[Elecraft] Can I measure antenna impedance with K2?

[Fred Jensen]

On 7/31/2014 1:23 PM, Per-Tore Aasestrand wrote:
> Hi Matt,
>
>> Of course you realise that the output impedance of the ATU will be the
>> complex conjugate of the impedance at the rig end of your feedline.
>
> I wonder if this is correct

Well, it is, however it wasn't the original question.
>
> IMO, the task of the tuner is to make the tx happy, so it can look into the
> load it is designed for. Thus, it will translate the impedance as seen from
> the antenna connector into something as close to 50 ohm as possible. The
> tuner will not change anything on the feeder. With or without the tuner,
> the feeder will have the same VSWR.

You are correct Per-Tore.  Looking into the male connector on your 
feedline, you will see *some* impedance which depends on the impedance 
at the feedpoint of your antenna, the characteristic impedance of your 
feedline, and the length of your feedline ... with one exception:

If the feedpoint impedance of your antenna is totally resistive, *and* 
is exactly equal to the characteristic impedance of your feedline, then 
length only matters when calculating loss.  This situation hardly ever 
occurs for any real hams however. :-)

The KAT2 "antenna tuner" does not "tune" the antenna, or anything else 
for that matter.  It is simply an L-C network [L-network in the KAT2] 
that transforms the complex impedance seen at the BNC connector to what 
the PA output filter wants to work into ... nominally 50+j0 ohms.  Once 
you get a match and find out the L, C, and configuration of the 
L-network in the tuner, you can use the L-network equations to determine 
the impedance transformation taking place in the "tuner" and thus the 
complex impedance looking into the feedline connector

If you know the complex impedance looking into the feedline connector, 
the characteristic impedance of the line, and it's length, you can use 
one of many programs to calculate the impedance of your antenna at the 
feedpoint.  I use N6BV's TLW which comes on the CD with the ARRL Antenna 
Book, but there are many other ways, including paper and pencil on a 
Smith Chart.

And yes, you are also correct that the presence of the tuner will not 
affect the VSWR on the feedline, nor the loss incurred by that VSWR. 
Hope this helps.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org