UHF (868 MHz) RFID external antenna
You could possibly get a small "pigtail" coax cable with an SMA cable at the end, and solder one side of the coax to your circuit, with the shield going to an appropriate ground and the center conductor to your signal. Then you could spec out a decent antenna for what you're trying to do and attach it to your SMA cable.

Though honestly I doubt it will work much better than the PCB antenna, unless you're trying to get an antenna with lots of directivity.