On Apr 14, 6:06 pm, Roy Lewallen wrote:
K7ITM wrote:
. . .
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.

In my opinion, the potential harm can be much worse. If it causes you to
buy into the notion that traveling waves interact in a linear medium,
that opens the door to a whole universe of invalid conclusions. We've
seen some of those promoted very vigorously in this newsgroup.

Roy Lewallen, W7EL

Yes, you're right, Roy. I guess I didn't consider that because I'm
not very likely to buy into it, but from the point of view of someone
just learning about linear systems, it's a danger.

The analogy may not be prefect, but I think it's a lot like the
usefulness of the idea of a "virtual ground" at the inverting input of
an op amp. But it's a virtual ground only under specific conditions:
strong negative feedback is active, and the non-inverting input is at
(AC, at least) ground potential. For it to be a useful concept
without too many pitfalls, the person using it has to be aware that
the conditions that make it a good approximation don't always hold.
Similarly for a "virtual short" on a line.

Again, though, it IS useful to me to think along these lines, when
looking to do something useful with stubs: I want to kill frequency
W, so I can put a stub across my line that's half a wave long at W,
shorted at the far end. At the same time I want to pass V, and the
stub I just put there to kill W has reactance X at frequency V. If I
put another stub with reactance -X at freq V across the line there, it
will let V through with minimum effect. Now go calculate how well it
will perform with particular lines.

So, to come up with a design to try, I do think about how stubs
behave, in a general sense, including things like "a half-wave line
shorted at the far end echos a short", but with the programs I have
readily available, it's silly to rely on approximations that drop the
line attenuation, when I want to know how my idea will actually work
when I build it.

Cheers,
Tom