# SMA to rp-SMA cable selection

I need to buy a 6-7m long SMA to rp-SMA cable to connect a Wifi radio unit (2.4 GHz and U.FL connector) and an external antenna (2.4 GHz and SMA connector). The longest ones on Farnell and Digikey are 300 mm long, but Google shows some suppliers. As I have never dealt with high speed cables and connectors, I would very appreciate if someone who knows these could advise me how I should select a cable that would cause less distortion on the signal: if I should pay attention to any parameter on the datasheet or should I avoid a cable that is made of a specific material, or what else?