FrSKY 150mm Rx antenna - new connector
(1) single replacement whisker antenna (not a pair!) This is the same antenna that the props chopped off your receiver. Fits the X4R-SB, D4R-II, S6R etc. Connector also fits R-XSR, XM, XM+ but the unshielded tip length is different than the original antenna.
The active element in the antenna is the unshielded part at the tip - if you have lost any of that portion of the antenna it's no longer tuned to 2.4GHz and should be replaced.
This is the new W.FL style connector. Production changed around August 2016. The older U.FL connector has a diameter of ~3mm. The newer connector has a diameter of ~2mm and uses a thinner wire.