[Amps] Input SWR revisited

[Robert Carroll]

Hopefully no one would run a multi quarter wave line
for a match in the shack since, if you look at the
Smith chart, you are back to where you started after a
360 angular rotation on the chart. The point is if you
match the transceiver and make it happy with a
particular length of line,  the SWR in the line is
close to meaningless from a loss standpoint. Spiraling
is not an option, it is just something observed on a
Smith chart only when the line is lossy and you are
driving a load far away.  These days no one of sound
mind would use a Smith chart to analyze such a
situation.  Of course at 10 meters if you are feeding
an antenna 100 meters away, you will probably want to
buy low loss transmission line and provide a good match
to it as you transition to the antenna.  But as you
point out that's not what we are talking about here,
which is mismatch in driving an amp relatively near to
a transceiver in your shack.  If you look at a Smith
chart (it's a normalized chart)  you will see the
wavelengths towards generator scale (the load view) as
well as a second scale marked towards load.  You'll see
that 180 angular degrees from the load on the chart
corresponds to a 1/4 wave length of transmission line.
As you learn to use the chart it is simple to see what
a mismatched load will look like at the generator
(transceiver) end.  And with a little more work you can
determine what length is needed to make the transceiver
happy-and take a look at the situation at other
frequencies.  These days such calculations are most
easily  done on a computer but an understanding of the
Smith chart  and the equations it is based on will help
in understanding how the match is taking place.  There
are many good and simple guides to the Smith chart on
the internet as well as in the ARRL handbook.  Digikey
has a high resolution Smith chart at this location
<http://www.digikey.com/~/media/Images/Article%20Librar
y/TechZone%20Articles/2014/March/The%20Smith%20Chart%20
A%20Graphical%20Tool%20Still%20Vital%20in%20RF%20Design
/article-2014march-the-smith-chart-an-ancient-fig2-full
size.jpg?la=en-US>  and a very nice 2014 guide to using
it at The Smith Chart: An
<http://www.digikey.com/en/articles/techzone/2014/mar/t
he-smith-chart-an-ancient-graphical-tool-still-vital-in
-rf-design> 'Ancient' Graphical Tool Still Vital in RF
Design.

 

73

Bob W2WG

 

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On
Behalf Of donroden at hiwaay.net
Sent: Wednesday, January 14, 2015 5:50 PM
To: amps at contesting.com
Subject: Re: [Amps] Input SWR revisited

 

In practice, few of us run amplifier input cables in
the multi-quarter wave length category, so spiraling
around the Smith is not an option.

Just Saying .......

 

Don W4DNR

 

 

 

 

Quoting Robert Carroll < <mailto:w2wg at comcast.net>
w2wg at comcast.net>:

 

> I suggest a look at this non-mathematical
paper---assuming this list 

> 

>> But, the swr inside the 75 ohm coax is still 1.333
at

> all points along

> 

 

> 

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