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Good Morning David,

Thank you for the link to the Amphenol Conversion Chart.

For reference, similar charts can be downloaded for free as PDF documents from Mini-Circuits. Here are some additional links.




And here is a link to my favorite, a dBm-Volts-Watts conversion table.


However, the fact remains that these charts are not necessary if your NanoVNA is set up to display all the data it is capable of showing. The issue here is that when one of the traces on the NanoVNA is displaying LOGMAG (or the Base 10 logarithm of the Magnitude of the signal) it is showing you the Return Loss. At the same time another trace can be set to display the VSWR. Both traces simply need to be set to take their data from the S11 Reflected Channel. I tried this last night and it works perfectly. The NanoVNA’s S11 LOGMAG and S11 VSWR displayed trace readings match the charts exactly.

Helmut is correct in showing the formulas for the mathematical relationships between Return Loss, Reflection Coefficient and VSWR. All three of these parameters are simply different ways of looking at the same data. If you have one, you can calculate the other two. For the NanoVNA this is simply a few additional lines of code in the firmware.

Now it’s at this point that we could get into an argument about whether or not Return Loss (or LOGMAG) should be diplayed with a minus sign or not. Some folks say, “It’s a loss, not a gain. So it should have a minus sign.” Other folks say , “Yea, but it’s name is Return LOSS! So the loss is implied and the minus sign is superfluous.” My understanding is that this argument started the day after Moses made the very first VNA — and it continues to this day. It is an argument that Rune Broberg got into with the original users of his NanoVNA-Saver program. To appease them he added a check-box to his software to add or remove the minus sign from the display of Return Loss. He left it to the end user to decide. And I will leave it to you to do the same.

73,

Larry, AE5CZ

On Sat, Nov 7, 2020 at 10:54 AM, Larry Goga wrote:

Now it’s at this point that we could get into an argument about whether or
not Return Loss (or LOGMAG) should be diplayed with a minus sign or not. Some
folks say, “It’s a loss, not a gain. So it should have a minus sign.”
Other folks say , “Yea, but it’s name is Return LOSS! So the loss is
implied and the minus sign is superfluous.” My understanding is that this
argument started the day after Moses made the very first VNA — and it
continues to this day. It is an argument that Rune Broberg got into with the
original users of his NanoVNA-Saver program. To appease them he added a
check-box to his software to add or remove the minus sign from the display of
Return Loss. He left it to the end user to decide. And I will leave it to
you to do the same.

Here is the IEEE perspective on this issue. PDF of "Definition and Misuse of Return Loss" by Trevor S. Bird



Roger

Roger I was very fortunate in that my second job out of college (I got a BS in electrical engineering) was with HP. Back when HP was the instrumentation company. In those days (1982) I doubt anyone would disagree that HP was THE premier designer of RF test equipment. VERY active in everything RF.

As part of the training for sales people, so we could intelligently talk with our customers, they spent 9 weeks, in 3, 3 week segments, teaching us the basics of rf and of their instruments.

While much of what I learned has been relegated to the dust bin in my brain, a few things remain. One was that return loss was always positive. If you do the math this will be born out. If you just think about it, its a ratio of incident vs reflected rf (or maybe the other way around. I just don't recall). A ratio of two rf power levels should always be positive. But again, do the math.

Sure many many folks make it negative. Many, in their explanation, first say one thing then later in their article, the other. Of course confusing! And some conflate return loss with insertion loss. But, insertion loss will also always be positive so I've not understood that one.

Anyway, as you said, its an argument that's been around forever. And, one side will always be wrong :)

Ron
N4XD