This is a radio which amplifies RF waves and converts them to pulses!
Most on-off-keying (OOK) AM receivers like this have a “data slicer”, this being an R/C integrator to produce the mean DC voltage level
of a bit stream. With this, the receiver can threshold while ignoring noise, when a bit string is being received.
To be practical, there needs to be a lot of amplifier gain prior to the diode.