At 11:30 PM -0800 1/28/03, Michael Tope wrote:
>The problem with antenna tuner specs is that they don't
>include qualifications about the power rating versus degree
>of mismatch.
I agree. Since the main purpose of a tuner is to accommodate
mismatch, the advertised power ratings of tuners should logically
include statements of the load impedances and frequencies for which
the power ratings apply. This could be done in various useful ways,
e.g., graphically, but it hardly ever is. The transmitting duty
cycle should also be specified, but rarely is.
Instead we have so-called "legal-limit" tuners that can't handle an
SWR of 3:1 on 80 meters even with 20% duty, and can't handle an SWR
of 1:1 at 100% duty.
>Another problem that was not addressed in
>the ARRL comparison article has to do with the nature of
>"T" network tuners. Most of these networks don't provide
>a unique matching "solution". There are in fact a number of
>different combinations of inductor and capacitor settings
>that produce a 1:1 VSWR at the tuner input. These
>multiple "solutions" don't, however, produce the same
>efficiency or power ratings....
This problem is well known, but is not adequately addressed in most
manufacturers' product literature.
An _excellent_ way of developing a feel for this issue is W9CF's
online T-network tuner simulator, at
<http://fermi.la.asu.edu/w9cf/tuner/tuner.html>.
Playing with this simulator beats playing with a real tuner because
the simulator is faster, because you can't damage it, because you can
give it any load impedance you like, and most importantly because it
provides numbers, e.g., percentage power loss.
This simulator doesn't tell you everything you'd like to know, e.g.,
the internal voltages and currents, but I suspect that other
tuner-simulation software is available that does. Can anyone can
point us to some?
>I am wondering if the folks who did the ARRL comparison
>test went thru this exercise of optimizing the tuner settings
>for efficiency, or if they just the spun the knobs until
>they got 1:1 VSWR and then measured efficiency. If this
>is the case, some of the tuners in the test may have been
>unfairly stigmatized as "low efficiency" when in fact the test
>operator just happened to randomly hit upon a bad
>combination of components settings.
I had the same question myself, and was disappointed not to find the
answer in the latest ARRL comparative review article. Although I
would expect the ARRL authors to know to tune for best efficiency, it
concerns me that they didn't say anything about it.
FYI, another article comparing four "high-power" tuners appeared in
March 1997 QST, and is downloadable from the Members Only section of
the ARRL website. One of the tuners reviewed there was the MFJ-989C
that you mentioned. On 80 meters this tuner was reported to
dissipate 22% of the applied power with an (unbalanced, coaxial) load
R of 12.5 ohms (SWR = 4), and 16% of of the applied power with a load
R of 25 ohms (SWR = 2). For 1500-W input, 22% and 16% represent 330
W and 240 W, respectively. IMO, a 2:1 is a very modest mismatch, and
even 240 watts is too much to dissipate in an inductor of the size
used in this tuner, so this tuner hardly seems to qualify as a
"legal-limit" tuner.
>BTW, I for one think that an "inductor temperature"
>readout would be a useful feature to have on an antenna
>tuner.
I agree. Because my antenna tuner is out of sight, I put a
remotely-readable thermometer atop its metal case. Obviously a
thermometer inside it would be much more useful, but so far I've been
too lazy to deal with the RFI problem that would entail.
73 de Chuck, W1HIS
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