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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