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
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