3.3 Voltage Standing Wave Ratio (VSWR)
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3.3 Voltage Standing Wave Ratio (VSWR)
The figures in section 2.8 can help us understand the reflection coefficient a bit better.
Consider Figure 3.3-1 for instance. The quoted reflection coefficient, , has a modulus of 0.33. This
means that one-third of the incident voltage gets reflected and two-thirds are delivered to the load.
Figure 3.3-1 Incident, reflected and total voltages for a 50 Ω line terminated by a 100 Ω load
From Figure 3.3-1, it is also apparent that, instead of using the modulus of the reflection
coefficient as an indication of how much voltage gets through to the load, we could look at the
maximum and minimum amplitude of the envelope of the total voltage along the line (dashed
purple curve). This is commonly done and this measure, which represents the ratio of maximum and
minimum amplitude of the voltage standing wave, is called VSWR
Of course
the maximum voltage along the line will be observed when incident and reflected voltages
are in phase and add up (A+B), Figure 3.3-2, whereas the minimum voltage will be observed when
such voltages are 180° degrees out of phase (A-B), Figure 3.3-1.
Lastly, let us point out that, as is to be expected, there exist a clear relationship between the
modulus of the reflection coefficient and the VSWR
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Time to play with this on Microwave Office and watch video 3.2 which illustrates this further.
Conquer Radio Frequency
133 www.cadence.com/go/awr
