On Sun, Jan 26, 2020 at 07:07 AM, Andy UA3RAW wrote:
As it was described by DJ7BA in #8655 message, you must find the lowest (=Lambda/4) resonance frequency. (Impedance there is about 0 Ohm.) One full revolution in the Smith diagram corresponds to a displacement along the line by a distance equal to half the wavelength in it.
So the half-way around, is equal to 1/4 wave length. This is a point of the lowest resonance and it is corresponding to the Marker 1 on your screenshot.
In order to find the characteristic wave resistance of a coax cable, you must find the point that is a half of 1/4 wave length, or 1/8 wave length, and read the reactance at this point. That's why it is called Lambda/8 method.
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Andy,
Thanks so much for the clarification. Its something I'll definitely be adding to my engineering notes. Electronics has been both my vocation and avocation for the past 40 years. I still enjoy learning new things, however; few subjects make my eyes glass over as the smith chart does.
At the beginning of my career the company I worked for won a government contract to RF immunity test ignitors to ensure they didn't unintentionally misfire. I had to design rf matching networks for use between our power amplifiers and the ignitors at each test frequency. We brought in the great Christopher Bowick to teach a course on using the smith chart to design matching networks. After that project was completed I never had another practical application to apply what I had learned and as they say "if you don't use it you lose it".
The Lambda/8 method is the first real world application, other than verifying the quality of my solt calibrations or measuring components, that I have used in a while. I wish there was a way of automating the process.
Again, thanks for taking the time to reply. Perfect example of the members helping members format.
- Herb
