Ham Radio General Class Test FREE Ham Radio General Class Test Antennas and Feed Lines Questions and Answers 2 — Questions and Answers
Question 1: What is the approximate length of a quarter-wave vertical antenna for 14.2 MHz?
- 16.5 feet (Correct answer)
- 33 feet
- 5.2 feet
- 67 feet
Correct answer: 16.5 feet
A quarter-wave antenna length is calculated as 234 divided by the frequency in MHz, giving approximately 16.5 feet for 14.2 MHz.
Question 2: What type of feed line loss increases significantly when the line is operated with high SWR?
- Coaxial cable (Correct answer)
- Open-wire ladder line
- Waveguide
- Hardline
Correct answer: Coaxial cable
Coaxial cable losses increase dramatically under high SWR conditions compared to open-wire feed lines.
Question 3: What is the primary advantage of a log-periodic dipole array (LPDA) antenna?
- Wide bandwidth with consistent gain and impedance (Correct answer)
- Highest possible gain on a single band
- Smallest physical size for any frequency
- Elimination of feed line losses
Correct answer: Wide bandwidth with consistent gain and impedance
An LPDA provides relatively constant gain and impedance characteristics across a wide frequency range.
Question 4: What happens to the feed point impedance of a half-wave dipole as it is raised from near ground level to a height of one wavelength?
- It fluctuates between approximately 50 and 90 ohms (Correct answer)
- It steadily increases to over 300 ohms
- It remains constant at 73 ohms
- It drops to near zero ohms
Correct answer: It fluctuates between approximately 50 and 90 ohms
The feed point impedance of a dipole oscillates as height changes due to ground reflection effects, varying roughly between 50 and 90 ohms.
Question 5: Which of the following describes the velocity factor of a feed line?
- The speed at which a signal travels through the line compared to the speed of light (Correct answer)
- The rate at which signal strength decreases per unit length
- The maximum voltage the line can handle before breakdown
- The ratio of capacitive to inductive reactance in the line
Correct answer: The speed at which a signal travels through the line compared to the speed of light
Velocity factor is the ratio of signal propagation speed in the feed line to the speed of light in free space.
Question 6: What is the purpose of a balun at the feed point of a dipole antenna when using coaxial cable?
- To convert between balanced and unbalanced systems and reduce common-mode current (Correct answer)
- To amplify the signal before it reaches the antenna
- To filter out harmonic emissions
- To increase the bandwidth of the antenna
Correct answer: To convert between balanced and unbalanced systems and reduce common-mode current
A balun converts the unbalanced coaxial feed line to the balanced dipole antenna, suppressing unwanted common-mode currents on the shield.
Ham Radio General Class Test FREE Ham Radio General Class Test Antennas and Feed Lines Questions and Answers 2
What is the approximate length of a quarter-wave vertical antenna for 14.2 MHz?