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KS3/P3 SWR Display Readings

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Ferris Jennings

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-

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Don Wilhelm-4

Re: KS3/P3 SWR Display Readings

Ferris,

I am assuming that you are referring to the SWR displayed from the P3
Transmit Monitor which is placed in your feedline to the antenna. If
other than that, please correct me.

The SWR displayed on the Transmit monitor is the SWR on the transmission
line to the antenna - in other words, it is the SWR at the *output* of
the tuner in the K3S.
The wattmeter in the K3S indicates the SWR at the *input* of the KAT3.
It tunes to 'keep the K3S finals happy'. That will not change the SWR
on the feedline to the antenna.

If you were to place the TX monitor sensor between the K3S PA and the
KAT3 (impossible to connect there), then the TX monitor and the K3S
should agree.
In other words, the term "Antenna Tuner" is not an entirely correct
term. It does not change the SWR on your antenna and feedline, but
instead transforms the SWR at the output into a low SWR at its input.
An ATU is a system of lumped components that compensates for the SWR on
the antenna and feedline and transforms it to a low SWR at its input to
match the 50 ohms resistive match wanted for maximum efficiency from the
transceiver PA transistors.

In other words, the K3S is showing you the SWR at the *input* of the
KAT3, but the SWR at the *output* of the KAT3 will not be changed. If
you want to change that, you will have to work on your antenna and
feedline system.

73,
Don W3FPR

On 12/18/2015 7:35 PM, Ferris Jennings wrote:

> 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?
>
>
>

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Nr4c

Re: KS3/P3 SWR Display Readings

In reply to this post by Ferris Jennings

No. You have just discovered the second law of SWR, "The confusion factor goes up exponentially with number of SWR meters in the system."

In other words the SWR reading in the K3S (via KAT3) is the result of "matching" antenna to the output of the amplifier in the radio (LPA or KPA3). Your sensor for the TXMON is between the antenna and the radio so does not know of the "match" and reports the un-matched SWR of the antenna.

Oh, the first law, "You can never have too many SWR meters, in the closet."

Sent from my iPhone
...nr4c. bill

> On Dec 18, 2015, at 7:35 PM, Ferris Jennings <[hidden email]> wrote:
>
> 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-
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

Ferris Jennings

Re: KS3/P3 SWR Display Readings

In reply to this post by Ferris Jennings

Thanks for all the replies.

Ah, it all makes sense ... now. I kept thinking that the SWR signal was
being sent from the K3S to the P3 via the RS232 port. Wrong.
So, guess I have, with the P3 SWR display, something of an antenna analyzer.
Nice to be able to have the K3S provide an "adjusted" SWR and the P3 provide
the real SWR at the antenna. Seems like my antenna does need a little work.
That project will have to wait a while.

Thanks again for the replies.
73
-Ferris NB6T-

-----Original Message-----
From: Ron D'Eau Claire [mailto:[hidden email]]
Sent: Friday, December 18, 2015 7:49 PM
To: 'Ferris Jennings'; [hidden email]
Subject: RE: [Elecraft] KS3/P3 SWR Display Readings

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:[hidden email]] On Behalf Of Ferris
Jennings
Sent: Friday, December 18, 2015 4:36 PM
To: [hidden email]
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-

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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