##  [Return Loss](/return-loss-0) 

 Definition

Return loss (RL) is a port-level measure, expressed in decibels, of the ratio between incident and reflected power caused by an impedance discontinuity; equivalently RL(dB) = -20·log10|Γ| = -10·log10(Pr/Pi), where Γ is the complex voltage reflection coefficient and Pi and Pr are incident and reflected powers at the same reference plane.

 

 

 

 

 

 





## Principle

Principle

Return loss quantifies the fraction of power reflected back toward the source: larger positive RL (dB) corresponds to smaller |Γ| and less reflected power; RL depends on frequency and the chosen reference impedance.

 

 

 

 

 





## Demonstration

Demonstration

Illustrative scenario: A 50 Ω feedline terminates on a load with |Γ|=0.1. Compute RL = -20·log10(0.1) = 20 dB. Recognition: measure at the port and observe a 20 dB return loss. Action: accept or improve the match depending on system requirements. Consequence: with RL=20 dB most power is delivered to the load and reflected power is 1% of incident.

 

 

 

 

## Misapplication

Misapplication

Treating RL sign or units incorrectly—for example, interpreting a numerically large negative dB value as better match—or conflating RL with insertion loss. Equivalently, comparing RL values measured at different reference planes or impedances without transformation produces misleading conclusions.

 

 

 

 

 





## Consequence

Consequence

A low RL (small dB value) indicates significant reflected power, leading to standing waves, reduced delivered power, possible transmitter stress, and increased measurement uncertainty; a high RL indicates a good port match but does not alone guarantee delivered power if other losses exist.

 

 

 

 

## Reversal

Reversal

When the transmission line between measurement plane and load is lossy or contains reactive elements, input-measured RL can be larger (appear better) than the actual load reflection; in active or non‑linear ports RL loses meaning if the device generates energy at the measurement frequency.

 

 

 

 

 





## Boundary

Boundary

Clearly within: RL measured at a defined coaxial connector referenced to the system impedance. Boundary case: RL measured at transmitter input on a lossy long cable versus at the load end—values differ and require transformation. Clearly outside: insertion loss (power lost in transmission) is not RL.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Return Loss ↔ VSWR: both quantify mismatch but use different numerical scales and sensitivities; choice depends on diagnostic or specification practice.

 

 

 

 

 





## Synthesis

Synthesis

Return loss provides a logarithmic, port-local scalar of reflected power that is convenient for specification and measurement, but its interpretation requires explicit reference plane, frequency, and knowledge of intervening loss or active behavior.