Lecture # 08 a: Standing Waves
and VSWR
Course: Microwave Engineering
Instructor: Engr. Zuneera Aziz
Excerpts from the sides of: Engr. Umair Mujtaba Qureshi
Introduction
• In free space, wave travels with velocity approx.
3 X 10^8 m/s.
• However, in a medium other than free space, it
gets reduced by the factor of 1/∈𝑟.
• Thus,
Standing Waves
• The ideal condition is that the load impedance
should be equal to characteristic impedance for
maximum power transfer
• However, this is not always the case
• Often a part of the incident wave is reflected
back on the transmission line
• The two waves travelling in the transmission line
form an interference pattern, called Standing
Waves
Standing Waves
• As the incident and reflected wave pass through
each other, there’s a pattern formed on the line
and that is the standing wave.
• The standing wave varies in amplitude only on a
fixed position
• The two traveling waves set up an interference
pattern known as a Standing Wave
• These stationary waves are called standing
waves because they appear to remain in a fixed
position on the line, varying in amplitude only.
Standing Wave Ratio (SWR)
• Ratio of maximum voltage to the minimum
voltage or the maximum current to the
minimum current of a standing wave on
transmission line.
• Often called the Voltage Standing Wave Ratio
(VSWR)
Standing Wave Ratio (SWR)
• Put Value of Vmax and Vmin in the SWR
equation
Standing Wave Ratio (SWR)
• Results:
▫ WORST CASE: If there’s a total MISMATCH
(i.e. reflected wave and incident wave amplitudes
are equal), then SWR = infinity
▫ BEST CASE: If there’s a complete MATCH (i.e.
ZL=Zo, that means Er=0), then SWR = 1
SWR – In terms of Reflection
Coefficient
Reflection Coefficient in terms of SWR
Standing Wave Ratio (Contd.)
• Homework:
• Derive this equation
• (Whichever gives an SWR greater than 1 is valid)
• For first case, Zo<ZL and for second case the vice
versa is true