Tricky Avionics Questions and Answers on “Transmission Lines as Circuit Elements”.
1. How does a shorted transmission line appear to a generator whose length is a quarter of the wavelength?
a) Very high impedance
b) Very low impedance
c) Short circuited line
d) RC tuned circuit
Answer: a
Clarification: At the load end of the shorted one quarter wavelength line, voltage is zero and current is maximum. But one-quarter wavelength back, at the generator, the voltage is maximum and the current is zero. To the generator, the line appears as an open circuit, or at least very high impedance.
2. How does a shorted transmission line appear to a generator whose length is a quarter of the wavelength?
a) Very high impedance
b) Very low impedance
c) Short circuited line
d) RC tuned circuit
Answer: c
Clarification: At the end of a shorted one half wavelength line, the generator sees the same conditions as at the end of the line, i.e., zero voltage and maximum current. This represents a short, or very low impedance.
3. When a shorted line does looks like a series resonant circuit to the generator?
a) Length of line is one quarter wavelength
b) Length of line is one half wave lengths
c) Sorted line
d) Open line
Answer: b
Clarification: When the length of the transmission line is one half of the wavelength, a standing wave pattern is produced where the current and voltage at the generator and the end of the transmission line is same. In this case, the line looks like a series resonant circuit to the generator.
4. When does a shorted transmission line look like an inductor to the generator?
a) Length is one quarter wavelength
b) Length is between one quarter and one half of wavelength
c) Length is more the one half of the wavelength
d) At all times
Answer: a
Clarification: If the line length is less than one-quarter wavelength at the operating frequency, the shorted line looks like an inductor to the generator. If the shorted line is between one quarter and one-half wavelength, it looks like a capacitor to the generator.
5. The characteristics of an open transmission line are just the opposite of shorted transmission line with respect to what the generator sees.
a) True
b) False
Answer: a
Clarification: To the generator, an open one-quarter wavelength line looks like a series resonant circuit and a one-half wavelength line looks like a parallel resonant circuit. This is just the opposite of a shorted line.
6. When does an open transmission line look like an inductor to the generator?
a) Length is one quarter wavelength
b) Length is between one quarter and one half of wavelength
c) Length is more the one half of the wavelength
d) At all times
Answer: b
Clarification: If the open transmission line is less than one-quarter wavelength, the generator sees a capacitance. If the line is between one-quarter and one-half wavelength, the generator sees an inductance. These characteristics repeat for lines that are some multiple of one-quarter or one-half wavelengths.
7. The impedance of a line is zero when ___________
a) It acts as inductor
b) It acts as capacitor
c) It acts as a resistor
d) Series resonant circuit
Answer: d
Clarification: If the line acts as a series resonant circuit, its impedance is zero. If the line is of such a length that it acts as a parallel resonant circuit, its impedance is near infinity. If the line is some intermediate length, it is reactive.
8. The special transmission lines constructed on PCBs to act as tuned circuits is called as?
a) Stripline
b) Loop lines
c) Printed lines
d) Transmission lines
Answer: a
Clarification: At UHF and microwave frequencies the values of inductance and capacitance become so small that it is difficult to realize them physically with standard coils and capacitors. Special transmission lines constructed with copper patterns on a printed circuit board (PCB), called microstrip or stripline, can be used as tuned circuits, filters, phase shifters, reactive components, and impedance-matching circuits at these high frequencies.
9. Which of the following is not true with respect to microstrip?
a) Used for transmission in UHF and microwave region
b) Copper is etched on an insulating base material
c) Point to point communications with wire are eliminated
d) Microstrip is ever onequarter or one-half wavelength long.
Answer: d
Clarification: Microstrip is a flat conductor separated by an insulating dielectric from a large conducting ground plane. The microstrip is usually onequarter or one-half wavelength long and is used for transmission in the UHF and microwave region.
10. What is the characteristic impedance of microstrip with the dimensions h = 0.0625 in, w = 0.1 in, t = 0.003 in, and ε= 4.5?
a) 84.35Ω
b) 42.6Ω
c) 53.9Ω
d) 65.21Ω
11. What is the transmission line in which a flat conductor is sandwiched between two ground planes?
a) Microstrip
b) Stripline
c) Coaxial line
d) Balanced line
Answer: b
Clarification: Stripline is a conductor sandwiched between two ground planes. It is more difficult to make than microstrip; however, it does not radiate as microstrip does.
12. Microstrip and striplines can be made by using monolithic, thin-film, and hybrid IC techniques to produce ________
a) Micro circuits
b) Microwave integrated circuits
c) Nano circuits
d) Embedded circuits
Answer: b
Clarification: Even tinier microstrip and striplines can be made by using monolithic, thin-film, and hybrid IC techniques. When these are combined with diodes, transistors, and other components, microwave integrated circuits (MICs) are formed.
13. A microstrip transmission line is to be used as a capacitor of 4 pF at 800 MHz. The PCB dielectric is 3.6. The microstrip dimensions are h = 0.0625 in, w = 0.13 in, and t = 0.002 in. What is the characteristic impedance of the line?
a) 48.9Ω
b) 12.3Ω
c) 54.3Ω
d) 65.7Ω