RF concepts

The ideas behind RF work, such as why 50 ohm, what S-parameters mean and how matching works. Expert detail first, then a plain-language version.

• Return Loss vs Insertion Loss: What Each Number Tells You

  Return loss, from S11, tells you how much power bounces back. Insertion loss, from S21, tells you how much fails to arrive. A 10 dB return loss reflects 10 % of the power, yet a matched 10 dB attenuator still has 10 dB of insertion loss.

  Updated 2026-10-13

• What S21 and S11 of a Low-Pass Filter Look Like

  At the -3 dB cutoff a Butterworth low-pass filter transmits half the power and reflects half, so S21 and S11 are both -3.01 dB. Above the cutoff S21 falls and S11 climbs towards 0 dB, because a lossless filter reflects what it cannot pass.

  Updated 2026-10-13