With the transmitter modded to take screw in antennas and while using a cheap 1dbi directional antenna I have gotten 1.3 miles with out losing signal. But being the antenna is directional you have to keep it pointed right at it. and with a 6 dbi omi directional im getting about .7-.9 miles depending on were i'm flying.
The mod to the transmitter is pretty easy and all you do is get a mini pcx rf to rp-sma pig tail cable and plug the mini connector in to the plug inside the transmitter and then fit the rp-sma connecter were the stock antennas hole. Getting it threw the hole is the harest part of the mod as you need to carve away some of the plastic to get it to fit in the hole right.
if you search utube for a "dji antenna mod" you should find a few videos others have made on how to do it and most have links to the stuff you need for it. and the mods can be done on any model transmitter.