[Elecraft] K2 KAT100 and BL2 questions

[David Woolley (E.L)]

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
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