HAM Antennas and Feed Lines 4 — Questions and Answers
Question 1: What type of feed line loss is dominant at lower HF frequencies?
- Dielectric loss
- Radiation loss
- Conductor (I²R) loss (Correct answer)
- Mismatch loss
Correct answer: Conductor (I²R) loss
At lower HF frequencies, conductor or I²R (resistive) loss dominates because skin depth is larger and conductor resistance is the primary loss mechanism.
Question 2: What is the purpose of a gamma match on a Yagi antenna?
- To increase the boom length mechanically
- To transform the low feed point impedance to 50 ohms and provide a coax connection (Correct answer)
- To allow the antenna to operate on multiple bands
- To reduce wind loading on the driven element
Correct answer: To transform the low feed point impedance to 50 ohms and provide a coax connection
A gamma match is an impedance-matching network that transforms the low impedance of a folded or standard driven element to 50 ohms for direct coax feed.
Question 3: What is NVIS (Near Vertical Incidence Skywave) propagation best achieved with?
- A vertical antenna with a good radial system
- A high-gain Yagi pointed at the horizon
- A horizontal antenna mounted at a low height (0.1 to 0.2 wavelengths) (Correct answer)
- A rhombic antenna oriented for long-path propagation
Correct answer: A horizontal antenna mounted at a low height (0.1 to 0.2 wavelengths)
NVIS requires high-angle radiation nearly straight up, achieved by a horizontal antenna mounted very low (0.1–0.2λ) above ground.
Question 4: What is the approximate length of a quarter-wave vertical antenna for the 20-meter band (14.2 MHz) in feet?
- 11 feet
- 16.5 feet (Correct answer)
- 33 feet
- 66 feet
Correct answer: 16.5 feet
A quarter-wave at 14.2 MHz is approximately 246/14.2 ≈ 16.5 feet, using the standard formula 234/f(MHz) for a quarter-wave.
Question 5: What is the primary reason that antenna efficiency decreases when a small loading coil is placed at the base of a short vertical antenna?
- The coil radiates out of phase with the antenna
- The coil's resistance dissipates power that would otherwise be radiated (Correct answer)
- The coil increases the antenna's radiation resistance
- The coil detunes the antenna from resonance
Correct answer: The coil's resistance dissipates power that would otherwise be radiated
A loading coil has series resistance that dissipates power as heat, reducing the fraction of input power that is actually radiated.
Question 6: What is the feed point impedance of a folded dipole antenna compared to a standard half-wave dipole?
- Half (about 36 ohms)
- The same (73 ohms)
- Four times higher (about 292 ohms) (Correct answer)
- Eight times higher (about 584 ohms)
Correct answer: Four times higher (about 292 ohms)
A folded dipole has approximately four times the impedance of a standard dipole, making it approximately 300 ohms — ideal for 300-ohm twin-lead.
Question 7: What is the function of a stub (shorted or open transmission line section) connected in parallel with a feed line?
- To increase the antenna gain
- To provide impedance matching or harmonic suppression (Correct answer)
- To act as a mechanical support for the antenna
- To equalize the signal on both conductors of a balanced line
Correct answer: To provide impedance matching or harmonic suppression
Transmission line stubs present a specific reactance or act as a trap to suppress harmonics or match impedances at a specific frequency.
HAM Antennas and Feed Lines 4
What type of feed line loss is dominant at lower HF frequencies?