HAM Antenna Theory and Design 2 — Questions and Answers
Question 1: What is the radiation resistance of a half-wave dipole antenna in free space?
- 36 ohms
- 50 ohms
- 73 ohms (Correct answer)
- 100 ohms
Correct answer: 73 ohms
A half-wave dipole in free space has a radiation resistance of approximately 73 ohms.
Question 2: What is the effect of adding a ground plane to a quarter-wave vertical antenna?
- It doubles the antenna gain
- It provides a low-impedance return path and creates an electrical image of the antenna (Correct answer)
- It increases the antenna's bandwidth by 50%
- It changes the radiation pattern to omnidirectional
Correct answer: It provides a low-impedance return path and creates an electrical image of the antenna
A ground plane provides a low-impedance return path and acts as a mirror, creating an electrical image that completes the half-wave dipole equivalent.
Question 3: What is the input impedance of a folded dipole antenna compared to a standard dipole?
- One-fourth the impedance (about 18 ohms)
- Half the impedance (about 36 ohms)
- The same impedance (about 73 ohms)
- Four times the impedance (about 292 ohms) (Correct answer)
Correct answer: Four times the impedance (about 292 ohms)
A folded dipole has approximately 4 times the impedance of a standard dipole, or about 292 ohms, due to the transformer action of the folded element.
Question 4: Which antenna type uses a driven element, a reflector, and one or more directors?
- Log-periodic dipole array
- Yagi-Uda antenna (Correct answer)
- Quad loop antenna
- Collinear array
Correct answer: Yagi-Uda antenna
The Yagi-Uda antenna uses a driven element, a slightly longer reflector behind it, and one or more shorter directors in front.
Question 5: What is the typical front-to-back ratio of a well-designed 3-element Yagi antenna?
- 3 to 6 dB
- 10 to 15 dB (Correct answer)
- 20 to 30 dB
- 40 to 50 dB
Correct answer: 10 to 15 dB
A well-designed 3-element Yagi typically achieves a front-to-back ratio of 10 to 15 dB.
Question 6: What happens to the radiation resistance of an antenna as its length becomes much shorter than a half wavelength?
- It increases significantly
- It remains approximately 73 ohms
- It decreases significantly (Correct answer)
- It becomes purely reactive
Correct answer: It decreases significantly
As an antenna becomes electrically short, its radiation resistance decreases significantly, making it less efficient at radiating power.
Question 7: What is the polarization of the electromagnetic wave radiated from a vertical antenna?
- Horizontal polarization
- Circular polarization
- Elliptical polarization
- Vertical polarization (Correct answer)
Correct answer: Vertical polarization
A vertical antenna radiates vertically polarized electromagnetic waves, with the electric field oriented vertically.
HAM Antenna Theory and Design 2
What is the radiation resistance of a half-wave dipole antenna in free space?