Ham Radio General Class Test Antennas and Feed Lines Questions and Answers — Questions and Answers
Question 1: An amateur operator is installing a new HF station and wants to feed a center-fed dipole antenna with 50-ohm coaxial cable. What is the primary function of a balun in this setup?
- To filter out-of-band signals
- To increase the antenna's gain
- To convert the balanced antenna feedpoint to the unbalanced coaxial cable (Correct answer)
- To provide a DC path to ground for static discharge
Correct answer: To convert the balanced antenna feedpoint to the unbalanced coaxial cable
A balun, which stands for 'balanced to unbalanced', is a device used to interface a balanced antenna (like a dipole) with an unbalanced feed line (like coaxial cable). It prevents the shield of the coax from becoming part of the antenna and radiating, which can distort the radiation pattern and cause RFI.
Question 2: A ham is using an SWR meter to check a newly installed antenna system. The reading at the low end of the 20-meter band is 2.5:1, and at the high end, it is 1.5:1. What does this indicate about the antenna's length?
- The antenna is too long for the band.
- The antenna is too short for the band. (Correct answer)
- The antenna is perfectly resonant.
- The SWR meter is likely defective.
Correct answer: The antenna is too short for the band.
When the SWR is lower at the higher frequency end of the band and higher at the lower frequency end, it indicates that the antenna is electrically short for the desired frequency range. Lengthening the antenna would shift the point of lowest SWR to a lower frequency.
Question 3: Which of the following feed lines generally has the lowest loss per unit of length at HF frequencies, especially when dealing with a high SWR?
- RG-58 coaxial cable
- RG-213 coaxial cable
- 450-ohm ladder line (Correct answer)
- 75-ohm hardline
Correct answer: 450-ohm ladder line
450-ohm ladder line, a type of parallel-conductor feed line, has significantly lower loss at HF frequencies compared to common coaxial cables. Its air dielectric and wide spacing between conductors contribute to this efficiency, making it particularly advantageous in systems with a high SWR, as it does not dissipate the reflected power as heat to the same extent as coax.
Question 4: A General Class operator wants to build a half-wave dipole antenna for the 40-meter band, centered on 7.150 MHz. Using the standard formula, what is the approximate total length of the antenna in feet?
- 32.7 feet
- 139.8 feet
- 65.5 feet (Correct answer)
- 131 feet
Correct answer: 65.5 feet
The common formula to calculate the approximate length of a half-wave dipole in feet is 468 divided by the frequency in MHz. For 7.150 MHz, the calculation is 468 / 7.150 = 65.45 feet.
Question 5: What is the primary advantage of a 3-element Yagi antenna compared to a simple dipole antenna for the same frequency?
- It is omnidirectional.
- It has a higher takeoff angle.
- It provides forward gain and directivity. (Correct answer)
- It is less affected by height above ground.
Correct answer: It provides forward gain and directivity.
A Yagi antenna uses parasitic elements (a reflector and one or more directors) to concentrate radiated power in a specific direction. This results in forward gain and a directional radiation pattern, allowing for stronger signals to and from the desired direction while reducing interference from other directions.
Question 6: An operator measures a high SWR of 3.5:1 at the transmitter. What is a potential consequence of operating the transmitter under this condition for an extended period?
- Improved signal-to-noise ratio on receive.
- The transmitter's final amplifier stage could be damaged. (Correct answer)
- The antenna's polarization will automatically switch.
- The feed line loss will decrease significantly.
Correct answer: The transmitter's final amplifier stage could be damaged.
A high SWR indicates a significant impedance mismatch, causing a large amount of power to be reflected from the antenna back to the transmitter. Modern solid-state transmitters have protection circuits that reduce power output to prevent damage, but older equipment or prolonged operation into a high SWR can cause the final amplifier transistors or tubes to overheat and fail.
Ham Radio General Class Test Antennas and Feed Lines Questions and Answers
An amateur operator is installing a new HF station and wants to feed a center-fed dipole antenna with 50-ohm coaxial cable.
What is the primary function of a balun in this setup?