On 6/22/2011 5:50 PM, John Smith wrote:
On 6/22/2011 3:24 PM, dave wrote:
On Jun 22, 1:49 pm, Cecil wrote:
Let's return to an earlier example and compare a single-port analysis
with a dual-port analysis.

100w
source--50 ohm--+--1/2WL 291.4 ohm--50 ohm load

The 50 ohm Z0-match point is at '+'. The forward power on the 50 ohm
line is 100 watts and the reflected power on the 50 ohm line is zero
watts. The forward power on the 291.4 ohm line is 200 watts and the
reflected power on the 291.4 ohm line is 100 watts. 100 watts is being
sourced and delivered to the 50 ohm load.

The voltage reflection coefficient, rho, at the load is (50-291.4)/
(50+291.4)=0.7071. The power reflection coefficient, rho^2, at the
load is 0.5, i.e. half of the power incident upon the load (200w) is
reflected (100w). Since the load is a single-port, these parameters
are consistent with a single-port analysis. In a single-port analysis,
we cannot tell the difference between a virtual reflection coefficient
and a physical reflection coefficient.

The problem comes when we use a single-port analysis on the Z0-match
point. Since the reflected power on the 50 ohm line is zero, a single-
port analysis would yield rho=0.0 and rho^2=0.0 when viewing the Z0-
match from the source side. When we perform a dual-port analysis, we
get different values for rho and rho^2, i.e. we get the complement of
the reflection coefficients at the load which is a characteristic of
any simple Z0-match similar to the above example.

For a dual-port analysis, rho looking into the Z0-match from the
source side is (291.4-50)/(291.4+50)=0.7071 and rho^2 looking into the
Z0-match from the source side is 0.5, the same as at the load. Looking
back into the Z0-match from the load side, the sign of rho is negative
just as it is at the load with rho^2=0.5, the same as at the load.

Since the two analyses yield different values for the reflection
coefficients, which analysis is correct? The answer gives the clue to
the resolution of this discussion.
--
73, Cecil, w5dxp.com

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?

Logic, immediately, suggests to me, that varying the frequency and
measuring voltage, amperage, and SWR would begin to immediately point
the answer(s.)

Regards,
JS


A 50 ohm, non-reactive/carbon load would complicate matters, so
naturally, I am assuming that is NOT the case ...

Regards,
JS