S-Parameters PCB Interconnect: Reading Return and Insertion Loss

Author: Vivi Peng | Published for icallin.com | Updated: October 21, 2026

Measuring an S-parameters PCB interconnect link describes how much of an incident wave is reflected at each port and how much is transmitted through. For a trace, connector launch or filter, they answer two questions: how much signal returns toward the source, and how much reaches the far end. Every value depends on frequency, port definitions and reference impedance.

A two-port network model is the starting point for a single-ended interconnect. Its four complex quantities, S11, S21, S12 and S22, each carry magnitude and phase. Read them together rather than choosing a board geometry from a single transmission curve.

Why waves help describe a PCB link

Copper is still a good conductor at radio frequencies. What changes is that a long enough interconnect behaves as a distributed network of resistance, inductance and capacitance, so propagation delay and reflections become significant. Electrical length and edge rate decide when this matters, together with the operating frequency.

A wave description characterizes that distributed network from its ports, without treating the trace as one lumped connection. A vector network analyzer (VNA) measures the ratios between incident and outgoing waves across a frequency sweep.

In Keysight's S-parameter notation, the first subscript is the receiving port and the second is the excited port. S21 is therefore the response at port 2 when port 1 is driven.

Read the four parameters under their defined conditions

Let a1 and a2 be the waves traveling into the network at ports 1 and 2, and b1 and b2 the waves leaving it. With real reference impedances, the waves can be normalized so that their squared magnitudes represent power.

Parameter	Defined wave ratio	Practical interpretation
S11	b1/a1 with a2 = 0	Reflection seen from port 1
S21	b2/a1 with a2 = 0	Transmission from port 1 to port 2
S12	b1/a2 with a1 = 0	Transmission from port 2 to port 1
S22	b2/a2 with a1 = 0	Reflection seen from port 2

Table 1. Each S-parameter is defined with the other port terminated in its reference impedance.

An incident wave of zero at the other port means that port is terminated in its reference impedance, typically 50 Ω. It is matched, not shorted. A real load that reflects energy changes the behavior of the connected system.

Figure 1. The four parameters describe separate excitation conditions. The arrows identify wave direction, rather than simultaneous test stimuli.

For a passive PCB link, S21 gives the forward attenuation and phase shift, and S12 the reverse. A passive interconnect is reciprocal, so S21 equals S12 under consistent definitions. An amplifier is not reciprocal. Reflections at the two ends, S11 and S22, can differ when the structure is asymmetric.

S11 return loss and S21 insertion loss in decibels

When unexpected dips in insertion loss occur, trace the physical geometry using high frequency PCB routing problems diagnostics.

Analyzers commonly plot S11 as log magnitude:

S11 in dB = 20 log10(|S11|)

Return loss uses the opposite sign:

Return loss in dB = −20 log10(|S11|)

An S11 of −20 dB is therefore a reflection magnitude of 0.1 and a return loss of 20 dB. With matched ports, reflected power is 0.1², or 1% of incident power. The remaining 99% is not all delivered: conductor and dielectric loss absorb part of it. S21 insertion loss follows the same arithmetic. S21 = −3 dB is a wave-amplitude ratio of about 0.708 and a power ratio of about 0.501.

dB value	Linear magnitude	Power ratio	Typical reading
S11 = −10 dB	0.316	10% reflected	Noticeable mismatch
S11 = −15 dB	0.178	3.2% reflected	Moderate match
S11 = −20 dB	0.100	1% reflected	Good match for many links
S11 = −30 dB	0.032	0.1% reflected	Very good match
S21 = −1 dB	0.891	79% transmitted	Low insertion loss
S21 = −3 dB	0.708	50% transmitted	Half-power point
S21 = −6 dB	0.501	25% transmitted	High insertion loss

Table 2. Decibel-to-linear conversion under matched reference conditions; acceptance limits come from the interface specification.

A dB value converts to a voltage ratio only when the reference impedances at both ports are the same, so confirm that before quoting gain or loss as a voltage.

Turn a plot into a design decision

Start from the budget. Define the useful frequency range and the system's loss, reflection and timing budgets, then assess the complete curve across that range. A narrow resonance can matter more than the value at one marker frequency, and phase carries delay information that magnitude hides.

Know what the measurement includes. Calibration sets the VNA reference planes. Connectors, launches and fixtures beyond those planes remain part of the measured network unless they are de-embedded. Comparing a simulated bare trace with a measured trace plus two launches can lead to the wrong material decision.

Document Touchstone port mapping. When exchanging a .s2p file, record the port map, reference impedance, frequency range and any de-embedded fixtures. Differential channels need a four-port (.s4p) model and mixed-mode parameters such as SDD21 and SCD21; a single-ended S21 curve does not show mode conversion.

Cascade with full network math. Evaluate matching networks or cascaded sections with a calculation that keeps reflections and phase, such as T-parameter or ABCD cascading.

Frequently Asked Questions

Why does the analyzer show negative S11 when return loss is positive?

The two quantities use opposite signs in their logarithmic definitions. An S11 log magnitude of −20 dB is a return loss of 20 dB. Check the selected display format before comparing against a specification.

Can I multiply the S21 values of two PCB sections?

Only when both sections are well matched, so that reflections between them are negligible. Otherwise waves bounce between the sections, and a full network cascade using the complete S-parameter models is required.

What return loss is good enough for a PCB trace?

It depends on the interface. Many designs treat S11 below −10 dB across the band as a minimum and aim for −15 to −20 dB on critical links. Use the limit mask in the interface standard as the acceptance criterion.

Conclusion

A useful S-parameter review ties each curve to its ports, reference planes and acceptance budget. That makes it possible to separate a launch mismatch from transmission loss before revising the PCB.

Written by Vivi Peng for icallin.com.