Internal antenna.
This device consists of Transmitter & Receiver with 2 functions.
1) Working distance: 1 to 20m adjustable
2) Dimensions:
a) Receiver: 60 x 36 x 15mm
b) Sender: Diameter 36.3mm x Thickness 13.2mm
3) Materials: ABS with Receiver and Transmitter
4) Frequency: 433.92MHz, alarm output: 80dB, power supply: AAA battery for receiver(not included), CR2032 for transmitter(included).