What's the main material of the HEISHA drone charging pad

The major part of the HEISHA drone charging pad is the pad, HEISHA selected the FR4 glass fiber as the main material of both the charging pad and the centralizing pole.
The primary factor of choosing FR4 glass fiber is because of its stable insulative. Even charging a drone fully automated, the safety issue is still prior. Especially when charging outdoor, being exposed to different kinds of environment, moist weather included, a high and stable insulative drone charging pad is essential.
Designated by the NEMA (National Electrical Manufacturers Association), FR4 glass fiber is made of knitted fiberglass cloth and the epoxy resin binder, and “FR” stands for flame retardant and enables this grade of glass fiber to a high flame resistant. Due to the almost zero water absorption, the FR4 glass fiber has been widely applied as an electrical insulator. 
Glazed and smooth surface is another factor as the whole charging will not start before the drone is completed landed and centralized on the pad, if the pad is not smooth enough, the landing gears with charging interfaces might be broke by the centralizing force. 
Considering the manual operation happens when the power is off, the centralizing pole is also adopted the FR4 glass fiber as material.