HAM Antenna Theory and Design 3 — Questions and Answers
Question 1: What is the purpose of a loading coil in an electrically short antenna?
- To increase the antenna's bandwidth
- To resonate the antenna by canceling the capacitive reactance (Correct answer)
- To increase the antenna's radiation resistance
- To improve the antenna's front-to-back ratio
Correct answer: To resonate the antenna by canceling the capacitive reactance
A loading coil adds inductive reactance to cancel the capacitive reactance of an electrically short antenna, bringing it to resonance.
Question 2: What is the gain of a large parabolic dish antenna primarily determined by?
- The feedpoint impedance of the feed element
- The diameter of the dish relative to the wavelength (Correct answer)
- The number of support struts
- The depth of the parabolic curve
Correct answer: The diameter of the dish relative to the wavelength
The gain of a parabolic dish is primarily determined by the ratio of the dish diameter to the operating wavelength.
Question 3: What is an NVIS (Near Vertical Incidence Skywave) antenna optimized for?
- Long-distance DX communications over thousands of miles
- Local and regional communications within about 300 miles (Correct answer)
- Satellite communications
- VHF/UHF propagation
Correct answer: Local and regional communications within about 300 miles
An NVIS antenna radiates at very high angles (near vertical) to use the ionosphere for regional communications within approximately 300 miles.
Question 4: What is the characteristic of an antenna with a very high Q factor?
- Wide bandwidth and low efficiency
- Narrow bandwidth and high efficiency for a resonant antenna (Correct answer)
- High gain and broad radiation pattern
- Low radiation resistance and high efficiency
Correct answer: Narrow bandwidth and high efficiency for a resonant antenna
A high-Q antenna has a narrow bandwidth, meaning it is very frequency-selective and operates efficiently only over a small frequency range.
Question 5: Which of the following describes an end-fire antenna array?
- Maximum radiation is broadside (perpendicular) to the array axis
- Maximum radiation is along the axis of the array (Correct answer)
- Radiation is isotropic in all directions
- Maximum radiation is at 45 degrees to the array axis
Correct answer: Maximum radiation is along the axis of the array
An end-fire array produces maximum radiation along the axis of the array, like a Yagi antenna.
Question 6: What is the purpose of a balun when connecting coaxial cable to a dipole antenna?
- To increase the antenna's gain
- To match the balanced antenna to the unbalanced transmission line (Correct answer)
- To tune the antenna to resonance
- To protect the antenna from lightning
Correct answer: To match the balanced antenna to the unbalanced transmission line
A balun (balanced-to-unbalanced transformer) prevents common-mode current on the coaxial shield by matching the balanced dipole to the unbalanced coax.
Question 7: What is the approximate gain of a typical 5-element Yagi antenna over a dipole?
- 2 to 3 dBd
- 6 to 9 dBd (Correct answer)
- 12 to 15 dBd
- 18 to 22 dBd
Correct answer: 6 to 9 dBd
A 5-element Yagi typically provides 6 to 9 dBd gain over a reference dipole, depending on element spacing and design.
HAM Antenna Theory and Design 3
What is the purpose of a loading coil in an electrically short antenna?