The diagram is referring to RADAR Beam diameter. At 20NM its 6900', which means if you set the tilt to 0 then the beam will be 3500' above and 3500' below (I think RADAR experts would say is the simple description), or alternatively set tilt to -3.5 degrees and now it is scanning 6900' below the aircraft at 20NM.
Looks like the beam width is 3.5 degrees. So you can estimate the beam width in feet versus distance (from the aircraft/range) with simple mathematics.