CAD CAD Antenna Designer Feed Networks and Impedance Matching 1 — Questions and Answers
Question 1: What is the primary purpose of an impedance matching network in an antenna system?
- To maximize power transfer by minimizing reflections between source and antenna (Correct answer)
- To increase the gain of the antenna element
- To reduce the physical size of the antenna
- To improve the radiation pattern directivity
Correct answer: To maximize power transfer by minimizing reflections between source and antenna
Impedance matching maximizes power transfer by ensuring minimum reflection between source and load, following the maximum power transfer theorem.
Question 2: What is the standard characteristic impedance of coaxial cable used in most RF antenna systems?
- 75Ω
- 100Ω
- 50Ω (Correct answer)
- 25Ω
Correct answer: 50Ω
50Ω is the industry standard characteristic impedance for most RF test equipment, cables, and antenna systems, balancing power handling and minimum loss.
Question 3: A quarter-wave transformer is used to match a 50Ω source to a 200Ω load. What characteristic impedance must the transformer section have?
- 100Ω (Correct answer)
- 125Ω
- 150Ω
- 250Ω
Correct answer: 100Ω
The required characteristic impedance is the geometric mean of source and load: Z = √(Z_S × Z_L) = √(50 × 200) = √10000 = 100Ω.
Question 4: What does a VSWR of 1:1 indicate about an antenna system?
- Maximum mismatch with all power reflected back to source
- Perfect impedance match with no reflected power (Correct answer)
- Half the incident power is reflected back to the source
- The antenna is resonant at twice the design frequency
Correct answer: Perfect impedance match with no reflected power
A VSWR of 1:1 indicates perfect impedance match between source and load, meaning all available power is delivered with no reflections.
Question 5: What is the primary function of a balun in an antenna feed system?
- To increase the antenna bandwidth
- To boost the transmitted power level
- To convert between balanced and unbalanced transmission lines (Correct answer)
- To provide impedance transformation only
Correct answer: To convert between balanced and unbalanced transmission lines
A balun (balanced-to-unbalanced transformer) converts between balanced transmission lines such as twin-lead and unbalanced lines such as coaxial cable to prevent common-mode currents.
Question 6: What is the reflection coefficient magnitude (|Γ|) when a 50Ω source drives a 100Ω load?
- 0.25
- 0.50
- 0.33 (Correct answer)
- 0.75
Correct answer: 0.33
Γ = (Z_L − Z_0)/(Z_L + Z_0) = (100 − 50)/(100 + 50) = 50/150 = 1/3 ≈ 0.33.
Question 7: Which transmission line type uses two parallel conductors separated by a dielectric and is classified as a balanced line?
- Coaxial cable
- Microstrip line
- Twin-lead (parallel wire) line (Correct answer)
- Rectangular waveguide
Correct answer: Twin-lead (parallel wire) line
Twin-lead parallel wire transmission line is a balanced line with two conductors separated by a dielectric spacer, historically used for television and FM antenna connections.
CAD CAD Antenna Designer Feed Networks and Impedance Matching 1
What is the primary purpose of an impedance matching network in an antenna system?