Microwave

VSWR, or Voltage Standing Wave Ratio.
When a transmission line (cable) is terminated by an impedance that does not match the characteristic
impedance of the transmission line, not all of the power is absorbed by the termination. Part of the power
is reflected back down the transmission line. The forward (or incident) signal mixes with the reverse (or
reflected) signal to cause a voltagestanding wave pattern on the transmission line. The ratio of the
maximum to minimum voltage is known as VSWR, or Voltage Standing Wave Ratio.
A VSWR of 1:1 means that there is no power being reflected back to the source. This is an ideal situation
that rarely, if ever, is seen. In the real world, a VSWR of 1.2:1 (or simply 1.2) is considered excellent in
most cases. In an EMC lab where many of thetests are very broadband in nature, a VSWR of 2.0 or
higher is not uncommon. At a VSWR of 2.0, approximately 10% of the power is reflected back to the
source. Not only does a high VSWR mean that power is being wasted, the reflected power can cause
problems such as heating cables or causing amplifiers to fold-back.
There are ways to improve the VSWR of a system. One way is to use impedancematching devices
where a change in impedance occurs. Baluns are used extensively in antennas to not only convert from
balanced to unbalanced signals but also to match the impedance of the source to the antenna. It is
common practice in EMC testing to include attenuators at any point where there is an impedance
mismatch. One emissions standard, for instance, specifies using an attenuator at theconnector of a
biconical antenna since it has a high VSWR at some frequencies. One of the conducted immunity
standards suggests using a 6dB pad at the input of the coupling device, which is commonly 150 ohms.
Attenuators obviously cause power loss, but they reduce VSWR by providing an apparently better
termination to a signal.
For example, lets look at a 6dB attenuator and its affect on circuitimpedance. Following is a schematic
for a 50 ohm 6dB attenuator:

If a 50 ohm termination is added to the output of this attenuator, the source will see a 50 ohm load. Two
extremes for terminating a transmission line are open and short circuits. In a completely open circuit, the
impedance would be infinite. Adding this 6dB pad to the output of a signal source, without terminating the
outputof the attenuator, would cause the source to see an 84 ohm termination (17 ohms in series with
67 ohms). Shorting the output of the attenuator would cause the signal source to see a 30.5 ohm
termination. In each case, the VSWR would be approximately 1.65:1. (The math will be covered later).
There are various ways of measuring and/or calculating VSWR. In the old days of open transmission
lines,the voltage could be measured along the length of the line until the maximum and minimum values
were found (which were ¼ wavelength apart) hence the reference to Voltage Standing Wave Ratio.
Thus, VSWR would be calculated by the following formula:
VSWR =

Emax
Ei + Er
=
Emin
Ei − Er

Where Emax = maximum measured voltage
Emin = minimum measured voltage
Ei = incident wave amplitude,volts
Er = reflected wave amplitude, volts
VSWR.DOC

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With the use of coax cables, measuring voltage along the cable is impractical. Dual-directional couplers
can be used to measure the forward and reverse power, and these values can then be used to compute
VSWR.

Prev
Pfwd
VSWR =
Prev
1−
Pfwd
1+

Where Prev = reverse power
Pfwd = forward power
VSWR can also berepresented other ways, such as Return Loss, Mismatch Loss and Reflection
Coefficient. Reflection Coefficient is common, can be calculated several ways, and ultimately used to
calculate VSWR. Here are some formulae for determining Reflection Coefficient (Ψ):
ρ=

Er
Where Er = reflected voltage, Ei = incident voltage
Ei

ρ=

Z1 - Z2
Where Z1 and Z2 are the mismatched impedances in ohms
Z1...