Review on Alfa AWUS036NHA High Gain Wi-Fi USB Adapter - Long-Range Network Booster with 5dBi and 9dBi Antenna - Ideal for Wardriving & Range Extension - Windows 7, XP/Vista 64-bit /128-bit - Atheros by Anne Wilson

I use these things to extend my wifi range from base station to drones. I use a battery-powered router as a base station and connect GCS (Solex, Tower, QGC, etc.) to the drone via UDP. The vehicles have a Raspberry Pi with SolexCC, which (among other things) handles the mapping between Mavlink and UDP messages. Using a standard Wi-Fi Pi, it's no surprise that the range isn't great. With them, I can keep in touch with a ground vehicle even after it has driven to the other side of a small hill and to the other side of my house. The only downside I've found is that it's difficult to fit this unit under my quad canopy, but that's more of an issue as the quad is narrow and already stuffed underneath. To get this to work with the Pi, all you have to do is blacklist the internal Wi-Fi driver and plug it into one of the USB ports. This then becomes wlan0. You can tell it's working if the blue light on it flashes quickly after the Pi boots up.