TT:
FYI: http://www.sss-mag.com/smith.html .
73 de
Gene Smar AD3F
> On Sun, 16 Oct 2005 21:49:38 -0400, Tom Rauch wrote:
>
> >You received good answers, I just wanted to point out the
> >problem with making measurements of resonance at the shack
> >end of a feedline that has standing waves.
>
> The analysis that helped me the most in understanding transmission
> line behavior is the Smith Chart. The center of the chart is a
> pure resistance equal to the Zo of the transmission line. At one
> end of the line, you "enter the chart" at ZL/Zo. The distance
> between this point and the center of the chart tells you the VSWR.
> The impedance will then vary along the line as it follows a circle
> whose radius is equal to the VSWR. Halfway around the chart is a
> quarter wave, a full circle is a half wave.
>
> For any real line (that is, one that has loss), the radius of that
> circle will gradually decrease as you go along the line, which
> means that the VSWR approaches unity. As you move along the line,
> the Z at any point will be some R+jX (or R-jX) value depending on
> the VSWR and where you are on the line. At resonance, you will
> have j0 at the antenna, and at every half wave repetition of that
> point, and at every quarter wave inverse of that point, but at
> every point in between you'll see some R and some X.
>
> The Smith Chart is a really powerful tool in understanding (or at
> least visualizing) what a transmission line does, and how we
> measure what's going on. I can't point to a specific place to
> learn it -- I learned it in EE school 40+ years ago -- but I
> recall seeing it in ARRL books on antennas. When I learned it, it
> was all on paper. Now you can do it with computer software, and
> many instruments generate Smith Chart displays of their data.
>
> You can, for example, take complex data for the Z at the
> transmitter end of a line, put it on a Smith Chart, and rotate
> around the chart for a distance equal to the line length to find
> the actual antenna impedance. I've done this using the AEA
> analyzer, to good effect. Better analyzers can do it even better
> (and probably faster, especially if they have a built-in Smith
> Chart).
>
> Jim Brown K9YC
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather
> Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
> and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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