CAD CAD Antenna Designer Feed Networks and Impedance Matching 2 — Questions and Answers
Question 1: On a Smith Chart, where is a perfect impedance match (Z = Z0) located?
- At the far right edge of the chart
- At the far left edge of the chart
- At the center of the chart (Correct answer)
- At the top of the chart
Correct answer: At the center of the chart
A perfect impedance match corresponds to a normalized impedance of 1 + j0, which is located at the center of the Smith Chart.
Question 2: What is the input impedance of a lossless transmission line terminated in a short circuit (Z_L = 0) and exactly λ/4 long?
- Zero (short circuit)
- Infinite (open circuit) (Correct answer)
- Equal to the characteristic impedance Z0
- Equal to twice Z0
Correct answer: Infinite (open circuit)
A λ/4 shorted stub presents an open circuit (infinite impedance) at its input because Z_in = Z0²/Z_L → ∞ as Z_L → 0.
Question 3: What is the approximate return loss for a system with a VSWR of 2:1?
- −3.0 dB
- −6.0 dB
- −9.5 dB (Correct answer)
- −20.0 dB
Correct answer: −9.5 dB
For VSWR = 2:1, Γ = (2−1)/(2+1) = 1/3, giving return loss = −20·log10(1/3) ≈ −9.5 dB.
Question 4: What is a Wilkinson power divider primarily designed to achieve?
- Equal power splitting with high isolation between output ports (Correct answer)
- Unequal power splitting for beam steering applications
- Impedance transformation between balanced and unbalanced lines
- Frequency diplexing for dual-band operation
Correct answer: Equal power splitting with high isolation between output ports
A Wilkinson power divider splits power equally between two output ports while providing high isolation between them, with all ports matched to the system impedance.
Question 5: What happens to the electrical length of a transmission line when the operating frequency is doubled?
- The electrical length is halved
- The electrical length remains the same
- The electrical length is doubled (Correct answer)
- The electrical length quadruples
Correct answer: The electrical length is doubled
Electrical length θ = βl = 2πfl/v_p is directly proportional to frequency, so doubling frequency doubles the electrical length of a fixed physical line.
Question 6: In a T-junction power divider, what is the primary disadvantage compared to a Wilkinson divider?
- T-junctions cannot split power equally
- T-junctions provide no isolation between output ports (Correct answer)
- T-junctions require resistive terminations that dissipate signal power
- T-junctions can only be used below 1 GHz
Correct answer: T-junctions provide no isolation between output ports
A T-junction power divider provides no isolation between output ports, meaning signals at one output couple directly to the other, while a Wilkinson divider provides high inter-port isolation.
Question 7: What does the outermost scale of a Smith Chart typically represent?
- Reflection coefficient magnitude
- Normalized resistance values
- Wavelengths toward generator or toward load (Correct answer)
- Frequency in GHz
Correct answer: Wavelengths toward generator or toward load
The outer perimeter scale of a Smith Chart represents wavelengths toward the generator or toward the load, used to track impedance changes as a signal moves along a transmission line.
CAD CAD Antenna Designer Feed Networks and Impedance Matching 2
On a Smith Chart, where is a perfect impedance match (Z = Z0) located?