The comparison of balanced tuners is in the September 2004 QST.
It is worth mentioning that there is a strong correlation between tuner
efficiency and money. According to the ARRL lab data, a $900 Palstar is
substantially more efficient (Surprised?) than a $200 MFJ.
Side note: I dimly recall that there is an outfit in Germany that has been
making balanced tuners since before balanced became cool. They are very
expensive ($1.5 K - $2K), but I suspect that they are much more efficient
than the specimens tested in the ARRL lab.
A comparison of unbalanced tuners is in the February 2003 QST.
Again, it is worth mentioning that there is a strong correlation between
tuner efficiency and cost. According to the ARRL lab data, a $600 Ameritron
is substantially more efficient than a $330 MFJ. (All the more remarkable,
seeing that apparently the same people make both.)
Comparing the two different reviews reveals another detail. Commercially
available balanced tuners, as a class appear to be lossier than unbalanced
tuners. (Speculation: Since both the loss and cost are dominated by the
inductor quality, and the balanced tuner needs two of them, I expect that
the manufacturers are tempted to cut corners on the inductor quality for
the balanced tuner.)
Other point worth noting: Losses are much more severe when the load
resistance is substantially lower than 50 ohms. All the tuners show their
worst performance at R = 6.25. With R = 400 all the tuners do much better,
despite the fact that the SWR is 8:1 in both cases. (Almost certainly this
is caused by the resistive voltage divider effect. The resistance in the
tuner inductor is in series with the load impedance.)
Comment on baluns: If you drive a ferrite core to saturation, it will
overheat. It doesn't really take much to do it; a few minutes of normal CW
operating with 100 watts into a 5:1 SWR on 20 meters will do the trick for
me. Once the core overheats, the inductance changes and you lose your match
(quite severely, in my experience). You are much more likely to drive a
balun core into saturation on the high SWR output of a tuner than on the
low SWR input of the tuner.
73,
Steve Kercel
AA4AK
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