In general, electromagnetic waves (radio waves) propagate in all directions. In this case, radio waves can be transmitted or received in any direction. However, as the distance increases, the strength of the radio wave decreases in proportion to the square of the distance.
Early radars (devices that use radio waves to detect distant metallic objects) also used concave mirror-shaped reflectors to compensate for this. Then, the reflector was rotated by a mechanical device to focus the radio waves in the desired direction.
You can still see the spinning radar system atop some ships.
Phase array antennas
When multiple antennas are arranged close to each other to emit radio waves, they are combined into stronger radio waves from a specific direction. And in the other direction, there is an interference effect that weakens the radio waves. By properly adjusting the phase difference between antennas like this, radio waves can be concentrated in the desired direction. In this case, it is not necessary to turn the direction using a mechanical device. In addition, the change of direction is also faster compared to the rotary type.