Rectenna - What's the Purpose of the Capacitors?
The capacitors and the diodes together form a voltage multiplier.

From the Wikipedia page:



That is a Villard voltage multiplier.  Your circuit is using a variant of it to provide a DC output from the received RF.

It uses a voltage multiplier in order to get a higher DC voltage than would be possible with a simple rectifier.

Using a single stage multiplier, you get (approximately) the peak to peak voltage of the RF instead of approximately the RMS voltage of the RF.

That comes at the cost of higher impedance.  The impedance of the capacitors adds up in a not immediately obvious way, such that the impedance increases rapidly with additional stages.

Your circuit uses two single stages stacked to provide about 2 times the RF peak to peak voltage as DC. I don't know how that will influence the total impedance.  I expect it will be lower than a typical two stage multiplier.

In conclusion, you must have C1 and C2.  With out them, the circuit will not work.