RF Energy Harvesting Project

Discussion in 'Homework Help' started by Pheezy, Jul 26, 2014.

  1. Pheezy

    Thread Starter New Member

    May 16, 2014
    11
    0
    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.

    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
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    I don't know how you expect to design a circuit from a block diagram without knowing a blessed thing about what you are doing. Before you do anything else I think you should do some calculations to see how much power you think is available to harvest just to get some idea of weather the effort will be worthwhile.

    I estimate a typical access point will have a power level of say -75 dBm. So how much DC power do you expect to harvest from that nearly insignificant amount of power? You're not hoping for some kind of over unity discovery are you?
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Pheezy
    You need to read about "free space loss". It will tell you how much you loose due to distance between the transmit antenna and the receive antenna at the frequency of interest. At one meter away from a 10 milliwatt access point your 3 db receive antenna will yield only single digit micro-watts of power.
     
  4. Pheezy

    Thread Starter New Member

    May 16, 2014
    11
    0
    The AP I am using transmits +23 dBm. I calculated using Friss Equation and FSPL that in the most ideal of cases, at one meter of distance, a 3 dBi antenna can capture -12 dBm (0.06 mW). I know it is not a lot and the goal is not to make any ground breaking discoveries. From some of the papers I have read, it seems it is possible to harvest energy but at small distances. I apologize. I should have made this clear in my post.

    Regards
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The output of a microwave detector diode is less than 50 milli-volts at -10dbm (HP8473B). That is not much power at 50 ohms.
     
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