[TowerTalk] (no subject)

K7GCO@aol.com K7GCO@aol.com
Wed, 27 Sep 2000 19:35:22 EDT

In a message dated 9/26/00 9:51:31 AM Pacific Daylight Time, 
w8ji@contesting.com writes:<< 
 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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