One standard 150mm FrSky receiver antenna - suitable for newer receivers.
Diameter of round gold connector ~2mm.
The active element in the antenna is the unshielded part on far end - if you have lost any piece of this part then it's no longer tuned to 2.4GHz and should be replaced.
** Older FRSKY receivers have a larger connector - see comparison pictures to make sure you order the correct one. See related items for the larger version. **