"Bruce W. Ellis" ha scritto nel messaggio
...
The various automatic antenna tuner/matcher manufacturers list a
tuning range in Ohms - e.g., 6 - 1000 Ohms. The question I have is to
whether that value is the magnitude of the impedance (which correlates
to SWR) or the real part of the impedance (since the tuner should be
able to cancel out the reactive part of the impedance). W0BF

Firstly, it should be noted that the magnitude of the impedance (Z) does NOT
correlate to SWR. As a matter of fact, the formula giving SWR as a function of
resistance (R) and reactance is such that R-X pairs corresponding to the same Z
magnitude may well yield different SWR figures and, conversely,.R-X pairs
yielding the same SWR figure generally correspond to different Z magnitudes.
This can be readily visualized on the Smith chart.

For instance the pair R=30 ohm, X=40 ohm yields an SWR of 3, that is the same
SWR produced by the pair R=150 ohm, X=0 ohm, despite the two Z magnitudes are
different (50 ohm in the first case, 150 ohm in the second one).

That said, an excerpt of the MFJ-993B autotuner manual reads: "... it is rated
at 300 watts to match 6 to 1600 ohms antennas (SWR up to 32:1)"

In my opinion, as a load of 1600 ohm would actually yield an SWR of just 32, the
MFJ sentence makes reference to a pure R of 1600 ohm (that is with X=0). As a
matter of fact, a Z magnitude of 1600 ohm, if not purely resistive, could
correspond to quite a wide range of SWR figures, and not necessarily to 32..

The cited sentence excerpt then tells nothing about the tuner ability to
neutralize the reactance of reactive loads.

On the other hand one must consider that, for the sake of limiting the internal
switching arrangement complexity, automatic tuners usually adopt an L-type
network in which the reactive element connected at the antenna (either in series
or in parallel, depending on whether a step-up or a step-down transformation of
the R is required) is not switcheable, i.e. it is either an inductor or as a
capacitor. This means that a tuner may inherently be more tolerant of capacitve
loads with respect to inductive loads, or viceversa, depending on the way it is
designed.

73

Tony I0JX