[TowerTalk] (no subject)
Michael Tope
Michael Tope" <W4EF@dellroy.com
Wed, 27 Sep 2000 19:34:56 -0700
Geez - this is getting really weird. We are talking about a metal box with some
inductors and capacitors in it - not a person whose honor has been called
into question. Its a fairly simple network. Pick a Hi-Z load, plug in the
numbers from the MB schematic, and and see if you can match to 50 ohms.
That should settle the argument rather handily one way or the other. Network
theory doesn't play any favorites.
Mike, W4EF............................................................................................................
----- Original Message -----
From: <K7GCO@aol.com>
To: <towertalk@contesting.com>
Sent: Wednesday, September 27, 2000 4:35 PM
Subject: [TowerTalk] (no subject)
>
> 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
> functions.
>
> 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
> Legend."
>
> 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
>
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
>
>
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com