HAM Radio Extra Class Test Antenna Theory and Design 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?

Accepted Answer

To create a directional radiation pattern with forward gain.

Answer

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?

Accepted Answer

Circular polarization

Answer

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?

Accepted Answer

73 ohms

Answer

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?

Accepted Answer

The antenna's radiation pattern is independent of distance in the far-field region.

Answer

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?

Accepted Answer

The tuner presents a 50-ohm load to the transmitter, improving power transfer from the radio.

Answer

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.

HAM - Radio Extra Class Test Practice Test

HAM - Radio Extra Class Test Practice Test

HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers