Jim Kelley wrote:

There can exist no real point where the characteristic line impedance is
both 50 ohms and 150 ohms.

I agree, and using the same logic, there also can be no such thing as real
steady-state conditions. That doesn't keep us from using it as a real concept.

+----------------
|
---------+
50 ohms 150 ohms
---------+
|
+----------------

You can draw a vertical line through the transition point. That vertical
line has zero width. Ergo, the 50 ohm to 150 ohm transition requires zero
length, conceptually, of course. But you knew that already. While you were
at it, why didn't you point out that there cannot exist a real characteristic
impedance exactly equal to 50 ohms or 150 ohms?

What about cancelled reflected waves, destructive interference and all
that?

I should have said there are no net reflections on the 50 ohm feedline.
There are two component reflections at point '+' that cancel each other
as illustrated by the s-parameter equation, b1 = s11*a1 + s12*a2 = 0
s11 is a reflection coefficient for a1 and s12 is a transmission coefficient
for a2, the voltage reflected from the load. Those two components have to
cancel for the b1 net reflections to equal zero.
--
73, Cecil http://www.qsl.net/w5dxp



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