What is a VSWR, reflection coefficient and reflection and forward power?

This article explains the definitions of basic terms such as VSWR, reflection coefficient and reflection and forward power and how to express and convert it using other physical values such as impedance, voltage, power and other. This therms are widely use in wireless and RF technology.

What is the definition of VSWR?

VSWR is an abbreviation for Voltage Standing Wave Ratio or sometimes in literature just SWR (Standing Wave Ratio). The value of VSWR presents the power reflected from the load to the source. It is often used to describe how much power is lost from the source (usually a High Frequency Amplifier) through a transmission line (usually a coaxial cable) to the load (usually an antenna).

How to express VSWR using voltage?

By the definition, VSWR is the ratio of the highest voltage (the maximum amplitude of the standing wave) to the lowest voltage (the minimum amplitude of the standing wave) anywhere between source and load.

VSWR = |V(max)| / |V(min)|

V(max) = the maximum amplitude of the standing wave
V(min) = the minimum amplitude of the standing wave

What is the ideal value of a VSWR?

The value of an ideal VSWR is 1:1 or shortly expressed as 1. In this case the reflected power from the load to the source is zero.

How to express VSWR using an impedance?

By the definition, VSWR is the ratio of the load impedance and source impedance.

VSWR = ZL/Zo

ZL = the load impedance
Zo = the source impedance

What is a reflection coefficient or reflection parameter or s11 parameter?

A reflection coefficient, sometimes called reflection parameter, defines how much energy is reflected from the load to the source of the RF systems. A reflection coefficient is also known as s11 parameter. By definition, a reflected coefficient is a ration of the reflected wave and the incident wave of the electric field strength. In the literature it is presented with the capital Greek letter gamma (Γ).

How to express a VSWR using reflection and forward power?

By the definition VSWR is equal to

VSWR = 1 + √(Pr/Pf) / 1 – √(Pr/Pf)

where:

Pr = Reflected power
Pf = Forward power

Conversion table – dBm to dBW and W (watt)

In this table we present how the value of power in dBm, dBW and Watt (W) corresponding to each other.

Power (dBm) Power (dBW) Power ((W)watt)
100 70 10 MW
90 60 1 MW
80 50 100 KW
70 40 10 KW
60 30 1 KW
50 20 100 W
40 10 10 W
30 0 1 W
20 -10 100 mW
10 -20 10 mW
0 -30 1 mW
-10 -40 100 μW
-20 -50 10 μW
-30 -60 1 μW
-40 -70 100 nW
-50 -80 10 nW
-60 -90 1 nW
-70 -100 100 pW
-80 -110 10 pW
-90 -120 1 pW
-100 -130 0.1 pW
-∞ -∞ 0 W

where:
dBm = decibel-milliwatt
dBW = decibel-watt
MW = megawatt
KW = kilowatt
W = watt
mW = milliwatt
μW = microwatt
nW = nanowatt
pW = picowatt

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