Pi Network & S - parameters

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