If I want to use XTend RF module(900 MHz - North America), which one of these antennas ( http://www.digi.com/products/accessories/antennas/whip-antennas ) should I use in order to get up to 40 mile transmission range?
use the 900MHz Yagi’s:
http://www.digi.com/products/accessories/antennas/whip-antennas
you’ll probably need the extra gain at that range.
The maximum distance was achieved using 6db gain yagii antennas placed on top of mountains. Any higher gain antenna requires the difference to be made up in Cable loss.