Compact BLE Antenna with a Modified PIFA Configuration for Wearable EMG Monitor

An antenna mounted underneath the top cover of a wearable device, fed by an ultra-low-profile connector, is proposed for Bluetooth-Low-Energy (BLE) communication at 2.4 GHz. As the antenna is to be used for a wearable device, it is essential that it should be compact in size and tolerant against the nearby medium change. The modified planar inverted-F antenna (PIFA) that we propose consists of a main patch and a parasitic element to broaden the antenna bandwidth. The end of the parasitic element is shorted to miniaturize the antenna to 25 by 10.8 millimeters, fitting well inside a watch-type wearable device. Also, the complete ground layer of the antenna makes it radiate well in the outward direction, and minimally interact with the back-side medium. This results in the reflection coefficient being insensitive to the medium change on the backside. The proposed antenna has a peak gain of 3.62 dBi along with 20% efficiency and the impedance bandwidth of 80 MHz (2.4 ~2.48 GHz). To examine the communicative operation of the antenna in practice, the received signal strength indicator (RSSI) of the complete prototype device with the antenna is measured in various postures and orientations, demonstrating reliable connectivity within a typical indoor distance of 10 meters. Lastly, the antenna is embedded in a wearable device, demonstrating electromyography's wireless monitoring.