1. visUal horizon from 13,000 meter is 220 NM and Radar horizon is 254 NM.
I have mentioned earlier also , Once it comes to Airborne electronic scanning horizon isnt a limitation.
2. and the range fUrther increases as the target aircraft increases its height.
2. the detection range in case of airborne radar is sUbject to its reciver sensitivity , clUtter control threshold , peak power of the radar, the freqUency it Uses. and many other variables
3. The main aim behind the development of a airborne radar was to cater for screening and radar horizon limitations
4. If you keep your radars polarity horizontal , it will give you a better range (keeping the rest constant)
5. There are certain form of EM waves that can curve with the curvature of earth to some extent