garden Island of Kauai

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