Is return loss positive or negative?

This is a source of endless confusion. Strictly defined, return loss is a positive quantity: RL = -20 log10(|gamma|). A perfect match has infinite return loss (no reflected power). A total reflection has 0 dB return loss. However, many engineers and instruments display S11 in dB, which is negative (S11 = 20 log10(|gamma|) = -RL). Some datasheets label this negative number as return loss. When someone says 'return loss is -15 dB' they almost certainly mean S11 = -15 dB, which is a return loss of 15 dB. Always check the sign convention when comparing specifications from different sources.

What return loss is good enough?

It depends on the application. For precision calibration standards: better than 40 dB. For VNA calibration loads: 30 to 40 dB. For amplifier input/output ports: 15 to 20 dB is typical. For antennas at the operating frequency: 10 to 15 dB is standard (corresponding to VSWR 1.5 to 2.0). For filter passbands: 15 to 25 dB. The minimum acceptable return loss also depends on what follows the mismatch: if the reflected signal passes through a 20 dB gain amplifier and reflects again from another mismatch, the resulting standing wave can cause significant gain and phase ripple.

How does return loss relate to insertion loss?

For a lossless device (like an ideal matching network or short cable), all power that is not reflected is transmitted: IL = -10 log10(1 - |gamma|^2). At 20 dB return loss, |gamma| = 0.1, so mismatch loss = -10 log10(1 - 0.01) = 0.044 dB. At 10 dB RL, |gamma| = 0.316, mismatch loss = 0.458 dB. For a lossy device (real cable, attenuator), the insertion loss includes both mismatch loss and dissipative loss. You cannot separate them from a simple S21 measurement alone; you need both S11 and S21 to distinguish how much power is reflected versus absorbed.

RF Design

Return Loss

Return loss answers the simplest and most important question in RF: how good is the impedance match? It is the ratio of power going into a port to the power bouncing back, expressed in decibels. A 20 dB return loss means 1% of the power reflects; the match is good. A 6 dB return loss means 25% bounces back; the match is poor. The number is always positive (despite widespread confusion with the negative S₁₁ display on network analyzers), and higher is always better. It is the single most commonly specified parameter on any RF datasheet, connector, cable assembly, or antenna.

Category: RF Design

Formula: RL = −20·log(|Γ|)

Perfect Match: ∞ dB

The Number That Defines Every Interface

Return Loss Requirements by Application

Application	RL Required	\|Γ\|	VSWR	Mismatch Loss	Why
Calibration standard	≥40 dB	≤0.01	≤1.02	≤0.001 dB	Reference accuracy
VNA port / test cable	≥30 dB	≤0.032	≤1.065	≤0.004 dB	Measurement uncertainty
Amplifier I/O	15 to 20 dB	0.1 to 0.18	1.2 to 1.4	0.04 to 0.14 dB	Gain flatness, stability
Filter passband	15 to 25 dB	0.06 to 0.18	1.1 to 1.4	0.01 to 0.14 dB	Low insertion loss ripple
Antenna (at resonance)	10 to 15 dB	0.18 to 0.32	1.4 to 2.0	0.14 to 0.46 dB	Efficiency, PA protection
Broadband termination	≥20 dB	≤0.1	≤1.2	≤0.04 dB	System isolation

Conversions:
RL = −20·log₁₀(|Γ|) = −S₁₁(dB)
|Γ| = 10^−RL/20
VSWR = (1 + |Γ|) / (1 − |Γ|)
Mismatch Loss = −10·log₁₀(1 − |Γ|²)

The sign convention trap:
RL (true definition): positive number, higher = better match
S₁₁: negative number, more negative = better match
RL = −S₁₁
When a datasheet says "return loss −15 dB" they almost always mean S₁₁ = −15 dB, i.e., RL = 15 dB.

Common Questions

Frequently Asked Questions

Positive or negative?

True return loss is positive (RL = −20 log|Γ|). S₁₁ is the negative number shown on VNAs. RL = −S₁₁. "Return loss of −15 dB" almost always means S₁₁ = −15 dB, i.e., 15 dB return loss. Always check the sign convention.

What value is good enough?

Calibration: ≥40 dB. Test equipment: ≥30 dB. Amplifiers: 15 to 20 dB. Antennas: 10 to 15 dB. Higher is always better. The required value depends on the sensitivity of the downstream system to reflections.

RL vs. insertion loss?

For lossless devices, mismatch loss = −10 log(1 − |Γ|²). At 20 dB RL: 0.04 dB. At 10 dB RL: 0.46 dB. For lossy devices, total IL includes both mismatch and dissipative loss; you need S₁₁ and S₂₁ to separate them.

Related Terms

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Quick Reference

Return Loss Conversion Chart

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