HAM Antennas and Feed Lines 2 — Questions and Answers
Question 1: What is the radiation resistance of a half-wave dipole in free space?
- 50 ohms
- 73 ohms (Correct answer)
- 300 ohms
- 450 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 primary advantage of a Yagi antenna over a dipole for HF communication?
- Lower feed point impedance
- Directional gain and front-to-back ratio (Correct answer)
- Broader bandwidth
- Easier construction
Correct answer: Directional gain and front-to-back ratio
A Yagi provides directional gain and improved front-to-back ratio compared to an isotropic or dipole antenna.
Question 3: What happens to the impedance of a transmission line when it is terminated in a purely resistive load equal to its characteristic impedance?
- It becomes purely reactive
- It equals the characteristic impedance at all points (Correct answer)
- It doubles at the source
- It drops to zero
Correct answer: It equals the characteristic impedance at all points
A line terminated in its characteristic impedance is a matched line, presenting that same impedance at every point along it.
Question 4: What type of antenna produces a circularly polarized signal?
- Half-wave dipole
- Yagi-Uda array
- Helical antenna in axial mode (Correct answer)
- Beverage antenna
Correct answer: Helical antenna in axial mode
A helical antenna operating in axial mode produces a circularly polarized wave, useful for satellite communications.
Question 5: What is the velocity factor of a typical solid polyethylene coaxial cable?
- 1.00
- 0.95
- 0.66 (Correct answer)
- 0.50
Correct answer: 0.66
Solid polyethylene dielectric coaxial cable has a velocity factor of approximately 0.66, meaning signals travel at 66% of the speed of light.
Question 6: What is the function of a balun at a dipole feed point when coaxial cable is used?
- To increase antenna gain
- To match impedance to 300 ohms
- To prevent common-mode current on the coax shield (Correct answer)
- To reduce SWR on the transmission line
Correct answer: To prevent common-mode current on the coax shield
A balun suppresses common-mode (shield) currents that would otherwise cause the coax to radiate and distort the antenna pattern.
Question 7: How does antenna height above ground affect the elevation angle of maximum radiation for a horizontal dipole?
- Greater height raises the main lobe angle
- Greater height lowers the main lobe angle (Correct answer)
- Height has no effect on elevation angle
- Greater height eliminates all low-angle radiation
Correct answer: Greater height lowers the main lobe angle
As a horizontal antenna is raised higher above ground, its angle of maximum radiation decreases toward the horizon, favoring DX communication.
HAM Antennas and Feed Lines 2
What is the radiation resistance of a half-wave dipole in free space?