[Elecraft] KS3/P3 SWR Display Readings

[Ron D'Eau Claire]

Ferris, is the sensor for the TX MON mounted directly on the ANT connector
on your K3S? (no coax jumper to the sensor). 

The reason for the question is this. The tuner in the K3S adjusts the
impedance the K3S's finals see to 50 ohms at the input to the KAT3 inside
the K3S, but the impedance the KAT3 sees at the ANT connector to the coax
might be quite different. That's what the KAT3 does. It transforms whatever
impedance it sees at the ANT connector to 50 ohms, non reactive or as close
as it can get and that's the SWR you see reported. It tells you the K3S
finals are delivering power most efficiently to your antenna system. But it
does nothing about the SWR on your coax feedline to the antenna. 

Unless your feed line is exactly 50 ohms and it is seeing exactly 50 ohms at
the antenna, there will be standing waves on the feed line. The idea is to
keep them low enough to prevent excessive feed line losses, but the SWR is
virtually never zero.

The impedance at any point along your feed line will vary as you move from
the rig to the antenna. That's dependent upon the SWR on the feed line and
the frequency. 

Any feed line between the sensor in the KAT3 and the sensor for the P3 TX
MON will cause them to see different impedances and so they will report
different SWRs.  

The best way to check the two is to mount your P3 TX MON sensor right at the
K3S ANT connector with a double male adapter so there's an absolute minimum
of feed line between the KAT3's SWR bridge in the one in the IX MON sensor. 

Ideally terminate the TX MON sensor in a good 50 ohm dummy load instead of
connecting it to the antenna. 

Then both SWRs should agree within a reasonable range. 

73, Ron AC7AC



-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ferris
Jennings
Sent: Friday, December 18, 2015 4:36 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] KS3/P3 SWR Display Readings

Hell-o Everyone -

 

Finished building my K3S and P3 (with TX Mon), and I'm starting to play with
the configuration/menus.

I have the KAT3 ATU installed, and it works great. But I do have a question
regarding the SWR displays.

 

I'm running a simple diploe, and the KAT3 tunes it down to 1.0-1 in a second
flat. However, the SWR meter on the PS display always reads way higher,
something like 2.9 - 1. I've swapped antennas and lead (another dipole), and
I get the same results.

 

Question is: why the discrepancy in SWR from the K3S display to the P3
display? Am I missing a configuration setting?

 

Thanks

-Ferris NB6T-