Practical_Antenna_Handbook_0071639586

C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 135
Example 4.6 A transmission line is connected to a mismatched load. Calculate both the
VSWR and the VSWR decibel equivalent if the reflection coefficient G is 0.25.
Solution
G
( a) VSWR = 1+
1– G
1+ 0.25
=
1– 0.25
= 1.25
0.75
= 1.67 :1
( b)VSWR = 20 log (VSWR)
dB
= (20) (log 1.67)
= (20)(
0.22)
= 4.4 dB
SWR on a transmission line is important for several reasons.
• The higher the SWR, the higher the maximum voltages on a transmission line
for a given power applied or delivered to the load. High SWR can cause
unexpected destruction of the line and any electronics equipment attached to it.
• Many of today’s solid-state transmitters, transceivers, and amplifiers reduce RF
output or shut down completely in the presence of high SWR.
• The reflected wave on a transmission line represents energy “rejected” by the
load. For a specified power delivered to the antenna, all components must be
“supersized” for a line with a high SWR.
• A transmission line operating with high SWR will exhibit greater loss per unit
length than its specified loss for a matched load impedance.
Mismatch (VSWR) Losses
The power reflected from a mismatched load represents a potential loss and can have
profound implications, depending on the installation. For example, one result might be
a slight loss of signal strength at a distant point from an antenna. A more serious problem
can result in the destruction of components in the output stage of a transmitter. The
latter problem so plagued early solid-state transmitters that designers opted to include
shutdown circuitry to sense high VSWR and turn down output power proportionately—a
practice that remains to this day.
In microwave measurements, VSWR on the transmission lines (that interconnect
devices under test, instruments, and signal sources) can cause erroneous readings—
and invalid measurements.