[TowerTalk] (no subject)
Wed, 27 Sep 2000 19:35:22 EDT
In a message dated 9/26/00 9:51:31 AM Pacific Daylight Time,
Here's what Walt asked me to forward!
*******For what reason?
To Tower Talk re Johnson Matchbox Tuners:
Legend has it, and thus it is generally believed
that the dual-differential capacitor at the output of the
Johnson Matchbox is an impedance divider, because it
is believed that the individual sections on
each side of neutral are voltage dividers. I'll
now explain why this is a misconception.
******This is not the topic under discussion and is
considered "Technical Rambling" by many although
I did like your discussion.
Observe that the four separate variable
capacitors of the dual-differential capacitor
turn simultaneously because they are all
mounted on a single shaft. Note also that the
two inner capacitors form a series capacitance
connected between the output terminals of the tuner,
which places this capacitance directly in
parallel across the input of the balanced
transmission line. Aside from balancing
the transmission line with respect to ground,
the only other effect of this capacitance shunting
the input to the feed line is to place a capacitive
load across the line input that decreases the
input impedance of the line. Consequently, as
this capacitance changes during the matching
procedure the effective line-input impedance also
changes. Thus, this capacitive line loading
plays no direct part in the matching process,
and as we will see later, it actually reduces the
impedance-matching rang(e). Now observe that the
two outer capacitors of the dual-differential capacitor
are connected in series, respectively, between the
ends of the tank inductance and the balanced output
terminals of the tuner. Ignoring the inductive coupling
between the input loop inductance and the tank
inductance for the moment, observe that the tank
inductance and the series capacitors comprise two
L networks, forming a dual-balanced L network.
Therefore, in conjunction with the mutual inductance
in the coupling between the unbalanced inductive link
and the balanced tank inductance, the balanced L
network performs the impedance matching and balun
Then what is the purpose of the capacitors that
shunt the input of the feed line? As long as these
capacitors turn simultaneously with the series
capacitors of the L networks they serve no
useful purpose, and are in fact, detrimental to the
range of impedance's the tuner can match. The
reason is that as the series capacitors are
varied to achieve the impedance match at the
input of the transmission line, the effective input
impedance at the input of the line is changing
simultaneously, because the shunt loading
capacitance is also changing. The matching
(r)ange of the tuner will be increased somewhat
if the shunt capacitors are disconnected, thus
allowing the full range of the series capacitors
in the dual L network to function into a constant
line-input impedance. As an experiment, with
the Matchbox in the original condition, tune for
a match into a balanced line. Now disconnect
the shunt capacitors. With a slight readjustment
of the main tuning capacitor the match will be
reestablished, but now with the input impedance
of the line unchanged by the shunt capacitors
while the main capacitors are adjusted.
However, if the shunt capacitors were on a
separate split-stator capacitor, permitting
adjustment independently of the main
capacitor, the impedance matching range
could be extended dramatically when the line
input impedance is higher than what the tuner
can match in its present form. This is because,
as stated above, the input impedance of the line is
reduced by capacitive shunt loading.
I have thus shown that the dual-differential
capacitor in the Johnson Matchbox is not
an impedance divider. Walt Maxwell, W2DU
Gee Walt-- I couldn't care less. Your presentation was informative but
"didn't address the main issue" that Tom should have asked you to bail him
out of. How did you get suckered into this tangent? He's made a big issue
out of the Match Box's supposed's inability to match typical Hi-Z's with open
wire lines, Bob Tails and 1/2 Wave Verticals and you have joined him. W6AM
used to use the KW MB at the end of his rhombics feedline--just for receive I
read in QST. He's turning over in his grave. You have joined him now in bad
mouthing the much maligned MB. Get off it's back. You post had some merit
but it "Served Absolutely No Useful Purpose What So Ever" and the "Wrong
Official TT Announcement: In an effort to improve technical coverage of
equipment and concepts, I'm introducing a "Brand New Concept". Lets do what
we can to explain what's "RIGHT" with equipment also--along with the bad.
Users of the Johnson Match Box have asked me to be the 'Technical Defender of
the MB" and I'm asking for donations to mount a "Spirited Defense" to Stop
this "Insidious Technical Campaign" against the MB. It's depleting my time
and money resources.
How the Dual Differential Capacitor works and what it does or doesn't do is
of interest to me as I'm into all aspects of performance and how to make the
most of it. But it's not of interest to virtually all of the rest of reading
this. In comparison this would be called "Technical Rambling" to everyone
but me as I like details. NOTE! When you go off on a "Technical Tangent"
supporting baseless concept (not yours--the other guys) you lose a notch or 2
of your Technical Credibility on this and other subjects.
All John Q Ham wants to know is "if he turns these 2 knobs in a certain way
it will give a 1:1 SWR and maximum transfer of RF into the feedline for a
wide range of impedance's and tune out a fair amount or reactance's of a
balanced or unbalanced load"--and it does.
With the Parallel or Series Tuners, they wanted to see Hi-Z's (2000-5000
ohms) or Lo-Z's (30-600 ohms). They won't handle in-between. The basic
configuration had to then be changed which was a big nuisance. In the
Parallel configuration you could tap down the coil for intermediate Z's with
some success. The ARRL versions didn't have series Xc's in the link either
and limited their matching range. I used to have a different tuner for each
band with proper switching so I wouldn't have to change coils and
connections. It was real "Hi-Tech Matching."
With the Match Box it was Shielded, had Calibrated 2 Knobs to tune and a Band
Switch (no plug in coils). There was no Series Xc as it didn't need it. It
matched a wide range of Lo, Med & Hi-Z's with only 2 knobs to twiddle and NO
CONFIGURATIONS TO CHANGE. Finally a "Shielded User Friendly Tuner" that
didn't require an Engineering Degree to know how to use for coax or balanced
lines. It worked best with resistive loads but could handle higher reactive
loads than other tuners except for the L Network. The 275 W (AM rating) will
handle a 600W 30L1 SSB output on 10M with barely perceptible
heating--progressively less on each lower band. It will work on 30,17&12M
also. I added another band switch with 4 more contacts and added another tap
for 75M also.
It's versatility can be enhanced by adding a series Xc in the link to ground
for certain odd loads. This requires taking the case off. Also add a switch
that allows selecting, inaddition to 2 turns of the link circuit, 3,4&5 turns
for even greater ranges of Z's it will match. I even added another parallel
tank variable Xc of about 20 uufd for finer tuning on 15-10M.
This is a great tuner and I respectively request that no further bad mouthing
of the Match Box be made. It's now "Officially Off Limits." When you do it
clearly illustrates lack of basic tuner skills and knowledge of Transmission
Line Theory 101. I have given it the "K7GCO Medal Of Tuner Honor with RF
Clusters" for Gallant Service Over and Above Existing Tuners in all Fields
Over 50 Years of Service along with a 21 Gun 1.5 KW Linear Salute. Too many
want to give it "Purple Hearts" with their nit picking arrows.
There is an interesting analogy of deficient technical expertise. Some
Engineer Analyzed the Flight Characteristics of the Bumble Bee. He concluded
that "the Bee should not be able to fly and the only reason it does is "no
one told it, it couldn't fly." If I was the Engineer that did this study, I
wouldn't go around telling everyone how dumb I was as we all know the Bee
does fly fairly well! I flunked him in Basic Air Flow 101. This is an
absolutely true story--I just made it up.
It is a true story that the MB does work very well and there are those who
say "it can't work properly." It does if you don't know all this "Tainted
Theory Total Nonsense".
The value of Match Boxes should be increasing progressively. I will be
releasing articles on how to build your own with all the K7GCO improvements.
During the mean time I request that the MB be "left in peace." I see a big
market for a book on "Antenan Tuners That Work" (for very low cost). The KW
jobs run aroud $500 when there are simple ways to avoid them even with open
wire line (buy and read my book how to do it). I suggest the Poo Poo'ers
suggesst a tuner that works better and at low cost instead.
From out if the West the "Lone Defender Riding My Silver Plated Horse Vector
and Wearing an Armored Asbestos Suit with Mask, Shield and a Cloud of RF
Dust," I seem to be the only one in Ham Radio Illustrating New and Better
Techniques and Defending Open Wire and Tuners. It's a tough row to hoe. I
can get this material along with good pictures printed in mags, get paid for
it and without all kinds of Arrows and Spears--no Armored Suit needed. That
sounds like a better way to ride.
Walt, stick to discussions on SWR or at least to the main subject--whether
the MB can match Z's typical of open wire line dipoles, Bob Tails and 1/2
Wave Verticals. You won't be accused of Technical Rambling" at least by me.
It says at the beginning you "asked your post to be added." What purpose did
it serve? K7GCO
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