Where is the bluetooth antenna located in iPhones?

I posted this question on the Apple support forum, but no one could give me an answer.

I am researching iBeacons for my final year project at University and I'm interested to know the position of the Bluetooth antenna in the iPhones.

Can anybody give me a reliable source explaining where the bluetooth antenna is located on the different versions of the iPhones? iPhone 4S up to 6 plus.

Cheers.