One does not fulfill his or her obligation of lighting the menorah if it is 20 Amos above since it is not easily seen.
The Rambam in Hilchos Shabbas 17:15 writes that a beam over a mavoi with pictures and designs works even above 20 Amos since it is recognizable and attracts attention.
Why don't we apply the same logic by the menorah. If one is 20 Amos above why can't he hang a big sign to attract people to it?