I'm trying to understand a basic crystal radio schematic I found in a book. I redrew part of the schematic and inserted it below. To the left, I omitted the antenna and tuning circuit. The diode below is labeled as the detector. The description in the book says the capacitor (ignore the value) "shorts the RF part of the signal to ground, leaving only audio-frequency energy at the output." My question is why does the capacitor do that? My understanding is that both RF and AF are a/c so why would one get shorted to ground and not the other?