Return Loss (RL)
Return Loss (RL) quantifies how much power is reflected back from a discontinuity in a transmission line, such as an impedance mismatch. It is expressed in decibels (dB) and indicates the ratio between the incident and reflected power.
A higher Return Loss implies better impedance matching and more efficient power transfer. Return Loss is always expressed as a positive value in dB, as it indicates power loss due to reflection.
Calculation of Return Loss
Return Loss is defined as:
RL = –20 × log₁₀|Γ|
where:
RL = return loss (dB)
Γ = reflection coefficient (unitless)
The reflection coefficient Γ is calculated by:
Γ = (Z_L – Z_0) / (Z_L + Z_0)
where:
Z_L = load impedance (Ω)
Z_0 = characteristic impedance of the transmission line (Ω)
Alternatively, RL can be defined based on power measurements as:
RL = 10 × log₁₀(Pᵢ / Pᵣ)
where:
Pᵢ = incident power
Pᵣ = reflected power
Relationship to VSWR
Return Loss and Voltage Standing Wave Ratio (VSWR) are directly related:
RL = 20 × log₁₀[(VSWR + 1) / (VSWR – 1)]
A low RL corresponds to a high VSWR, indicating poor impedance matching. High RL values (e.g., >20 dB) correspond to low reflection and good match conditions.
Example
A Return Loss of 20 dB means that only 1 % of the incident power is reflected. This corresponds to a VSWR of approximately 1.22:1, which is typical for well-matched RF systems.
Industry Standards
Typical Return Loss thresholds by application:
>10 dB: General RF systems
>15 dB: Commercial telecommunications
>20 dB: High-precision lab and test setups
These limits help ensure efficient signal transmission and system stability.
Applications
Return Loss is a critical parameter in:
Antenna measurements
Cable and connector evaluation
Filter and matching network design
Compliance testing for RF systems
Return Loss is also relevant in optical fiber systems and digital networks, where reflections can degrade signal quality.
Compared to VSWR, Return Loss offers higher resolution for small reflections due to its logarithmic scale, making it the preferred metric in professional RF and EMC measurements.