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On your other aspect of return loss:
Hi Dana and all, Yes, indeed at low frequencies lossy transmission lines can have a complex characteristic line impedance. In that case even some 2.141 > Gamma1 > 1 is possible, as the Gamma1 formula, correct for lines, for lines will yield. This was described already in the late sixties by Robert A. Chipman in "Theroy and problems of Transmission Lines" as well as by others. At a close look one will see, that anyway that is not the invention of a "perpetuum mobile". For describing lumped element mismatch caused Gamma2, application of the Gamma1 formula, however, leads to false results. On transmission lines (Title and contents of Chipman's book is limited to these) we have actual reflections and thus the Gamma1 formula is correct. In lumped element mismatch caused cases "reflections" do not occur and accordingly the Gamma1 formula does'nt apply - though looking similar. So: Let's distinguish these two cases rather than use the ATIS (former ANSI) type of false assumption, that these were all the same. By distinguishing the two Gamma cases, we avoid just one (less known) reason of a number of different possible pitfalls, all leading to false negative Return Loss on passive lumped element mismatch circuits. 73, Hans DJ7BA --Urspr¨¹ngliche Nachricht----- Von: [email protected] <[email protected]> Im Auftrag von Dana Whitlow Gesendet: Mittwoch, 3. Juni 2020 23:18 An: [email protected] Betreff: Re: [nanovna-users] out of "Presentation on the NanoVNA for the raileigh Radio Society", Now: "False Return Loss Calculations" Hello all, What you do in your own shop (or ham shack) is of course your own business. But if you teach, please, please, teach that a passive load's return loss is a positive number of dB, as it is the only one of the two practices that is logically consistent with the meaning of the word "loss". Of course you should probably bring this whole controversy to the attention of your students and warn them to take a close look at numbers presented in papers etc to be sure what the author really means. Now on to another aspect of return loss. I tend to define the system impedance Zo as the impedance of transmission lines commonly used in the environment, and I tend to assume that the line impedance is a pure real number. This assumption (or "approximation" if you prefer) is excellent in normal RF applications, but falls apart rather badly at very low frequencies. Now here's my quandary: Only a pure real load impedance can absorb all the power incident on it. So if one is dealing with a system Zo that is complex, it seems to me that there is no load impedance that results in zero reflected power back into the line. Is this reasoning too simplistic? When I took the transmission lines course at UMich in the late 1960's, this issue was not addressed and I was not yet sharp enough to think to ask. Dana -- Diese E-Mail wurde von Avast Antivirus-Software auf Viren gepr¨¹ft. |
