On Jun 23, 12:43*pm, Cecil Moore wrote:
On Jun 22, 5:24*pm, dave wrote:

ok, i'm afraid i'm going to have to ask the simple question... if you
blackbox the load and stub and look at just the one connection to it
and that gives you no reflected power... where do you define the
second port, and why?

For the two-port analysis, only the impedance discontinuity at point
'x' is in the black box. One port is the source side of the impedance
discontinuity. The second port is the load side of the impedance
discontinuity. It allows the standard s-parameters to be measured and
the standard s-parameter equations to be used.

On the source side of the impedance discontinuity:

b1 = s11(a1) + s12(a2)

On the load side of the impedance discontinuity:

b2 = s21(a1) + s22(a2)

Those are the normalized voltage equations. Squaring those equations
shows what happens to the component powers including interference
components. Reference:

http://www.sss-mag.com/pdf/an-95-1.pdf
--
73, Cecil, w5dxp.com

p.s. if the separation between the two ports is just the discontinuity
connection 'point' then the voltages must be the same and the currents
are exact opposites only because of the direction convention defined,
there can be no difference measuring on one side of a point to the
other.