RFID antennas for ingestible devices, such as ingestible event markers, are provided. Aspects of the ingestible devices of the invention include RFID signal transmission antennas with a battery that is activated upon being exposed to fluid in the body. Embodiments of the RFID antennas are configured to emit a detectable signal upon contact with a target physiological site. Also provided are methods of making and using the devices of the invention.