[Johnson] Musings on the matchboxes

Tue, 23 Dec 2003 08:19:07 -0500

Gents,

IMHO, one of the reasons for using any kind of matching device is so that a
proper 50 ohm low-pass filter can be in the line between transmitter and tuner
with proper terminations to avoid damage and achieve full attenuation. Any
additional harmonic attenuation from the tuner is likely to be small compared to
the filter.

My conclusion,  better to focus on matching capabilities needed in the tuner and
count on the filter for harmonic suppression.

Just my $.02

73, Joel Hallas, W1ZR

Richard Peterson wrote:

> 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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