[Elecraft] K3 SWR Accuracy - reprise

[Wayne Burdick]

Hi Stewart,

The KAT3, like all wide-range ATUs, has significant stray reactance,  
especially on the higher bands. When you tuned it into 50 ohms, you  
created an L-network on each band that tunes out this reactance--at  
that one impedance.

In your test, the KAT3 is between the K3's SWR bridge and the LP100,  
so the two bridges are looking at different points in the network. So  
changing the load Z to 25 ohms affects the two readings differently.

The disproportionate error on 20-15 m reflects the nature of the  
KAT3's strays. On these bands, the stray-cancellation values that it  
automatically finds results in an L-network. On still higher bands, it  
may look more like a Pi network, which could explain why your SWR  
error delta  is lower on 6 meters. On the lower bands, the strays are  
less significant, so the readings match.

If you removed the KAT3, the readings would be in closer agreement.

73,
Wayne
N6KR


----
http://www.elecraft.com

On Nov 4, 2009, at 8:44 AM, Stewart <stewart at baker.nildram.co.uk> wrote:

> Hi,
> Now traffic on the new K3 Macro functions has somewhat died down,
> I would like to raise again the issue of the K3's SWR measurement
> accuracy.
>
> I did some tests using dummy loads and a LP-100 meter as follows.
>
> The dummy load is a precision 100W one which I measured with a VNA
> and has a VSWR of <1.03 from 1 to 30 Mhz. To simulate a 2:1 VSWR
> I used another dummy load (not quite as good) in parallel.
>
> The K3 output was from ANT1 using the internal ATU. The ATU was
> automatically tuned to 1:1 on all bands.
> The results I got for 2:1 VSWR are as follows:-
>
>               LP-100           K3
> 160         1.99                1.8
> 80           1.99                1.8
> 60           1.99                1.8
> 40           1.95                1.8
> 30           1.97                1.6
> 20           1.97                1.4
> 17           1.97                1.3
> 15           1.96                1.2
> 12           1.96                1.5
> 10           1.96                1.4
>
> As can be seen my K3 SWR indication consistently under reads.
> However, it really gets bad on 20,17 & 15.
>
> From previous replies I understand that this under reading would
> be compensated for in the firmware, sometime in the future .
>
> Is this still on the list of problems to be addressed ?
>
> 73
> Stewart G3RXQ
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