HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers — Questions and Answers
Question 1: A Yagi antenna is designed for the 2-meter band. If the director elements are slightly shorter than the driven element, and the reflector is slightly longer, what is the primary effect of these length differences?
- To increase the antenna's bandwidth.
- To decrease the SWR on the feed line.
- To create a directional radiation pattern with forward gain. (Correct answer)
- To enable multi-band operation.
Correct answer: To create a directional radiation pattern with forward gain.
In a Yagi-Uda antenna, the reflector element is typically about 5% longer than the driven element, and the director elements are progressively shorter. This specific arrangement of parasitic elements causes the currents induced in them to have a phase relationship that reinforces radiation in the forward direction (towards the directors) and cancels it in the reverse direction, resulting in a directional pattern with significant forward gain.
Question 2: An amateur radio operator is setting up a satellite communication link. Which type of antenna polarization is most advantageous to mitigate signal fading due to the satellite's rotation and changing orientation?
- Horizontal polarization
- Vertical polarization
- Circular polarization (Correct answer)
- Slant polarization
Correct answer: Circular polarization
Circular polarization (either RHCP or LHCP) is ideal for satellite communications because the rotating electric field of the wave means that a signal can be received regardless of the antenna's orientation, as long as both the transmitting and receiving stations use the same rotation sense. This negates the effects of spin and tumbling that can cause the satellite's antenna orientation to change relative to the ground station, preventing deep signal fades that would occur with linear polarization.
Question 3: What is the theoretical free-space radiation resistance at the feed point of a center-fed half-wave dipole antenna?
- 50 ohms
- 300 ohms
- 600 ohms
- 73 ohms (Correct answer)
Correct answer: 73 ohms
The theoretical radiation resistance of a half-wave dipole antenna in free space is approximately 73 ohms. This value represents the portion of the antenna's feed point impedance that is due to the radiation of electromagnetic waves. It is a fundamental characteristic used in antenna design and impedance matching.
Question 4: In antenna theory, what distinguishes the far-field region from the near-field region?
- The far-field is where the E and H fields are in phase, while in the near-field they are 180 degrees out of phase.
- The antenna's radiation pattern is independent of distance in the far-field region. (Correct answer)
- The near-field region is dominated by radiated power, while the far-field is dominated by reactive power.
- Impedance measurements are only valid in the near-field region.
Correct answer: The antenna's radiation pattern is independent of distance in the far-field region.
In the far-field (or Fraunhofer) region, the angular distribution of the radiated power (the radiation pattern) is essentially independent of the distance from the antenna. In the near-field (Fresnel and reactive regions), the field structure is more complex, and the shape of the radiation pattern changes with distance.
Question 5: A radio operator uses an antenna tuner to match a non-resonant antenna to a 50-ohm transmitter. Which statement accurately describes the effect of the tuner?
- The tuner physically alters the antenna's length to make it resonant.
- The tuner eliminates all power loss in the feed line.
- The tuner corrects the SWR on the feed line between the tuner and the antenna.
- The tuner presents a 50-ohm load to the transmitter, improving power transfer from the radio. (Correct answer)
Correct answer: The tuner presents a 50-ohm load to the transmitter, improving power transfer from the radio.
An antenna tuner (or matching unit) is an impedance-matching network. It does not change the antenna or the SWR on the feed line past the tuner. Its function is to transform the complex impedance at the tuner's input to the 50 ohms the transmitter is designed to see. This allows the transmitter to deliver its full power without being damaged by a high SWR, even though significant SWR and associated losses may still exist on the coax between the tuner and the antenna.
Question 6: If a 100-foot length of coaxial cable with a velocity factor of 0.66 is used to construct a quarter-wave matching stub at 14.1 MHz, what is its approximate physical length?
- The conductivity of the center conductor and shield
- The type of dielectric material used between the conductors (Correct answer)
- The overall diameter of the cable
- The frequency of the signal being transmitted
Correct answer: The type of dielectric material used between the conductors
The velocity factor (VF) of a coaxial cable is the ratio of the speed of signal propagation in the cable to the speed of light in a vacuum. This speed is primarily determined by the dielectric constant of the insulating material separating the center conductor and the shield. Materials like foam polyethylene have a higher velocity factor (e.g., 0.82) than solid polyethylene (e.g., 0.66) because their dielectric constant is lower, allowing the wave to travel faster.
HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers
A Yagi antenna is designed for the 2-meter band.
If the director elements are slightly shorter than the driven element, and the reflector is slightly longer, what is the primary effect of these length differences?