Brian Lloyd wrote:
current node (odd multiple of a 1/4 wave)? Yes, I know that the ability
to achieve a conjugate match depends on the capacitance and inductance
range of the tuner. Also this affects the range of the real component
(resistance) that can be matched and what frequency you are trying to
match it on.
The ablity to match the real component depends on the imaginary
component. For a high impedance match, I believe you want the C at the
antenna side. If you convert 50 ohms in series with the extreme
inductance values (maximum, I think) to the equivalent parallel
combination, at the frequency, you will get the maximum possible real
part transformation. This might not be achievable for a particular
frequency and antenna reactive component, because of limits in the range
of the C value, it which case you may need to iterate to get an L value
compatible with the limiting C value.
At the extreme limits, the SWR steps will be rather high, so you might
want to consider doing the calculation with an L value that is beyond
the range on the grounds that the resistance match will be no worse than
that halfway between the limit value and the next possible value.
2. Does the BL2 have a problem when looking at a really high impedance,
i.e. when center feeding a full-wave doublet? And yes, I am aware that I
really wouldn't want to feed the balun with coax from the tuner but
rather should put the balun as close to the tuner as possible and then
run a low-loss balanced line from the balun to the doublet and that the
feedline will act as a transformer to change the impedance (resistance
AND reactance) as seen by the balun and, therefore, the tuner.
All current baluns don't like high common mode impedances. In 4:1 mode,
they also don't like high differential mode impedances. In both cases,
this is because the leakage inductance reactance is not significantly
higher than that from the relevant feedpoint impedance. The best place
for a current balun is before the ATU. (A high common mode impedance
with a low differential one is probably not a problem, because most of
the power will go into the differential one; the balun won't help much
but you probably didn't need one.)
I guess the real question is whether I want to get a KAT100 and a BL2 or
just stick with my SGC-231. The nice thing about the KAT100 is that it
The SGC will also have frequency and reactance based variations in its
resistance matching range. It's balun may well not like high impedances.
doesn't draw power unless it is tuning. My SGC-231 draws power all the
The KAT2 uses latching relays; the KAT100 uses continuously powered ones.
time to hold in the relays. That sucks up more battery. Also it is
physically bigger. Looking at the schematics for both I can see that the
SGC-231 has a greater range of C and L than the KAT100 does but maybe
the KAT100 is enough, especially when ganged up with the BL2 to provide
a 4:1 impedance ration to start with. I know the theory but was looking
That implies operating the balun well away from its design impedance.
Particularly on lower bands, the shunt impedance will be too low.
for a quick answer from the field of people here who have more
experience with this than I do.
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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