Avionics MCQs – Standing Waves
1 - Question
Which of the following does not cause standing waves?
a) Short circuit
b) Open circuit
c) High frequency high power signal
d) Impedance is not matched
Explanation: If the load at the end of a line is an open circuit or a short circuit or has an impedance other than the characteristic impedance of the line, the signal is not fully absorbed by the load. This causes the signal to get reflected back the line and cause a standing wave.
2 - Question
Which of the following is not true regarding standing wave?
a) In a standing wave the energy moves towards the power source
b) In a standing wave power loss occurs
c) Standing waves do not affect signal strength
d) Standing waves are not desirable
Explanation: Standing waves are not desirable. Some of the energy is reflected from the end of the line and actually moves back up the line. This reflection indicates that the power produced by the generator is not totally absorbed by the load. When there is a power loss naturally the strength of the signal goes down.
3 - Question
What is the load on a transmission line?
a) 0 Ω
c) Some value between 0 and infinity
d) Some value between0 and 1
Explanation: In practice, however, the load on a transmission line is neither infinite nor 0Ω; rather, it is typically some value in between. The load may be resistive or may have a reactive component.
4 - Question
The transmission line in which the resistive impedance is equal to the characteristic impedance is called _____
a) Matched lines
b) Paralleled lines
c) Balanced lines
d) Unbalanced lines
Explanation: Ideally, a transmission line should be terminated in a load that has a resistive impedance equal to the characteristic impedance of the line. This is called a matched line.
5 - Question
How would the graph of wavelength vs voltage look like if the transmission line is matched (neglect resistive loss)?
a) Linearly increasing
b) Linearly decreasing
c) Exponentially decreasing
Explanation: If a voltmeter is moved down a matched line from the generator to load and the rms voltage values are plotted, the resulting wavelength versus voltage line will be flat.
6 - Question
In an unmatched line the actual signal on the line is ___________
a) The sum of forward and reflected signals
b) The difference of forward and reflected signals
c) The product of forward and reflected signals
d) The modulus of forward signal
Explanation: In an unmatched line, standing waves are formed which is a combination of forward and reflected signal. The signal actually on a line is simply the algebraic sum of the forward and reflected signals.
7 - Question
What is the Standing wave ratio if a 75Ω antenna load is connected to a 50Ω transmission line?
Explanation: Standing wave ratio = SWR = Load impedance/ Characteristic impedance = 75/50 = 1.5.
8 - Question
The ratio of the incident voltage wave Vi to the reflected voltage wave Vr is called the reflection coefficient.
Explanation: The ratio of the reflected voltage wave Vr to the incident voltage wave Vi is called the reflection coefficient. The reflection coefficient provides the current and voltage information on the line.
9 - Question
What is the reflection coefficient If a line is terminated in its characteristic impedance?
Explanation: The reflection coefficient of a line that is fully terminated in its characteristic impedance is 0. This is because there is no reflected voltage on the line.
10 - Question
What percentage of power is reflected if the reflection coefficient is 0.5?
Explanation: Reflected power= Γ2= 0.52=0.25 = 25% of initial power.
11 - Question
What is the resistive load if SWR= 3.05 and Zo =75Ω?
Explanation: Zl = Zo/SWR = 75/3.05 = 24.59Ω.