wrote:
We have a choice of two rho for this situation:

Correction: We have a choice of two reflection coefficients
each with its own unique definition.

black box - 0, computed from the surge impedance of the line and
the steady state impedance of the load

Actually, Sqrt(Pref/Pfwd), the definition of rho.

open box - 0.5, computed from the surge impedance of the line and
the surge impedance of the load

Actually, (150-50)/(150+50), the definition of s11.

In any case, what we have in this experiment is a case where there
IS an impedance discontinuity and yet there is no reflection (if
you use the "black box" rho, as is often done).

This is technically not true. The NET reflections are zero. There
are two non-zero component reflections as seen from the s-parameter
equation:

b1 = s11*a1 + s12*a2

These three terms are all reflections. b1 is the NET reflections
toward the source. Since b1 = zero, s11*a1 = -s12*a2, i.e. the two
component reflections are of equal magnitude and opposite phase and
therefore cancel. This is explained in the last three paragraphs on
the Melles-Griot web page.

So, if we are allowed to say in the first experiment that rho is 0
despite an impedance discontinuity, we are equally allowed to say
for the second that rho is -1 despite the absence of a discontinuity.

There is NOT an absence of a discontinuity. The discontinuity is as
large as it can possibly be, an unterminated transmission line. There
is an infinite SWR on the 1/4WL line. The apparent zero impedance
at the input is simply a V/I ratio where Vnet=Vfwd+Vref=0
The forward wave is carrying Vfwd^2*Z0 watts associated with the
forward Poynting vector and the reflected wave is carrying Vref^2*Z0
watts associated with the reflected Poynting vector. A directional
wattmeter will indicate the same thing.

There is NO physical discontinuity at the black box. The only physical
discontinuity is the open end of the stub. That's where 100% of the
reflection action takes place.
--
73, Cecil http://www.qsl.net/w5dxp



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