HAM HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers 2 — Questions and Answers
Question 1: What is the approximate radiation resistance of a half-wave dipole antenna in free space?
- 73 ohms (Correct answer)
- 50 ohms
- 36 ohms
- 100 ohms
Correct answer: 73 ohms
A half-wave dipole in free space has a theoretical radiation resistance of approximately 73 ohms.
Question 2: What type of antenna pattern does a collinear array produce?
- Omnidirectional with gain concentrated at low elevation angles (Correct answer)
- Highly directional in one direction
- Bidirectional figure-eight pattern
- Hemispherical pattern above ground
Correct answer: Omnidirectional with gain concentrated at low elevation angles
A collinear array stacks radiating elements vertically to compress radiation toward the horizon, producing omnidirectional coverage with low-angle gain.
Question 3: What is the primary advantage of a log-periodic dipole array (LPDA)?
- Wide bandwidth with relatively constant gain and impedance (Correct answer)
- Extremely high gain on a single frequency
- Circular polarization across all bands
- Elimination of feedline losses
Correct answer: Wide bandwidth with relatively constant gain and impedance
The LPDA provides consistent gain and impedance characteristics across a wide frequency range due to its scaled element design.
Question 4: How does the beamwidth of a parabolic dish antenna change as the dish diameter increases relative to wavelength?
- Beamwidth decreases (Correct answer)
- Beamwidth increases
- Beamwidth remains the same
- Beamwidth oscillates unpredictably
Correct answer: Beamwidth decreases
A larger dish diameter relative to wavelength produces a narrower beamwidth and higher gain.
Question 5: What is the effect of adding a reflector element to a Yagi-Uda antenna?
- It increases forward gain and improves the front-to-back ratio (Correct answer)
- It decreases the antenna impedance to near zero
- It eliminates all sidelobes from the radiation pattern
- It converts the antenna to circular polarization
Correct answer: It increases forward gain and improves the front-to-back ratio
A reflector element directs energy forward, increasing gain and reducing radiation off the back of the antenna.
Question 6: What is the purpose of a gamma match on a Yagi antenna?
- To match the low impedance of the driven element to a 50-ohm feedline (Correct answer)
- To increase the bandwidth of the antenna by 50%
- To eliminate the need for a balun
- To add circular polarization capability
Correct answer: To match the low impedance of the driven element to a 50-ohm feedline
A gamma match transforms the typically low impedance at the driven element feed point to match standard 50-ohm coaxial cable.
HAM HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers 2
What is the approximate radiation resistance of a half-wave dipole antenna in free space?