HAM FREE HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers 2 — Questions and Answers
Question 1: What is the primary advantage of using open-wire (ladder line) feed line instead of coaxial cable?
- Much lower loss at high SWR (Correct answer)
- Higher power handling at UHF frequencies
- Better shielding from RF interference
- Easier to route through walls and conduits
Correct answer: Much lower loss at high SWR
Open-wire feed line has significantly lower loss than coax, especially when operating with high SWR conditions on multi-band antennas.
Question 2: What is the approximate feed point impedance of a half-wave dipole antenna in free space?
- 73 ohms (Correct answer)
- 50 ohms
- 300 ohms
- 36 ohms
Correct answer: 73 ohms
A half-wave dipole in free space has a theoretical feed point impedance of approximately 73 ohms.
Question 3: Which antenna type produces a circularly polarized signal?
- A crossed-Yagi fed 90 degrees out of phase (Correct answer)
- A collinear vertical array
- A half-wave dipole mounted at an angle
- A ground-plane antenna with drooping radials
Correct answer: A crossed-Yagi fed 90 degrees out of phase
A crossed-Yagi with quadrature (90-degree) phasing between elements produces circular polarization, commonly used for satellite work.
Question 4: What happens to the radiation resistance of a shortened antenna compared to a full-size resonant antenna?
- It decreases significantly (Correct answer)
- It increases significantly
- It remains the same
- It becomes purely reactive
Correct answer: It decreases significantly
Shortening an antenna below its resonant length causes the radiation resistance to decrease, reducing efficiency.
Question 5: What is the purpose of a balun at the feed point of a dipole antenna fed with coaxial cable?
- To prevent common-mode current on the coax shield (Correct answer)
- To increase the gain of the antenna by 3 dB
- To match 50-ohm coax to 300-ohm feed line
- To filter harmonics before they reach the antenna
Correct answer: To prevent common-mode current on the coax shield
A balun prevents unwanted common-mode currents from flowing on the outside of the coax shield, which can distort the antenna pattern.
Question 6: What is the velocity factor of a typical solid polyethylene dielectric coaxial cable?
- Approximately 0.66 (Correct answer)
- Approximately 0.95
- Approximately 0.82
- Approximately 0.50
Correct answer: Approximately 0.66
Solid polyethylene dielectric coax has a velocity factor of approximately 0.66, meaning signals travel at 66% the speed of light.
HAM FREE HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers 2
What is the primary advantage of using open-wire (ladder line) feed line instead of coaxial cable?