Transmission Lines
High Frequency
- At low frequencies, the circuit elements are lumped since voltage and current waves affect the entire circuit at the same time.
- At microwave frequencies, such treatment of circuit elements is not possible since voltage and current waves do not affect the entire circuit at the same time.
- The circuit must be broken down into unit sections within which the circuit elements are considered to be lumped.
- This is because the dimensions of the circuit are comparable to the wavelength of the waves.
Worker on power transmission lines by Bikeboy, CC BY-SA 2.0, via Wikimedia Commons
Solved Example: 9910-01
For a transmission line if $\dfrac{L}{C} = \dfrac{R}{G}$, then which of the following is incorrect?
A. $Z_0^2 = \dfrac{R}{G}$
B. The line is called as distortionless line
C. If a series of pulses are transmitted they arrive undistorted.
D. The line is lossless
Correct Answer: D
Solved Example: 9910-02
A single-phase transmission line of impedance j0.8 Ohm supplies a resistive load of 500 A at 300 V. The sending end power factor is:
A. Unity
B. 0.8 lagging
C. 0.8 leading
D. 0.6 lagging
Correct Answer: D
Solved Example: 9910-03
A transmission line having 50 $\Omega$ impedance is terminated in a load of (40 + j30) $\Omega$. The VSWR is:
A. 1.333
B. 0.8
C. 1
D. 2
Correct Answer: D
Solved Example: 9910-04
A 100 V peak signal at 1 GHz is applied to a transmission line of characteristic impedance 100 $\Omega$, terminated with an antenna of 75 $\Omega$. The peak values of reflected and incident currents are:
A. 0.142 A, 1 A
B. 0.284 A, 1 A
C. 0.142 A, 2 A
D. 1 A, 0.142 A
Correct Answer: A
Solved Example: 9910-05
If a transmission line is terminated with a resistance equal to its characteristic impedance:
A. Reflection coefficient will be unity
B. Standing wave ratio will be minimum
C. The line loss will be maximum
D. The input impedance will be twice the terminating resistance
Correct Answer: B
Solved Example: 9910-06
A transmission line of surge impedance 300 Ω is connected to a load of 300 $\Omega$. The reflection coefficient of transmission line at the load end will be:
A. 0
B. -1
C. 2
D. +1
Correct Answer: A