# Pi Network & S - parameters

Discussion in 'Electronic Basics' started by [email protected], Dec 16, 2004.

1. ### Guest

Hi,

I am new in the RF field and I have a question regarding Pi Networks
and S parameters.

I was recently reading an article about Pi networks and it uses in RF.
I am confused about a couple of things. The article said the k=1/S21,
where k is the attenuation constant. How do you derive this relation
between k and S21. I know that k=10^(A/20), where A is the value of
attenuation, does it have something to do with this?

My second question is, the article says that the load reflection
coefficient gammaL=1/k^2. How do I derive this?

Thanks,
Nam

2. ### John WoodgateGuest

I read in sci.electronics.design that wrote (in
No, that equation converts between A in decibels and k as a pure number.

S21 is the forward 'gain' (a number less than 1 for a passive network),
so the attenuation coefficient is obviously its reciprocal.
You will find the answer on the pages up to page 17 of the document:

http://www.sss-mag.com/pdf/an-95-1.pdf

Remember that in general s21 is a complex number (there is phase-shift
through the network) and that s21 = 1/k.  