反射损耗、反射系数和VSWR

三者的定义、比较和关系

From April 2008 High Frequency ElectronicsCopyright © 2008 Summit Technical Media LLC

DESIGN NOTES

Return Loss, Reflection Coefficient and VSWR

Here is a brief review of three related RF trans-mission parameters,along with a reference table oftheir values.

At high frequencies,the actual voltages,currentsand phase relationships that define an impedance aredifficult to measure directly.Instead,we use steady-state measurements of the traveling waves in trans-mission lines,which can be separated by a directionalcoupler into those traveling in “forward”and “reverse”(or “reflected”) directions.

Return loss,reflection coefficientand voltage stand-ing wave ratioare different numerical values for theseradio frquncy transmission masurmnts.Although they are related mathematically,each refersto a different means of measurement or analysis.

Reflection coefficient (Γ)is the fraction of a forwardtraveling wave that is reflected from a mismatchedload,expressed in polar coordinates (|Γ|(θ).The mag-nitude has no units,since it is a ratio,eeeand is alsoee

referred to as rho (ρ).The phase angle may be given as0 to 360 degrees or +180 to –180 degrees,depending onthe analytical system that will use the data.

Return loss (RL) is the ratio of the magnitude of theforward and reverse traveling waves,squared to rep-resent power and converted to dB.Thus,it is related toreflection coefficient by

RL = 10log(|Γ|2)

Voltage standing wave ratio (VSWR) dates to thetime when slotted lines were the primary means oftransmission line measurements.VSWR is simply theratio of the standing wave maxima and minima,asmeasured by moving the probe along the line.Being avoltage measurement,it is related to Γby

VSWR =

Vfwd+ Vref1+|Γ|Vmax==Vfwd– Vref Vmin1–|Γ|

VSWR is typically used with installed systems

such as transmitter and antenna installations.RL andΓ are more common in laboratory practice.

The accompanying table is a handy reference toequivalent values of RL,VSWR and |Γ| over a rangeof RL values from 1 to 60 dB.

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