Apple to case makers: "no metal touching the antenna"

In data that Apple gives to the manufacturers of cases We can find all the measurements of the phone and some instructions, such as that neither the camera nor the headphone, microphone, speaker or proximity sensor inputs can be covered; but the most interesting is the one prohibits anything metallic in contact with the edge where the antenna is located, Ya que any driver will decrease coverage.

Here you have the rest of the phone measurements so that you can take a look, click to see bigger.