On Apr 13, 11:49 pm, Ian White GM3SEK wrote:
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.
....

Indeed. I was thinking about this in terms of short-cuts before
reading Ian's post. What if you take a short-cut and it just takes
you off into the woods? I'm not sure my posting about this made it
into the thread in an intelligible way. (I fear Google may have sent
it off on a "short-cut.")

The gist of it was that, although there are examples where considering
points an even number of half-waves from a short as being shorts
themselves work fine, there are plenty of counter examples too. I
fear that people new to the use of stubs will be lulled into a false
sense of security using that concept, when indeed it fails miserably
at times. Especially in this age of computers and readily available
programs to deal with lines, INCLUDING their loss, why would I use a
concept that may take me on a short-cut that turns out to be the long
way around?

What IS useful to me about the concept is NOT the calculation of the
performance of a particular network of stubs, but rather in coming up
with the trial design to test with full calculations. My example was
the use of two stubs to give me a null on one frequency and pass
another frequency; I can get a null by putting a "virtual short" at
that frequency, and that's a line that's a half wave long on that
frequency, shorted at the other end. But on a slightly lower
frequency, it looks capacitive, so I can put another stub that's
inductive in parallel with it to create an open circuit at the
frequency I want to let pass. THEN I pull out the calculations with
line attenuation included, and discover that in some situations it
works fine, and in others, the performance is terrible.

It's a useful visualization tool and design aid; it's a poor analysis
tool at best. At worst, it will lull you into building something that
just won't work, wasting time and resources.

Cheers,
Tom