[Elecraft] K3 SWR Accuracy - reprise

[Wes Stewart]

I suggest a reading of Larry's original paper on the design of the LP-100 to understand the heroic efforts needed to make these measurements with any accuracy.

see: http://www.telepostinc.com/Files/phipps-1.pdf

It should come as no surprise that the K3's built-in directional coupler lacks the directivity of the ones in Larry's LP-100, and directivity is extremely important in this application.

For an example left to the reader to work out, measure a load with 20 dB return loss using a coupler with 20 dB directivity and see what the uncertainty is.

Wes Stewart  N7WS

--- On Wed, 11/4/09, Stewart <stewart at baker.nildram.co.uk> wrote:

From: Stewart <stewart at baker.nildram.co.uk>
Subject: [Elecraft] K3 SWR Accuracy - reprise
To: "Elecraft Reflector" <elecraft at mailman.qth.net>
Date: Wednesday, November 4, 2009, 1:44 AM

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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