HAM Antennas and Feed Lines 3 — Questions and Answers
Question 1: What is the characteristic impedance of open-wire (ladder) line typically used with balanced antenna tuners?
- 50 ohms
- 75 ohms
- 300 to 600 ohms (Correct answer)
- 1000 ohms
Correct answer: 300 to 600 ohms
Open-wire or ladder line has a characteristic impedance typically ranging from 300 to 600 ohms depending on conductor spacing and diameter.
Question 2: What is the effect of adding a director element to a Yagi antenna in front of the driven element?
- It increases the front-to-back ratio and forward gain (Correct answer)
- It broadens the antenna bandwidth
- It raises the feed point impedance
- It rotates the polarization 90 degrees
Correct answer: It increases the front-to-back ratio and forward gain
A director, being slightly shorter than the driven element, re-radiates energy forward, increasing forward gain and front-to-back ratio.
Question 3: What is a loading coil used for in a shortened mobile HF antenna?
- To increase the antenna's bandwidth
- To add inductance and resonant the electrically short antenna (Correct answer)
- To lower the antenna's feed point impedance to 50 ohms
- To protect the antenna from lightning
Correct answer: To add inductance and resonant the electrically short antenna
A loading coil adds the inductance needed to resonate an electrically short antenna at the desired operating frequency.
Question 4: What is the input impedance of a quarter-wave matching section (Q-section) used to transform a 200-ohm antenna to 50 ohms?
- 50 ohms
- 100 ohms (Correct answer)
- 150 ohms
- 200 ohms
Correct answer: 100 ohms
The characteristic impedance of a quarter-wave matching transformer equals the geometric mean: √(50 × 200) = 100 ohms.
Question 5: What is the approximate gain of a three-element Yagi antenna relative to a dipole?
- 3 dBd
- 5 dBd (Correct answer)
- 7 dBd
- 10 dBd
Correct answer: 5 dBd
A well-designed three-element Yagi typically achieves approximately 5 dBd (5 dB gain over a dipole) of forward gain.
Question 6: Which transmission line type has the lowest loss per unit length at VHF frequencies?
- RG-58 coaxial cable
- RG-8 coaxial cable
- Hardline coaxial cable
- Open-wire ladder line (Correct answer)
Correct answer: Open-wire ladder line
Open-wire ladder line has the lowest loss per unit length because its low-density dielectric minimizes dielectric losses, especially at VHF.
Question 7: What is a collinear antenna array designed to achieve?
- High directivity toward the horizon by stacking vertical elements in phase (Correct answer)
- High front-to-back ratio for directional communication
- Circular polarization for satellite work
- Very low angle radiation for NVIS propagation
Correct answer: High directivity toward the horizon by stacking vertical elements in phase
A collinear array stacks vertical elements fed in phase, compressing the radiation pattern toward the horizon for increased omnidirectional gain.
HAM Antennas and Feed Lines 3
What is the characteristic impedance of open-wire (ladder) line typically used with balanced antenna tuners?