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
On 7/25/2016 2:35 AM, Don Wilhelm wrote:
John,
Yes, the "earthing" connection to the SO-239 jacks is important.
That connection is provided by the mechanical connection to the standoff
between the right side panel and the KAT3.
The assembly manual instructions will provide adequate connection -
first insert the side panel screw through the 2-D connector and then
tighten the standoff to the 2D connector, and lastly, put the screw and
lockwasher into the standoff.
The problem is that *if* that long screw in the upper rear corner of the
right side panel is removed for any reason, it cannot be tightened
properly without going through the entire procedure - tighten the 2D
connector, then tighten the standoff and finally tighten the screw into
the standoff.
If one tries to remove that screw from the right side panel, it is
impossible to re-tighten everything without going through the entire
process in order. The threads of the 2D connector cannot be properly
tightened to the threads of the standoff.
A shortcut is to drill out the screw threads on that 2D connector
allowing the screw to pass through the 2D connector without threads.
That will permit the screw threads to pass through the 2D connector and
be tightened onto the standoff.
I recommend that anyone removing the right side panel from their K3/K3S
to drill out the threads in this hole of the 2D connector so it can be
assembled tightly without going through the entire procedure of first
tightening the 2D connector, then the standoff and finally the screw and
lockwasher.
73,
Don W3FPR
On 7/23/2016 9:09 PM, [email protected] wrote:
Guys,
I've run into something on 80M trying to use the internal antenna tuner.
It fails to tune to a low SWR. Brian K3KOI had a similar problem but
never quite got to the bottom of it because partly intermittent,
possibly multiple faults.
In the end to fix it:-
a) I rewound one of the swr sensor toroids (centre wire had possibly
arced to ferrite because I disconnected antenna by mistake and tuned),
and
b) fixed a new earthing wire between KAT3 chassis and SO259 Ant1
socket body. (possible poor earth connection via KAT3 securing pillar
to side panel)
John G3PQA
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