[Elecraft] K3 (not S) Antenna tuner strangeness

[Wes Stewart]

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