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Engineering Notes


RF POWER AMPLIFIERS TECHNICAL REFERENCE MATERIAL


VSWR & Return Loss (Impedance)


Impedance = Z = Ei/Ii

50 Ohm or 75 Ohm is normally used as the amplifier interface to other equipment where: Ei and Ii are the incident voltage and current.

VSWR = E max / E min = (1 + |P|) / (1 - |P|)
where P = reflection coefficient = (Z - Z0) / (Z + Z0)

Return Loss in dB = 20 log |P|

COMMON TRANSFORMATIONS
VSWR RETURN
LOSS
POWER
REFLECTED
POWER
TRANSMITTED
REFLECTION
COEFFICIENT
1.0
1.1
1.25
1.5
1.75
2.0
3.0
6.0
-
-
26.4 dB
19.1 dB
14.0 dB
11.3 dB
9.5 dB
6.0 dB
2.9 dB
0 dB
0 %
0.2 %
1.2 %
4.0 %
7.4 %
11.1 %
25.0 %
51.0 %
100 %
100 %
99.8 %
98.8 %
96.0 %
92.6 %
88.9 %
75.0 %
49.0 %
0 %
0.00
0.05
0.11
0.20
0.27
0.33
0.50
0.71
1.00

Notes:

1. A purely capacitive or inductive load has an infinite VSWR.
2. VSWR = ZL / ZO or ZO / ZL for resistive loads only.



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