Helium Miner antenna range?

The city I live in is a very flat, no hills, no elevation changes to worry about. I live in the suburban area of city, mainly bi-level and 2 story homes and trees to contend with, maybe a few 3 story apartment buildings as well. With that said, I am planning on mounting antenna on the peak of roof of my house (20 ft above ground approx.). If I am mainly up against trees and homes as obstacles what db gain antenna would perform best?