Looking at various ARA prototype pictures I noticed they all seem to feature pluggable connectors.
Those are SMA connectors, perhaps the most common 50 ohm coaxial rf connector these days. Two variants exist: SMA and RP-SMA. The former has the center pin on the cable, while the latter has it on the PCB/chassis end:
The FCC wanted to prevent consumers from using third party antennas on WiFi equipment, so they designed a functionally identical yet incompatible connector (RP-SMA) so that consumers wouldn't have a choice. This of course backfired immediately (China exists), and both types are readily available.