GROL Antennas and Transmission Lines 3 — Questions and Answers
Question 1: What is the approximate gain of a half-wave dipole antenna relative to an isotropic radiator?
- 0 dBi
- 2.15 dBi (Correct answer)
- 6.0 dBi
- 10.0 dBi
Correct answer: 2.15 dBi
A half-wave dipole concentrates radiation broadside to the element in a figure-eight pattern, yielding approximately 2.15 dBi gain over an isotropic (point) radiator.
Question 2: What is the primary advantage of a Yagi-Uda antenna for fixed point-to-point communication links?
- It operates on all frequencies without retuning
- It provides high directional gain toward a specific direction (Correct answer)
- It requires no feedline because it is self-resonant
- It maintains a perfect 1:1 SWR across a wide bandwidth
Correct answer: It provides high directional gain toward a specific direction
A Yagi-Uda antenna uses a reflector, driven element, and one or more directors to focus radiated energy in a single direction, providing substantial gain.
Question 3: What determines the polarization of a transmitted radio wave?
- The operating frequency of the transmitter
- The orientation of the electric field, determined by the physical orientation of the radiating element (Correct answer)
- The output power of the final amplifier
- The electrical length of the transmission line feedline
Correct answer: The orientation of the electric field, determined by the physical orientation of the radiating element
Antenna polarization is defined by the orientation of the transmitted electric field vector, which corresponds to the physical orientation of the antenna element.
Question 4: What is the -3 dB beamwidth of a directional antenna?
- The angle at which antenna gain equals the gain of an isotropic radiator
- The angular width between the two points on the main lobe where radiated power falls to half its peak value (Correct answer)
- The total angular coverage of all radiation lobes combined
- The angle between the main lobe and the first null in the radiation pattern
Correct answer: The angular width between the two points on the main lobe where radiated power falls to half its peak value
The -3 dB beamwidth is measured between the two half-power points on either side of the main radiation lobe peak, indicating how tightly the antenna focuses its energy.
Question 5: What is the radiation resistance of a half-wave dipole antenna in free space?
- 50 ohms
- 75 ohms
- 73 ohms (Correct answer)
- 300 ohms
Correct answer: 73 ohms
A half-wave dipole in free space has a radiation resistance of approximately 73 ohms, which represents the resistive component of its feedpoint impedance accounting for radiated power.
Question 6: What is the front-to-back ratio of a directional antenna?
- The ratio of antenna physical length to the operating wavelength
- The ratio of gain in the forward direction to the gain directly behind the antenna, expressed in dB (Correct answer)
- The ratio of forward transmitted power to the power received from behind
- The ratio of antenna height above ground to feedline length
Correct answer: The ratio of gain in the forward direction to the gain directly behind the antenna, expressed in dB
Front-to-back ratio compares the antenna's maximum forward gain to its gain exactly 180 degrees opposite, indicating how well the antenna rejects signals from the rear.
Question 7: Which antenna type is most commonly used for satellite earth station communications at microwave frequencies due to its very high gain?
- Half-wave folded dipole
- Yagi-Uda array
- Parabolic reflector dish (Correct answer)
- Vertical quarter-wave monopole
Correct answer: Parabolic reflector dish
Parabolic dish antennas focus microwave energy to or from a point feed at the focal point, achieving very high gain necessary for the long path lengths of satellite communication.
GROL Antennas and Transmission Lines 3
What is the approximate gain of a half-wave dipole antenna relative to an isotropic radiator?