# RF Voltage Multiplier

My project partner and I are working on making a rf-dc energy harvesting circuit capable of lighting an LED (even intermittently) or trickle charging a coin cell battery over a long time. We are looking to use 2.4Ghz and 5 Ghz wifi bands to convert as much energy as possible using a Cockcroft-Walton voltage multiplier. I know that distance will be extremely low but we are more interested in showing it is possible than it being useful as of now. As many stages as necessary to power the led briefly, I think the minimum stages is 4-5. I have a dual band antenna 50 ohm impedance, schottky diodes, ceramic caps(various sizes), and access to a network analyzer to build a matching circuit. Supposing -20dbm or better is this possible?

How do I calculate the necessary capacitor sizes to use for the multiplier and the size of a load capacitor for instance to charge the battery? What is a good source for pcb for surface mounting that is better than fr-4?

• Hello and welcome! Please embed your picture in your post rather than using a link. Thanks! – Selvek Mar 28 '18 at 1:31
• "PCB for surface mounting that is better than FR-4"... FR-4 is the industry standard for most PCBs without specific requirements. If you want "better", what's wrong with FR-4? – Selvek Mar 28 '18 at 1:33
• From journal articles it seemed to suggest that FR-4 was not good enough for low input-high frequency circuits. – Alexander Hoffer Mar 28 '18 at 1:39
• Air. As it's a prototype, just dead-bug it and avoid the loss altogether. (But PCB dielectric loss is the least of your worries) – Brian Drummond Mar 28 '18 at 9:08

You must have enough voltage to overcome the forward drop of the diodes. (ignoring the reality of lots of capacitance at 2.4GHz)

There are plenty of ways of transforming any DC voltage, from mV, up to 3V.

So you want to get your RF impedance as high as possible, so the voltage will as high as you can get. Then rectify with a Schottky or point-contact diode.

Folded dipoles have a high impedance, maybe 200ohm, so might be a good place to start.

If you want to reduces losses at 2.4GHz or 5GHz, RO4xxx could be a starting point instead of FR-4.

For diodes SMS7630-061, seems to be the best compromise with low VF, low capacitance and small package which reduces the parasitics.

for the caps, you need to minimise the self discharge especially since you are running at -20dBm.

for the load cap, you need to also consider that without impedance matching you will have a hard time charging to full power.

There is a paper on Arxiv describing a similar project which uses active circuitry to maximise power extraction as well as changes on the Wi-Fi routeur to maximise the power to be harvested.