For a 3-element Yagi antenna with horizontally mounted elements, how does the main lobe takeoff angle vary with height above flat ground?

The takeoff angle (or angle of radiation) of a horizontally polarized antenna is heavily influenced by its height above ground. Reflections from the ground interact with the direct wave radiated by the antenna. As the antenna is raised higher above a flat ground plane, the main lobe of the radiation pattern is compressed downward. This phase interaction between the direct and reflected waves causes the takeoff angle to decrease. A lower takeoff angle is highly desirable for long-distance DX communications, as it allows the signal to strike the ionosphere at a shallower angle and travel further per skip. Therefore, raising a horizontal Yagi improves its long-haul performance.