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Yes, "Return Loss" s o m e t i m e s is a misnomer: If used in context with simply two different impedances meeting at a connection.
This can be understood as a complex generator's inner (serial) impedance Z2 being terminated with a (serial) complex load Z1 (parallel to
be recalculated to serial equivalent).

This can be built from lumped components.

Everybody will agree, that there is absolutely no "returning" of a "wave", and no "reflection", in what is just a complex voltage divider.
Physically it is a different (no wave) phenomenon. In such cases, all the following are "misnomers" I n t h e s t r i c t s e n s e of the wording:

Standing "Wave" Ratio,
"Reflection" Factor,
"Return" Loss.


Reflections will, however, appear on Lines: there is a forward wave and a reflected wave.
There is a (wave typical) 180° phase shift by reflection involved, that makes valid

Gamma = (Z1 - Z2) / (Z1 + Z2) (a)

But where there are no waves, there are also no reflections and thus no 180° phase shift. This is why we get a different formula.
Please note, that Z2 above is the line's w a v e impedance - that does not exist in lumped circuits.

There we need to use a different Gamma:

Gamma = (Z1 - Z2*) / (Z1 + Z2) (b) with the asterisk meaning conjugate complex.

Z2 here now means generator's inner impedance (Some call it Thévenin Impedance.)

Unfortunately so many well established papers and publications do not care for that difference between (a) and (b) , just very few do.

And the many careless publications are being quoted and repeated ever so often. That is the problem. (Just the same as with fake news ...)


So - for complex voltage dividers - that is where there are simply no waves involved -
but also if one had previously calculated equivalent impedances on places with waves, like at line ends -
thus whenever we are using a load impedance Z1 and an (equivalent) generator impedance Z2, the above (b) is the correct formula.

The problem with the Smith Chart is, that the Z (or Y) Plane is mapped to the Gamma plane only by formula (a).

It would be extremely bad to "reinvent" the Smith Chart.

So we must live with (a) - like it or not. I see no choice.

The good thing is: The same (Smith chart type) mapping formula can stay, if both (serial) reactances are combined together at the load side.
Then the (thus new) generator impedance is real without reactance and the (thus new) load includes the sum of the two (serial) reactances.
Then the conformal mapping formula (a) can be applied again as usual in the Smith Chart.


We need the voltage divider type formula (b) when dealing with power transfer.


But when will that become common knowledge?

Debating fake physics (calculating with wave formulas where there are no waves and thus no no phase shift) is like fighting against windmills ...


73, Hans
DJ7BA




-----Ursprüngliche Nachricht-----
Von: [email protected] <[email protected]> Im Auftrag von Oristo
Gesendet: Freitag, 4. Oktober 2019 17:32
An: [email protected]
Betreff: Re: [nanovna-users] Return Loss

its become the consensus that return loss is a negative number

IMO, "return loss" is a poor term.
Arguably, >>any<< return is loss, when sending power is the goal. In which case, "return loss" might be power sent (since lost from return)..




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