2D circles is not the case. Forget them. They will work only for locations at the center of the viewscreen. All others will look like inclined ellipses (probably partly hidden, if went hidden side of globe).
I've looked at xcom code, and found how they calculating detection. ShortRange=500, LongRange=750, Hyperwave=800, XCom Craft=200 (yeah it has its own detection range). Detection probability depends on ufo size and altitude and number of radars of same type (yeah number of radars actually affect on probability, as they say in ufo user manual - each radar makes full scan every 5 minutes, so increasing number of radars will increase detection capability of base). Units of distance are direct 3D distance between 3d coordinates (sqrt(x*x+y*y+z*z)). Earth diameter is 2048 units. So one hemisphere is covered by range of 1448. It's like, borderline of such radar is a intersection circle between earth globe and detection 3d __sphere__ of base/craft.
And by the way, UFOs are detected only every 30 minutes.