[Johnson] Musings on the matchboxes

Mon, 22 Dec 2003 19:27:09 -0700

Kevin is right -- totally right. The T-Match is a high-pass device and will 
offer little in the way of harmonic suppression. This is something to worry 
about if we need harmonic suppression, at least with a moderin rig. I am not 
convinced that we do. But there is no denying that you get additional 
suppression with certain types of transmatches.

Now, the Lew McCoy Ultimate transmatch was pretty-much determined by 
engineers over the years to be a modified high-pass "T" network...that is, 
two series capacitors and a shunt coil. I have heard, however, that a "T" 
that uses series inductance and a shunt capacitor (still back-to-back "L" 
networks, I suppose, much like a "T" is) is actully a low-pass device, yet 
can still handle some fairly wild and crazy impedance excursions.

Whatever works!

Richard, WB5NEN

From: Kevin J Ward <[email protected]>
To: [email protected], [email protected]
Subject: [Johnson] Small Johnson Matchbox has its limits
Date: Mon, 22 Dec 2003 00:35:02 -0500

Richard -

There is one very good reason to use a balanced tuner like the Johnson;
harmonic rejection!  This type of tuner can significantly reduce the
harmonic content of your signal, especially when used in conjunction with
a good low-pass filter.  The T-network tuner or "transmatch" offers no
harmonic rejection whatsoever.  Yes, it will match a wider range of
impedances, but even the lowly L-network (low pass configuration) will
outshine the Ultimate Transmatch in harmonic rejection.

A second fact to consider is that the "transmatch" is basically an
unbalanced device.  It requires a balun at the output to feed an open
wire line.  Those baluns will saturate at some power level.  When that
happens the result is a clipped waveform or flat toping, part of the
signal turned to heat, and lots of harmonics.  This combination has
actually made your signal worse!  The balanced tuner won't saturate.  Any
tuner can spit and smoke when fed too much power, but I'm talking about
similar components fed the same amount of RF.

The T-network tuner is a useful device, but not an unmixed blessing.

Kevin  N2IE

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