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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.

http://postimg.org/image/bxaaj3ywt/68b48acb/

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

Regards

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  • \$\begingroup\$ λ/4 is suitable for capturing the signal. You need longer for capturing the energy. All 200mW of it. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 27 '14 at 3:28
  • \$\begingroup\$ Thanks for the reply. But doesn't a quarter wavelength half the energy I am capturing as compared to a half-wavelength dipole? \$\endgroup\$ – Pheezy Jul 27 '14 at 3:33
  • \$\begingroup\$ Do a sanity check first - how much energy do you think you'll be able to recover (or need to recover), and at what range? \$\endgroup\$ – W5VO Jul 27 '14 at 3:42
  • \$\begingroup\$ If you have a WiFi adapter that can display dBm (most can with appropriate software) you could attach the antenna and convert that into mW for a rough estimate of power. Although I suspect the answer might come out in the order of nW. \$\endgroup\$ – PeterJ Jul 27 '14 at 3:45
  • \$\begingroup\$ Very low levels of energy. Using the Friss Equation and the FSPL, I calculated that in the most ideal of cases using 3 dBi gain antennas, At a meter's distance from the Wi-Fi source, an antenna can capture -12 dBm (0.06 mW). \$\endgroup\$ – Pheezy Jul 27 '14 at 3:48
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1) Which antenna is more suitable for this project? A half wavelength dipole antenna or a quarter wavelength whip antenna with a ground plane?

The choice of quarter wave or half wave depends on how good your ground system is. If you can supply a good solid ground plane then the quarter wave will provide about the same level of performance. A bigger gain would be with a directional antenna that you could point at the radiation source either actively by phasing it or physically by rotating it.

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

Whatever fits your design, but SMA is good for high frequency use.

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

If you are designing your own antenna you need a network analyzer to measure your antenna and fine tune the network. Even if you aren't making your own antenna a network analyzer is the best way to verify your impedance match is correct.

Don't expect to "harvest" much power as you'll be lucky to get miliwatts. Better off going for a large nearby FM or TV signal than WiFi

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You really also want to consider what loading you put on the antenna. If you terminate into a 50 ohm resistor to GND almost all of the acquired energy gets absorbed as heating in the 50 ohm termination!!

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