Can you explain the performance of a (Bluetooth) antenna?
Parameter S11 (aka return loss) is a measure of how accurately the antenna "looks like" 50 ohms. A perfect return loss scenario is -infinite dB but anything lower than -10dB is likely to be acceptable in most circumstances.

If you want a more detailed explanation note the following: -



Where,



See this wiki page where I stole the picture formula. It's also related to VSWR: -



Image stolen from this wiki page.