Ultrasonic HC-SR04 sensor, why does it even have a dead-zone of 2cm?
Probably the physical layout and size of the transducers. Sound gets more directional as frequency rises, the received needs to be able to get the echo. Looks like the transducers are about 3cm apart (judging by the 0.1&quot; connector). The transducers have a polar pattern which is almost certainly the cause of the range restriction.