Roy Lewallen wrote:

Please let me emphasize again that not I or anyone else who has posted
is disputing the validity of your matching methods or the utility of
the "virtual short" concept. The only disagreement is in the contention
that the "virtual short" actually *effects* reflections rather than
being solely a consequence of them.


The key word there is "utility" - the virtual short/open concept is
*useful* as a short-cut in our thinking. But concepts are only useful if
they help us to think more clearly about physical reality; and
short-cuts are dangerous if they don't reliably bring us back onto the
main track.

We know that in reality both the forward and the reflected waves take a
side-trip off the main line into the stub, and from the far end of the
stub they are reflected back to rejoin the main line at the junction.
Since an open- or short-circuited stub has a predictable effect at the
junction where it is connected, then we could save a little time by
noting that a stub is present, and simply assuming what its effect will
be.

Within those limitations, I don't have any particular problem about
calling the effect a "virtual short" or "virtual open". As Richard
said, it is only a metaphor. We are using the word "virtual" as a label
to remind ourselves that the effect at the junction is not the same as a
genuine physical short or open circuit on the main line.

Where the concept goes off track is if anyone forgets about the
limitations, and begins to believe that a metaphor has physical
properties of its own. (It doesn't, of course - all of the physical
effects on the main line are caused by the stub, and the stub is the
only place where the root causes can be found.)

If there is any problem in using a short-cut, then simply forget it -
step back and analyse the complete physical system including the stub.


Walt said:
Incidentally, there has been mention of 'virtual' reflection coefficients. I can't agree with this
terminology.
Roy replied:
I don't use "virtual reflection coefficient" by name or in concept,
although it might have some utility in the same vein as "virtual
short".

Agreed. It all comes back to "usefulness" or "utility" again. As I said,
concepts are only useful if they help us to think more clearly about
physical reality - and "virtual reflection coefficient" has exactly the
opposite effect.



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73 from Ian GM3SEK