What describes the principal direction of radiation in an end-fire antenna array?

The principal direction of radiation in an end-fire antenna array is along the plane of the array and perpendicular to the antenna elements. This design allows the array to focus its energy in a specific direction, maximizing the gain in that direction while minimizing radiation in other directions.

End-fire arrays are typically configured with elements oriented such that they produce a directional radiation pattern, effectively aligning along the axis of the array. This capability is particularly important in applications where targeted communication or surveillance is required, as it enhances signal strength and quality in the desired direction.

The other options do not accurately capture the physical characteristics of an end-fire array. For instance, radiation that is perpendicular to the plane of the array would describe a broadside array instead, which radiates energy outwards from the sides rather than along the length of the array. An angle to the array or circular patterning also does not align with the intended directional focus of an end-fire configuration, which is specifically designed for linear, forward radiation.