There is something wrong with the unmatched SWR values you measured/posted.
If your load is actually 23.5 +j0 then the beginning SWR is ~2.1:1, not the 1.6
you list. That's not to say that the tuner shouldn't do something with it but
it does point to something amiss. Likewise the values you give for 4:1 are
closer to 3.6:1
On 7/25/2016 6:30 AM, brian wrote:
Hi Don,
Further insight needed.
John and I have looked into the 80M tuner strangeness problem further.
We both have qualitatively seen the same thing. It is related to trying to
tune Z's that are on the order of 25-30 ohms on 160M/80M.
John already addressed the grounding issue in his K3.
The original problem raised was trying to tune a vertical antenna on 80M which
had a measured R=30 ohms and XL =18 ohms. The tuner only slightly reduced the
SWR. Repeated presses of ATU Tune did not improve things.
A calculation of the L network component values needed to match this load
shows L/C values well within the range that the KAT could switch in.
I then tried matching a 25 ohm pure resistive load-- two 50 ohm dummy loads in
parallel. One would think this kind of load could easily be matched. Not so.
It would match only on the upper end of 80M and not at all on 160M. The tuner
did not reduce the 2:1 SWR at all on 160M and only brought it down a few
tenths on the low end of 80.
It is interesting to note that the high end of 80 match used L/C values
greater than needed in the vertical antenna to match it.
John tried other resistive and combined resistive/reactive loads. Here are his
results.
Freq Load value measured SWR on K3 SWR on K3 after KAT3 tune
1.836 23.5 (via 1 m. RG213) 1.6 1.6
1.836 32+j28 2.5 1.0
3.522 23.5 1.6 1.3
3.522 56+j55 3.0 1.0
7.006 23.5 1.5 1.0
7.006 132-j76 4.0 1.0
10.100 23.5 1.3 1.0
10.100 30-j48 4.0 1.0
So we have two different K3's having the same qualitative matching problem
with lower Z loads on 80/160.
Of course it is possible that both KAT tuners have other component failures.
It would be interesting if others would try the two paralleled dummy load
experiment.
Elecraft suggested shorting out the 8.215 MHz trap in series with the
ATU/output. They say the trap has been removed in later KAT versions.
The trap consists of parallel 96 nH and 3900 pf components.
It is not clear that would fix this particular issue. It undoubtedly was a
fix for other issues.
Looking at the component values and parallel configuration:
1) Both shorted would be the Elecraft fix
2) Open C: Parallel inductor would shunt the RF around the failed cap. On
80/160 the added series reactance would be <2 ohms.
3) Open inductor: Parallel C would introduce a series reactance of less than
an ohm.
4) Both open, the tuner would not match on any band.
So we are left with a mystery.
Why are lower Z (~25-30 ohm) loads so hard to match on 80/160?
It would be instructive if others have luck matching the 25 ohm paralleled
dummy load case on 80/160.
Insight in where else to look would be welcomed.
73 de Brian/K3KO
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