RF Energy Harvesting Project

Hello there,

I am presently working on an energy harvesting system using RF signals. I have read some of the other posts made on this topic and have learnt quite a few things. However, I have some questions that I would like answered just to clarify some doubts.

The intention is to acquire 2.4 GHz wi-fi signals from an AP (access point) to an antenna.

That antenna that I am planning to use is either a quarter wave whip antenna (with a ground plane) or a half wavelength dipole antenna tuned to 2.4 GHz. Since its omni-directional, I can expect the gain to be around 3 dBi. I am still unsure about which antenna would be most suitable. According to my theoretical study, a half wavelength is more appropriate as I can acquire more energy but then someone suggested a quarter wavelength is more suitable. Also, does it matter what type of end connection I require, e.g. SMA connection or a BNC? So I would like someone to shed some light on this or maybe link me to a web address so I can do my own research.

Next, assuming I have selected my antenna, I can expect it to have 50 ohm impedance (as this is generally the case). Now, I need to design an impedance matching circuit to ensure max power is obtained. In order to do this, I have to know what load impedance I have on my harvesting circuit. But here is the issue. Since I am designing the harvesting circuit, I actually don't know what the load impedance is. I understand how to design the impedance matching circuit mathematically speaking, however, without knowing the load impedance, I don't know how to work on this. A friend suggested I work on the next part of my harvesting circuit (which is the voltage multiplier circuit) and based on this, I will have a load impedance which I can use to calculate the impedance matching circuit. Once again, I need some assistance on this. Another source suggested I connect up a "dummy load", however, I need to do some more research on this and how it works.

So to summarize,

1) Which antenna is more suitable for this project? A half wavelength dipole antenna or a quarter wavelength whip antenna with a ground plane?

2) What kind of end connection is needed? E.G. SMA?

3) How do I calculate the load impedance in order to calculate the inductor/capacitor value required for the impedance matching circuit?

The circuit that I intend to design is given in the diagram.

**broken link removed**

Any tips or ideas I should think about would help. I am still learning. If there are any questions, please let me know.

Regards

Impedance matching can be seen as adjusting the V/A characteristic.

Your LC tank can resonate with higher V, less A...
Or less V, higher A.

This is assuming each case has the same amount of power from the antenna. (Watts = V * A)

If power is great, you can use small coil value, high capacitor value. These present small impedance to the resonant f. It allows greater current flow.

However if power is small, you probably want to obtain higher voltage from the resonating action. Increase coil value. Decrease capacitor value. This is associated with smaller current flow (higher impedance presented to the resonant f).

Experimentation will no doubt be necessary, to find the L and C values which work best.

BradtheRad said:

Impedance matching can be seen as adjusting the V/A characteristic.

Your LC tank can resonate with higher V, less A...
Or less V, higher A.

This is assuming each case has the same amount of power from the antenna. (Watts = V * A)

If power is great, you can use small coil value, high capacitor value. These present small impedance to the resonant f. It allows greater current flow.

However if power is small, you probably want to obtain higher voltage from the resonating action. Increase coil value. Decrease capacitor value. This is associated with smaller current flow (higher impedance presented to the resonant f).

Experimentation will no doubt be necessary, to find the L and C values which work best.

Click to expand...

Thank you for your reply. I intend to simulate an impedance matching circuit design on a software with variables Power IN and V/I.

You do understand how tiny the signal is from a Wifi access point on 2.4GHz ?

you do understand you wont even light a LED from right beside the TX antenna, let alone a few feet away ?

Dave

http://www.ipcbee.com/vol4/41-ICESD2011D10053.pdf

hope this helps !!!

shashy.br said:

http://www.ipcbee.com/vol4/41-ICESD2011D10053.pdf

hope this helps !!!

Click to expand...

well that was good read for some late nite entertainment !!

sounds like you have never heard of the invers square law that determine the drop off of the RF signal level as distance from the transmitting antenna increases
Your paper also gives no indication on how close your TX and RX antennas were to each other to get your 6mV

It seems you and your colleagues also don't understand that yes you can use a capacitor/diode network to increase the voltage, but in doing so you decrease the available current.
That is, you cannot get more power ( in Watts, mW, uW) out than what goes in. So every time your multiplier doubles the voltage, you halve the available current.
And since no circuit is perfect, there are all the other losses to contend with.

The RF voltage levels available to be plucked out of the air around you on any given frequency band are only a microvolt or so ( if you are lucky)
Unless you live right beside a high power commercial radio or TV transmitter

your paper stated that the system didn't succeed in its goal .... the above reasons I gave are the primary reasons why

regards
Dave

well that was good read for some late nite entertainment !!

Click to expand...

A serious project concept or paper about RF power harvesting would start with an estimation of available field strength/power density, expected efficiency and intended power output. Unfortunately the original post and the quoted paper are both lacking a meaningful calculation.

Maybe the OP knows about these things and skipped the point. Otherwise I suggest to start working.

FvM said:

A serious project concept or paper about RF power harvesting would start with an estimation of available field strength/power density, expected efficiency and intended power output. Unfortunately the original post and the quoted paper are both lacking a meaningful calculation.

Maybe the OP knows about these things and skipped the point. Otherwise I suggest to start working.

Click to expand...

Yup totally agree :smile:

It is seen so many times all across many forums on the net
The proponents just don't seem to realise/understand just how small the RF signal levels are in the air around them ...
a few mV and a few uW when you are more than a dozen or so metres away from a transmitter tower


Its like a couple of guys in a physics forum I belong to trying to argue that Tesla's non-Hertzian waves exist
They are completely ignorant of the well understood physics and propagation of EM radiation.
Something that wasn't known or realised in Tesla's time

regards
Dave

davenn u are right ...we did this paper when we were Noobs and did not fully understand what we are doing...i have just stated the paper here for reference purposes only...not telling this will work 100%...!!!

just trying to helpout people..!!!