Free Ham Radio General Class Test Antennas and Feed Lines Questions and Answers — Questions and Answers
Question 1: What is the primary purpose of an antenna tuner (transmatch)?
- To absorb harmonic radiation and prevent its transmission.
- To match the impedance of the transceiver to the impedance of the feed line and antenna system. (Correct answer)
- To increase the antenna's gain and directivity.
- To filter out noise and interference before it reaches the receiver.
Correct answer: To match the impedance of the transceiver to the impedance of the feed line and antenna system.
An antenna tuner, also known as a transmatch or impedance matcher, is used to ensure maximum power transfer from the transmitter to the antenna system. It does this by matching the output impedance of the transceiver (typically 50 ohms) to the impedance presented by the feed line and antenna. This minimizes reflected power (high SWR) and allows the transmitter to operate efficiently.
Question 2: In a Yagi antenna, what is the typical relationship in length between the driven element, the reflector, and the directors?
- The reflector is the longest, the driven element is shorter, and the directors are the shortest. (Correct answer)
- The driven element is the longest, the reflector is shorter, and the directors are the shortest.
- All elements are the same length for maximum resonance.
- The directors are the longest, the driven element is shorter, and the reflector is the shortest.
Correct answer: The reflector is the longest, the driven element is shorter, and the directors are the shortest.
In a typical Yagi-Uda antenna design, the reflector element is slightly longer than the driven element, and the director elements are progressively shorter than the driven element. This arrangement of element lengths is crucial for achieving the antenna's characteristic gain and directional pattern, with the signal being focused in the direction of the directors.
Question 3: A technician measures the SWR at the transmitter end of a long coaxial feed line and gets a reading of 1.1:1. However, the antenna itself is known to be a poor match for the operating frequency. What is the most likely reason for the low SWR reading?
- The SWR meter is malfunctioning.
- The long feed line is attenuating the reflected wave, making the SWR appear lower than it is at the antenna. (Correct answer)
- The transmitter's output power is too low for an accurate reading.
- The antenna has been correctly matched by the length of the feed line.
Correct answer: The long feed line is attenuating the reflected wave, making the SWR appear lower than it is at the antenna.
A long run of coaxial cable has inherent loss (attenuation) that increases with frequency. This loss affects both the forward power going to the antenna and the reflected power coming back. The reflected wave is attenuated on its way back to the SWR meter, resulting in a reading that is artificially low and not representative of the actual SWR at the antenna feed point.
Question 4: What is the primary radiation pattern of a simple quarter-wavelength vertical antenna installed with a good ground-plane system?
- A figure-8 pattern with nulls off the sides.
- Primarily directional, with maximum radiation off the top of the antenna.
- Omnidirectional in the horizontal plane. (Correct answer)
- A cardioid pattern with a deep null in one direction.
Correct answer: Omnidirectional in the horizontal plane.
A quarter-wavelength vertical antenna, when used with an effective ground plane (like a set of radials), radiates equally well in all horizontal directions. Its radiation pattern is omnidirectional in the azimuth (horizontal) plane, making it a popular choice for general-purpose communication where you want to contact stations in any direction.
Question 5: The velocity factor of a coaxial cable is primarily determined by which of its physical properties?
- The conductivity of the center conductor and shield.
- The type of dielectric material used between the center conductor and shield. (Correct answer)
- The overall diameter of the cable.
- The tightness of the outer jacket.
Correct answer: The type of dielectric material used between the center conductor and shield.
The velocity factor represents the speed at which a radio wave travels through the feed line compared to the speed of light in a vacuum. This speed is determined almost entirely by the dielectric constant of the insulating material separating the center conductor and the shield. Materials like foam polyethylene have a higher velocity factor (closer to the speed of light) than solid polyethylene.
Question 6: An amateur operator is setting up an HF station and finds the SWR is 3:1 on their desired frequency. Which of the following is the most likely consequence of operating with this high SWR?
- Increased signal clarity and audio fidelity.
- Improved antenna efficiency and greater radiated power.
- The transceiver may reduce its output power to protect its final amplifier circuits. (Correct answer)
- The feed line will radiate more effectively than the antenna.
Correct answer: The transceiver may reduce its output power to protect its final amplifier circuits.
A high SWR indicates a significant impedance mismatch, causing a portion of the transmitter's power to be reflected back down the feed line. Modern solid-state transceivers have protective circuitry that detects high SWR and automatically reduces the output power to prevent the reflected power from damaging the final amplifier transistors. While some power is lost and the system is inefficient, the primary immediate effect is often this protective power reduction.
Free Ham Radio General Class Test Antennas and Feed Lines Questions and Answers
What is the primary purpose of an antenna tuner (transmatch)?