[TowerTalk] Dipole resonates lower than expected.

[ersmar@comcast.net]

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
> 
> 
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