Voltage Doubler Capacitor Values
The impedance of a voltage multiplier isn't just the impedance of the capacitors.  Because of the multiplication effect, the impedance of the multiplier is a multiple of the capacitor impedance.  It can be calculated using the formula for the impedance of a Cockcroft-Walton multiplier with the number of stages set to 1.

The impedance of a Cockcroft-Walton multiplier is given by:

$$\frac {1}{2 \pi fC} (4n^3 + 3n^2 - n)$$

Where:


\$n\$ is the number of stages (1 for a Greinacher doubler.)
\$C\$ is the capacitance in farads.
\$f\$ is the frequency in hertz of your input signal.


For a single stage, that works out to 6 times the impedance of a single capacitor.

The impedance the antenna "sees" should be the sum of the multiplier impedance and the load impedance.

If you want maximum power from the antenna, then you would try to make the total impedance (multiplier and load) match the antenna impedance.



Pretty much all my experience with voltage doublers is at low frequencies and low voltages.  RF and impedance matching aren't exactly my strong suit.

That said, I have built a multiplier for detecting cellphone and WiFi signals.  It was completely passive - powered entirely from the antenna.  It could light an LED to indicate a  cellphone transmitting at a "range" of a couple of centimeters.

Things you'll want to keep in mind:


Use diodes rated to operate at your frequency (or rather, frequency range.)
Use diodes that have a low forward voltage.  Germanium diodes aren't usable up to GHz frequencies - they give up somewhere over 100MHz.  The forward voltages varies with current.  Lower current, lower forward voltage.  Look for diodes whose datasheets give you a curve relating forward voltage to current.  I used a pile of BAT63 diodes since I wanted to detect up to 2.4GHz.
At high frequencies, you'll have to pay attention to the parasitic inductance of the capacitors.  If the capacitor datasheet doesn't mention the inductance or doesn't cover your frequency range then you should find a different capacitor that does.


I'm sure there are others here who can give you better advice on the RF parts, and on how to decide whether to match the antenna impedance to the multiplier impedance.  Corrections and suggestions are welcome.