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Return Loss, AW: [nanovna-users] Accuracy of calculated values - Nano VNA and Saver
Hi all,
why blame someone for complaining controversial RL signs !? Keep cool, please. Yes, Return Loss has been an issue and some use it in a wrong way. This caused Trevor S. Bird, former IEEE chief editor, to write an article concerning that. Most remarkable, however, is the fact that even Trevor S. Bird in that very article was trapped himself, when he gave this "Return loss definition": Why ? What’s wrong with that? His formula for Gamma is the reciprocal part of what is between the absolute lines: Gamma = (Z1 – Z2) / (Z1 + Z2) (a) which here is falsely quoted for power transfer purposes, not (what would be true) for reflexionless transfer purposes. But the formula Bird quotes, unfortunalely is the one for reflexionless purposes. For power match calculating purposes it should be: Gamma = (Z1 – Z2*) / (Z1 + Z2) (b) the asterisk meaning conjugate complex. Otherwise for example a perfect complex match would not yield an SWR = 1.0 ! (a) would yield |Gamma| results > 1 and even negative SWR for a strictly passive network with lumped components for simple complex Z1 and Z2: Try this: Inner Generator Impedance Z2 = 100 – j 200 Ohm Load Impedance Z1 = 200 + j 300 = Ohm. Result: (a) ?strangely“ yields |Gamma| = 1.612 and SWR = -4.266 and RL = -4.149 dB (b) ?correctly” yields |Gamma| = 0.447 and SWR = 2.618 and RL = 6.989 dB so: Trevor S. Bird’s use of (a) inside his RL definition indeed yields negative RL, even though he just makes it a point that this cannot be and is wrong ! There are more trapped occasions than just Trevir S. Bird’s paper using an inadequate type of Gamma for power transfer considerations. Even the most wonderful SimSmith at one point does. By the way: SimSmith is the absolutely best Smith Chart program I ever have seen. (It also accepts .s1p sweep files as saved by NanoVNA saver without any problems.) I HIGHLY RECOMMEND SimSmith to all NanoVNA users and that is why I mention it here: Just google for ‘AE6TY SimSmith’ to find Ward Harriman’s home page There scroll down a bit to find in dropbox : It is very advanced. Look at least the Manual. And … Enjoy ! 73, Hans DJ7BA -----Ursprüngliche Nachricht----- Von: [email protected] <[email protected]> Im Auftrag von Martin via Groups.Io Gesendet: Freitag, 4. Oktober 2019 14:39 An: [email protected] Betreff: [nanovna-users] Accuracy of calculated values - Nano VNA and Saver Hi All, Owen Duffy has recently posted a note about the Nano VNA on his blog. <> He makes a few points about the accuracy of calculated values with both the Nano VNA and Saver in particular. If we first deal with the issue of Return Loss, which is probably the most problematic par, and has certainly caught me out on several occasions. Just about every instrument I can remember using, has RL shown as a negative curve, even if the units themselves are positive. This is handy if for example you are tuning a filter, as you can see the insertion gain on the uppermost trace and the RL loss on the lower one without them overlapping. It also matches the convention of SWR plots and when measuring the RL of cables it matches the convention of more attenuation being negative. However Owen makes the point that negative loss is actually gain (double negative) and vice versa, and the existing conventions do indeed lead to confusion and mistakes being made. Maybe return loss should really be called return gain, and then everyone would be happy (well maybe - but this is not a serious suggestion). However if we put this to one side, there is still the issue of how the values are being calculated, and if they are in fact correct. If not then I think this should be investigated in more detail and fixed, as there would seem to be an opportunity to do this before it propagates further. Regards, Martin - G8JNJ -- Diese E-Mail wurde von Avast Antivirus-Software auf Viren geprüft. |
