Why An envelope detector recovers AM audio by half-wave rectifying the modulated RF with a diode, then feeding the pulses to an RC low-pass filter. The capacitor charges to the peak of each RF cycle and discharges slowly through the resistor, so the output voltage follows the amplitude envelope of the carrier while the RF itself is filtered away. The RC time constant must be long compared with one RF cycle but short compared with the highest audio frequency.
Watch out Zener breakdown describes voltage regulation, not detection, and the diode in an envelope detector is forward biased on signal peaks, not operated in reverse breakdown. Sensing reactance versus frequency describes a varactor, used in FM modulators rather than AM detection.
Envelope detector = rectify then smooth: diode chops the RF, RC follows the audio outline.