After recording this video, I closed down the bonnet with the intent to go around and collect some data. But as soon as I closed the bonnet, AirMobile sensor disconnected from the smartphone. I reopened the bonnet, and connection was back
The problem was clear, the reason less. I didn't expect the steel of the bodyshell to shield Bluetooth 2.4 GHz frequencies to that extent
In my opinion, the possible causes could be
Antenna / module position: the Bluetooth module was placed between the LaunchPad board and the AirMobile board. While the LaunchPad has a ground plane, the AirMobile hasn't. But the LaunchPad in on the "line of sight" between the Bluetooth module and the smartphone, so the ground plane could actually act as a shield
Bodyshell grounded: the steel the bodyshell is made of shields Bluetooth frequencies more than I expected because the bodyshell connected to the ground. I assumed that the simple equation "tyre = rubber = insulation" is not completely true. I found out that tyre's compound contains a certain amount f graphene (to reduce internal friction and hence fuel consumption) and graphene is a conductor...
High-frequency noise due to sparks: the last hypothesis is that high frequency noise are produced by the electrical system of the engine, in particular by the high voltages involved in generating sparks
Because I was not able to determine the root cause, I moved the Bluetooth module outside the case and placed it directly on the bonnet in a protective bag
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