> QST very sanctimoniously proclaims that they won't allow antenna
> advertising that includes gain figures; yet every month QST carries
> ads for so-called "legal limit" and "high power" antenna tuners that
> -- ARRL's own lab testing reveals -- dissipate 30 to 40 percent of
> the applied RF power if the antenna is a G5RV driven at 3.5 MHz.
Hi Chuck,
The problem with antenna tuner specs is that they don't
include qualifications about the power rating versus degree
of mismatch. 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. A case in point is the MFJ
Tuner we have at the local club station here (I think its
the 989C). On 75 meters we use it to match an 80 meter
delta loop. With that tuner I can use a number of different
roller inductor settings to get a 1:1 VSWR. Unfortunately,
depending on the settings, the capacitors in the MFJ tuner
will arc (high voltage) or the roller inductor will overheat
(high shunt RF current). If I pick the right "in-between"
settings, the roller inductor doesn't overheat and the
capacitors don't arc. With that combination of tuner settings,
the tuner handles the 1500 Watt output of our AL-82
without any trouble.
If you take the cover off the MFJ tuner and observe the
settings of the capacitors and the roller inductor for the
various tuning combinations, it becomes clear what is going
on. When you use a combination of a small capacitance and
large inductance, the voltage drop across the capacitor
becomes large (V=I*Xc), and the capacitor arcs. The shunt
inductor current on the other hand is given by I = V/Xl.
Since Xl is large in this case, the shunt RF inductor current
is small. This tuning combination probably results in the best
overall efficiency since most of the losses in a "T" network
are due to the finite Q of the shunt inductor. This combination
is power limited, however, by the breakdown voltage of the
series capacitors. If I retune the network for larger capacitor
settings (low Xc), the RF voltage across the capacitors
drops, so the arcing goes away. Unfortunately, to get a 1:1
VSWR with the larger capacitor settings I need to lower
the shunt L. This causes the RF current in the inductor to
increase, thereby increasing the losses in the tuner. If the
inductor setting is too low, the inductor losses will power
limit the tuner (e.g. the coil will smoke).
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. This is what happened
to me the first time I tried to tune up the MFJ989C on
75 meters. I concluded it was a piece of junk, when in
fact it was quite capable of handling 1500 watts if
adjusted properly.
If you look at the ARRL data, all of the low efficiency
numbers correspond to very narrow VSWR bandwidths.
This suggests a very high loaded Q for those tuner settings,
which would be consistent with higher circulating current
in the inductor and the resulting poor efficiency. It will
be interesting to find out if the ARRL controlled for this
in their testing, or if the tuner knobs were treated as
roulette wheels.
BTW, I for one think that an "inductor temperature"
readout would be a useful feature to have on an antenna
tuner.
73 de Mike, W4EF................................................
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