Loss parameter in RF MEMS Switches

hi,

I have started reading about RF MEMS switches. I was wondering if anyone could help me regarding Loss parameter.
Loss is defined by Loss = 1 - (S11)^2 - (S21)^2

I understand how to measure S21. but How do I measure S11 in a MEMS Switch?

Thanks

with the VNA "vector Network Analyzer" as u measure the S21 , make it measure the S11 , under to conditions , when the switch in the two different positions

khouly