Learn what S11 and S21 mean, how return loss and insertion loss relate to them, and how to read RF filter plots without confusing reflection, transmission, and gain.
Reviewed against public product information on 2026-07-29.
S11
input reflection
S21
forward transmission
Typical unit
dB vs frequency
Reference
defined port impedance
What an S-parameter describes
An S-parameter describes how a traveling RF wave is reflected or transmitted at a network port. For a two-port device, the first index identifies the receiving port and the second identifies the excited port. S11 therefore measures the wave returning to port 1 when port 1 is excited, while S21 measures the wave reaching port 2 from that excitation. The values only make sense with the reference impedance and frequency sweep stated.
S11 is a reflection coefficient at the input port
S21 is forward transmission from port 1 to port 2
S12 describes reverse transmission
S22 describes reflection at the output port
How S11 becomes return loss
S11 is complex, so it contains magnitude and phase. Plots often show its magnitude in decibels as 20 log10(|S11|), which is normally zero or negative for a passive network. Return loss is commonly reported as the positive value -20 log10(|S11|). That sign convention causes many mistakes: an S11 trace at -20 dB corresponds to 20 dB return loss and about one percent reflected power.
More-negative S11 magnitude means less reflected power
S11 = -10 dB corresponds to 10 dB return loss
Return loss is not the same quantity as insertion loss
Always confirm whether a plot label shows S11 dB or positive return loss
How S21 relates to insertion loss and gain
S21 shows forward transmission. A passive filter with S21 near 0 dB in its passband transmits most available power, while a negative value indicates attenuation. Insertion loss is often written as -20 log10(|S21|), so an S21 magnitude of -1 dB corresponds to 1 dB insertion loss. An active network may produce positive S21 dB, which is forward gain rather than negative insertion loss.
Use S21 to locate passband and stopband behavior
A passive passband should be close to 0 dB, not above it
The cutoff definition depends on the filter specification
Positive S21 can be valid for an active device with gain
Read S11 and S21 together
A filter decision is stronger when transmission and reflection are inspected together. A falling S21 trace shows that less power reaches the output, but it does not by itself say whether power is reflected, dissipated, or redistributed by loss and mismatch. S11 reveals the input match at the same frequencies. Place markers on both traces at the passband edge, design frequency, cutoff target, and critical stopband points before changing components.
Check passband S21 and S11 at the same marker frequencies
Separate mismatch loss from intended stopband rejection
Compare the full sweep before and after tuning
Record reference impedance, sweep limits, and marker values
Primary sources
Verify the details
Technical definitions and competitor statements are grounded in the public documentation below. Product capabilities can change after the review date.
They describe the same reflection behavior with different sign conventions. S11 magnitude is often plotted as a negative dB value; return loss is usually reported as the corresponding positive value.
Is S21 the same as insertion loss?
For a passive two-port, insertion loss is commonly the negative of S21 magnitude in dB. For an active network, positive S21 represents forward gain, so the language must be chosen carefully.
Why is port impedance important?
The incident and reflected waves are defined relative to a reference impedance. Changing that reference changes the reported reflection coefficient and therefore S11.
Can a filter have good S21 and poor S11?
Yes. A response can show acceptable forward transmission at one frequency while the input match is worse than the design target. Both traces should be evaluated against the specification.
