Insertion Loss from S21

Formula

Description

Insertion loss quantifies how much signal power is lost when a component (filter, cable, connector, PCB trace) is inserted between a source and load. It is derived from the S-parameter S₂₁, which represents the forward transmission coefficient. |S₂₁| = 1 means no loss (IL = 0 dB), |S₂₁| = 0.5 means 6 dB loss (half the voltage, quarter the power). Insertion loss is positive by convention (a "loss"). It is the primary specification for evaluating cables, connectors, PCB vias, and filter passband performance.

Variables

• IL — Insertion loss (dB, positive value)
• |S₂₁| — Forward transmission coefficient magnitude (0 to 1)

Practical Notes

Typical insertion losses: SMA connector < 0.03 dB, 1 m RG-58 coax at 1 GHz ≈ 0.7 dB, FR-4 via (through board) at 5 GHz ≈ 0.5-1 dB, PCB SMA launch ≈ 0.1-0.3 dB. For passive components, |S₂₁| can never exceed 1 (0 dB). For amplifiers, |S₂₁| > 1 represents gain, and IL becomes negative (i.e., there is actually gain, not loss).