reflection coefficients and impedance matching

[pepewaw]

Hi,

I´ve been reading the AN154 from Agilent to understand the S parameters. However, There´s one thing I can´t figure out:

One one part, it defines the reflection coefficient (gamma) as the following:

"Another very useful relationship is the reflection coefficient, Gamma. This is a measure of the quality of the impedance match between the load and the characteristic impedance of the line. The reflection coefficient is a complex quantity having a magnitude, rho, and an angle, theta. The better the match between the load and the characteristic impedance of the line, the smaller the reflected voltage wave and the smaller the reflection coefficient."

Later on, refering to a network made of a source, an input matching network, a device, an output matching netowork, and a load (in that order), it states that "Maximum unilateral transducer gain can be accomplished by choosing impedance matching networks such that Gamma s = S11* and Gamma L = S22*" (where Gammas are the reflection coefficients of the input matching network and the output matching network).

I understood that maximun power transfer was when gamma = 0 (no wave reflected). So what am I missing?

Any insight is welcome

 

[BigBoss]

No reflection occurs when that equation above is satisfied that doesn NOT mean Gamma=0
This comes from "maximum transfer is available under conjugate matching" principle.

But in transmission lines, if the transmission line has purely characteristic impedance, input relection will be zero meaning gamma=0.
I guess you confuse two phenomena..

 

[PeterMartin]

Yes Bigboss is correct but I'd add that it depends on just where the reflection coefficients are being measured.

This diagram may help:
and there is a matching network calculator on this page:
**broken link removed**

 

[pepewaw]

Thanks!, think I get it now