6-WHAT IS THE IMPORTANCE OF VSWR IN TELECOM? DESCRIBE WITH EXAMPLE

6-WHAT IS THE IMPORTANCE OF VSWR IN TELECOM? DESCRIBE WITH
EXAMPLE

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6-WHAT IS THE IMPORTANCE OF VSWR IN TELECOM? DESCRIBE WITH EXAMPLE ..

Answer / ashu

For a radio (transmitter or receiver) to deliver power to
an antenna, the impedance of the radio and transmission
line must be well matched to the antenna's impedance. The
parameter VSWR is a measure that numerically describes how
well the antenna is impedance matched to the radio or
transmission line it is connected to.

VSWR stands for Voltage Standing Wave Ratio, and is also
referred to as Standing Wave Ratio (SWR). VSWR is a
function of the reflection coefficient, which describes the
power reflected from the antenna. If the reflection
coefficient is given by , then the VSWR is defined by the
following formula:

The reflection coefficient is also known as s11 or return
loss. See the vswr table below to see a numerical mapping
between reflected power, s11 and VSWR. If you don't want to
go through complicated equations to understand the
relationship between VSWR, mismatch loss, s11/gamma and
would like a calculator to do it for you, check out our
VSWR calculator page and we'll do the VSWR conversion for
you.

The VSWR is always a real and positive number for antennas.
The smaller the VSWR is, the better the antenna is matched
to the transmission line and the more power is delivered to
the antenna. The minimum VSWR is 1.0. In this case, no
power is reflected from the antenna, which is ideal.

Often antennas must satisfy a bandwidth requirement that is
given in terms of VSWR. For instance, an antenna might
claim to operate from 100-200 MHz with VSWR<3. This implies
that the VSWR is less than 3.0 over the specified frequency
range. This VSWR specifications also imples that the
reflection coefficient is less than 0.5 (i.e., <0.5) over
the quoted frequency range.

Physical Meaning of VSWR
VSWR is determined from the voltage measured along a
transmission line leading to an antenna. VSWR is the ratio
of the peak amplitude of a standing wave to the minimum
amplitude of a standing wave, as seen in the following
Figure:

Figure 1. Voltage Measured Along a Transmission Line.
In industry, VSWR is sometimes pronounced "viz-wer".

When an antenna is not matched to the receiver, power is
reflected (so that the reflection coefficient, , is not
zero). This causes a "reflected voltage wave", which
creates standing waves along the transmission line. The
result are the peaks and valleys as seen in Figure 1. If
the VSWR = 1.0, there would be no reflected power and the
voltage would have a constant magnitude along the
transmission line.

VSWR, Reflected Power, and s11
Is a VSWR of 3 bad? How bad is a VSWR of 12? Well, there
are no hard rules. In this section, we'll try to put the
VSWR number in context. Below is a table showing the
relationship between VSWR, total reflected power, and
(also known as s11), and total reflected power. Note that
the reflected power is simply the reflection coefficient ()
squared.

Table I. VSWR, Reflected Power, and (s11)
VSWR (s11) Reflected Power (%) Reflected Power (dB)
1.0 0.000 0.00 -Infinity
1.5 0.200 4.0 -14.0
2.0 0.333 11.1 -9.55
2.5 0.429 18.4 -7.36
3.0 0.500 25.0 -6.00
3.5 0.556 30.9 -5.10
4.0 0.600 36.0 -4.44
5.0 0.667 44.0 -3.52
6.0 0.714 51.0 -2.92
7.0 0.750 56.3 -2.50
8.0 0.778 60.5 -2.18
9.0 0.800 64.0 -1.94
10.0 0.818 66.9 -1.74
15.0 0.875 76.6 -1.16
20.0 0.905 81.9 -0.87
50.0 0.961 92.3 -0.35

In the above table, a VSWR of 4 has 36% of power delivered
by the receiver reflected from the antenna (64% of the
power is delivered to the antenna). Note that a reflected
power of 0 dB indicates all of the power is reflected
(100%), whereas -10 dB indicates 10% of the power is
reflected. If all the power is reflected, the VSWR would be
infinite.

Note that VSWR is a highly non-linear function of the
reflection coefficient . That is, there is very little
difference in reflected power when the VSWR increases from
9 to 10; however there is an 11% change in reflected power
when the VSWR changes from 1 to 2.

In general, if the VSWR is under 2 the antenna match is
considered very good and little would be gained by
impedance matching. As the VSWR increases, there are 2 main
negatives. The first is obvious: more power is reflected
from the antenna and therefore not transmitted. However,
another problem arises. As VSWR increases, more power is
reflected to the radio, which is transmitting. Large
amounts of reflected power can damage the radio. In
addition, radios have trouble transmitting the correct
information bits when the antenna is poorly matched (this
is numerically defined in terms of another metric, EVM -
Error Vector Magnitude).