At 09:52 AM 11/1/97 -0500, L. B. Cebik wrote:
>I have heard the Johnson Matchbox described as a modified Z match, which is
>not quite right. Having just obtained one, I thought I might describe the
>circuit, which appears to be very similar to the link coupler offered by
>Annecke in Germany.
>The secondary coil is tapped at reasonable positions for 80/40/20/15/10
>meters, shorting out the unused turns toward the outer ends. Across the
>outer limits of the coil is a split stator capacitor, center grounded,
>which is used to set the tank at resonance. The required value of
>capacitance will vary somewhat as the reactance and resistance at the
>antenna terminals is varied.
>The terminals are not connected directly to the outer limits of the tank.
>Each side of ground passes to a butterfly capacitor. The center of each
>butterfly goes to the antenna terminal. A butterfly capacitor is a split
>stator variable arranged so that as capacitance on one side goes up, it
>decreases on the other. The antenna terminal on each side of ground is
>thus set at a certain reactance from ground and certain series reactance
>from the tank, while the reactance across the entire capacitor remains
>roughly constant. This arrangement forms a voltage divider. It also forms
>a means of compensating for reactance at the antenna terminal of the
>tuner, allowing it to match a wide range of R+/-jX combinations that might
>be present at the antenna terminals and still present the requisite high
>impedance to the tank circuit ends.
>I have one question others might be able to answer for me. My unit has a
>U-channel directional coupler contact cemented to the bottom of the
>chamber, while the paperwork calls for the external directional coupler
>(which I also have).
Hi LB and the group, My KW Matchbox does not have this internal
coupler, just an external coax line directional coupler.
Finished the V-beam installation last weekend, during the CQWW.
Had a chance to listen with the V's as the contest wound down.
The Matchbox must be tuned right where you want to listen.
Still have way too much V-beam wire laying across tree tops,
and sagging down between tree tops. Not sure exactly how to
remedy this situation; the trees are very high, the wires
600 foot long each, and tough to pull enough tension on them
to be rid of the sags, and try to clear the trees. Will think
of something, I hope. Yes, the wire is polyvinyl covered,
Now, my first days' of use of the V's was using the Johnson
KW Matchbox. What a beast to tune! As LB points out in
the quotes above, setting the butterfly, split stator
variable capacitor to find the correct transmission line
matching impedance de-tunes the secondary tank. But, if
you then try to tweak the secondary via the primary variable,
the vswr goes even higher! You must first detune the butterfly,
then tweak the tank, then see if you get a lower overall
vswr. It seems you cannot get the best match tuning the
butterfly last, you must "find" it's proper setting by
finding a better setting for the tank variable after guessing
at a setting of the butterfly -- as I wrote before: I find
Also I could only get the big Matchbox to tune up the V's on
80, 40 and 30 meters (could use the 40 meter coil taps to
tune up on 30.) And was the tuning critical. A few kHz above
or below, and the vswr seen by the rig skyrockets!! Also,
the receiver is "deaf" until you get the matchbox set up,
tuned properly, as signals will not pass through the match-
box if the big tank is not at or very near the desired frequency
resonance. There is no tuner by-pass switch available, as
exists on "today's" single ended tuners.
On the bands above 30, I could find no way to get a tune
using the Matchbox.
So, I obtained 4:1 balun, as suggested by Paul, N4XM,
and tried tuning up using his XMatch(tm) tuner. I
chose to use the big Amidon high voltage balun for
use with higher power antenna tuners. Folks at Amidon
got it out here to Kauai in two days -- super service!
The V's now tune 160 meters up through 10 meters, no problem.
Get 1.0:1 matches (never could get better than 1:1.4 or so
using the Matchbox). And the tuning is broad. If I tune up
the V's on, say 7050 for 1.0:1, get 1.1 at 7001 and all the
way up to 7250 where I get 1.3! With the tuner bypassed, 4:1
balun still in-line with the V-beam transmission twin line,
the vswr runs from 2.6:1 to 2.2 to the 50
ohm rig across that band range.
Yes, I am suspicious of the low vswr's across pretty broad
band ranges. Probably means I am coupling in loss from
the great green menace on which much of the V beam wire
lays! But, if that is true, why was the Matchbox so
fussy to tune almost exactly to the kHz? The
Amidon balun wouldn't be lossy and cause a flatter line
match, would it?
Also, sorry about my "Some Thoughts" post to the towertalk
reflector; I have been duly cautioned/chastised!
73, Jim, KH7M
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