I thought some of you might find this interesting.
I swept a FlightAware 1090MHz mode S filter across my VNA at work
S-parameters describe the input-output relationship between ports (or terminals) in an electrical system.
In general, Snm represents the power transferred from Port m to Port n in a multi-port network.
If we have 2 ports (or terminals), like a filter with two terminals, or a coax with two ends between antenna & receiver, generally called Port 1 and Port 2, then:
S12 represents the power transferred from Port 2 to Port 1.
S21 represents the power transferred from Port 1 to Port 2.
S11 represents how much power delivered to port 1 is reflected back to power source, and hence is known as the reflection coefficient or return loss (sometimes written as gamma Γ). If S11=0 dB, then all the power is reflected from the port 1 and nothing passes to other ports. If S11=-10 dB, this implies that if 3 dB of power is delivered to the port 1, -7 dB is the reflected power. The remainder of the power was “accepted by” or delivered to the port 2.
If anyone is interested in digging into the data, I am including a link to the s2p files