RF Survival Guide — Part 2

S-Parameters Demystified

Generic input reflection coefficient measurements for LoRa antennas.

Welcome back, RF hobbyists!
In Part 1 of the RF Survival Guide, we explored the chaotic beauty of RF jargon. Today, we dive into a deceptively simple yet powerful tool in any RF engineer’s toolbox: scattering parameters.

If you’ve ever used a VNA (Vector Network Analyzer) spitting out a bunch of curves labeled S11, S21, and thought, “this must be sorcery,” you’re not alone! But fear not — by the end of this guide, you’ll see that S-parameters are just a clever way of describing how RF components behave.

What Are S-Parameters?

S-parameters, or scattering parameters, tell you how signals scatter when they hit a device. In plain terms:

• How much of each input signal gets reflected back?
• How much of each input signal gets transmitted through to each output?

For a 2-port device (like an amplifier, filter, or cable), you typically see:

• S11: Input reflection coefficient at port 1 (input return loss);
• S21: Transmission coefficient from port 1 to port 2 (forward gain/forward loss/isolation)
• S12 : Transmission coefficient from port 2 to port 1 (reverse gain/reverse loss/reverse isolation)