Negative elevation angles only result in a decreasing beam height relative to the ground for a limited number of kilometres away from the radar. Beyond that point the beam height increases again, relative to the ground below. This is mainly due to the curvature of the earth. In a vacuum, the radar beam would follow a straight line at whatever elevation angle you transmitted it at. However due to the earth's atmosphere, it "normally" curves downward with a curvature of 4/3 the earth's radius. But of course the earth's surface curves downward even more sharply than this, at a curvature equal to the earth's radius. Attached is a beamheights.txt file that shows some sample negative elevation angles and expected beam heights above ground at various radiuses (in kilometres) from the radar for normal atmospheric conditions.