A mobile device peripheral that incorporates a repeater antenna inside of a protective case for a mobile device to extend the range of short-range communications while minimizing interference caused by the thickness or materials of the protective case. The repeater antenna can be capable of receiving a wireless signal from an antenna in the mobile device and transmitting the same signal or an amplified version of the same signal. The repeater antenna can be capable of receiving a wireless signal from an external antenna and transmitting the same signal or an amplified version of the same signal to the mobile device. The mobile device peripheral can also include a battery that provides additional power the mobile device. Without a repeater antenna, the materials and/or thickness of the additional battery and/or the case can partially or completely block a short-range, low-frequency, or low-power communication signal such as Near Field Communication (NFC).