Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?

A quarter-wave ground-plane vertical antenna is designed with its radiating element positioned upright, and the ground plane beneath it acts as a reflective surface. This configuration produces an omnidirectional radiation pattern in the horizontal plane (azimuth), meaning that the signal is transmitted equally in all directions around it.

This omnidirectional characteristic is advantageous for communication because it allows the antenna to pick up signals from any direction, making it suitable for varied operational scenarios where the signal source may not be consistently in one direction.

The other options suggest specific directional characteristics which do not accurately describe the behavior of this type of antenna. For instance, a unidirectional pattern would imply that the antenna radiates primarily in one direction, which is not the case for a quarter-wave ground-plane design. Similarly, while the antenna does direct some energy towards the horizon, describing it as directional or conical is misleading as it does not focus energy in a single direction but rather disperses it uniformly across a 360-degrees plane.