[Elecraft] K3 (not S) Antenna tuner strangeness
Ron D'Eau Claire
ron at cobi.biz
Mon Jul 25 13:16:56 EDT 2016
FWIW, my K3 (not S) matches 25 ohms resistive (two 50 ohm loads in parallel
as you described) on both the low end of 160 and 80. In both cases the
matched conditions shows an SWR of 1.5:1.
73, Ron AC7AC
-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of brian
Sent: Monday, July 25, 2016 6:30 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 (not S) Antenna tuner strangeness
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, g3pqa at onetel.com 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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