All radars are limited by a pretty basic shortcoming. They can't see around corners. Given this, it makes sense that the biggest limiting factor on how far a radar can see or detect objects is the curvature of the earth. In fact, about 60 miles from any radar sight, a radar with its beam trained relatively parallel to the earth's surface is already only able to see objects one mile above the surface. In other words, the globe's curving surface is already sloping so far away from the radar's position that its signal begins to point toward space.