Bluetooth technology uses Radio Frequency (RF) to transmit signal. It uses the same frequency band (2.4GHz) as WiFi and other RF technologies.
The signal strength is highly dependent on working environment and the reception performance is also dependent on the mobile device used. Any blockage i.e. (a metal or concrete between ECPin and the mobile device) the signal strength can be reduced significantly. ECPin will not work in microwave oven as RF signal is completely blocked by shielding.