best antenna for 15. story building.

self.HeliumNetwork1m ago
hello there in 6 months im about to move to a 20 story building with me living on the 15th. floor. i there will have a balcony that covers 2 sides of the building so my thought was to put the antenna up in the exact corner. im living in vienna, near the center with lots of buildings around/beneath for kilometers to come. what antenna do u guys reccomend for such a position? thanks