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RF Energy Harvesting Project

Discussion in 'Electronics Homework Help' started by Pheezy, Jul 27, 2014.

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  1. Pheezy


    May 17, 2014
    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.

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

  2. Gryd3


    Jun 25, 2014
    I personally think your barking up the wrong tree...
    Here's why:
    My wireless AP is currently transmitting at 40mW which is incredibly small if you wish to drive any kind of load. Remember that Watts = Voltage x Current... so 40mW will in theory provide you with 5V and 0.008Amps. This is far too little to do anything useful with continuously unless you plan to store this and use it intermittently. (I have the option of bumping it up to 250mW, but this requires a hardware modification and additional cooling... you will not find this very often in other homes)
    An argument can be made that you can harvest from multiple APs at a time... This is partially true. If two APs are operating on the same frequency, they may be out of phase. The closer they get to being 180 degrees to each other, the less you will get as two waves 180 degrees out of phase from each other will cancel each other out. (Or at least decrease the amplitude of the larger wave) In practice however, multiple APs operate on slightly different frequencies which means that the wide-band signal you are receiving will fluctuate greatly as the multiple signals intertwine and the gains you will receive will be negligible. Remember that with AC, 1V + 1V is not always 2V. It depends entirely on the frequency and phase of both signals.
    That energy I had posted above... the 40mW is the Transmit power of my AP. This does not directly relate to the power at the receiving end.
    Take a look at
    As you move further away from the source of the signal, the power drops off at a considerable rate... so you would need a very large antenna to compensate, or be nearby.

    Here is some homework for you.
    Figure out how much energy you 'want' for this project to be worth it.
    Figure out how much energy you could expect to get from a 'single' AP if you were in an ideal world 20 feet from it. (You cannot capture 100%, even with a directional antenna. 10% is still far too high even if you pointed a small satellite dish at it.)
    Figure out how many AP you would require within a certain range of your harvester (At this point you should realize this may not work)

    The signal from an AP is far too small for you to do anything useful with other than stealing someone's internet connection. I could have been more blunt, but I want you to understand why rather than walking away thinking some dude with a purple cat picture said it was impossible.

    That being said, this is based off of my knowledge. I can only guarantee you that I know this is right. This is not stopping you or anyone else from discovering or inventing something new
  3. davenn

    davenn Moderator

    Sep 5, 2009
    Hi there pheezy

    you need to read gryd3's response and questions to you very carefully. Several times if necessary
    come back with the answers to his questions and go from there

    just to give you an indication .... you will not light a LED from the antenna of a 100W transmitter at anything greater than around 5 metres
    As gryd3 pointed out, the power out of AP's is extremely low

    Last edited: Jul 27, 2014
    KrisBlueNZ likes this.
  4. OLIVE2222


    Oct 2, 2011
    To have an idea you need around 4W EIRP to read a passive UHF RFID tag located few meters away!
    Last edited: Aug 9, 2014
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Dave Jones thinks it's perfectly acceptable to get power from a wireless access point. He calculates the time it would take to charge a fairly small battery using back of the envelope calculations.

    Easily achievable.

    If you have 4 years to wait

    See the third video on this page.
    OLIVE2222 and KrisBlueNZ like this.
  6. Pheezy


    May 17, 2014
    I wanted to thank you all for your replies. I appreciate you not blowing up at me. Like I said, I am still trying to learn about all of this. I just find it interesting.

    So here is a little update on what I am doing and to clarify a few things.

    1) WiFi AP transmits +23 dBm so 200 mW. Goal is to capture only at a distance of maybe a meter or even less. Going to be storing it all in a supercapacitor or THINergy MECs.
    2) Using a 9 dBi antenna to capture signals.
    3) I have done a bit of research using the Friss Equation and the FSPL. I understand that at one meter of distance, -5 dBm is received. This was calculated using a software and formula substitution.
    4) Multiple antennas has been thought

    Now for some questions,
    1) Is this project still not viable?
    2) Can someone suggest a free software or a software I could obtain on trial, other than ADS which I can use to design the voltage multiplier circuit?
    3) How would I calculate the input impedance of a voltage multiplier circuit?

  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Assume you have an iPhone 5 with a 1440 mAh battery. Then assume you can capture 100% of the energy from the router. It will take about 7 hours to charge the battery.

    If you have to stay that close to the router (and incidentally render it useless, why not plug the phone in?

    But let's also be practical. The router is not transmitting continually, you can't capture all of the energy. That energy you intercept cannot be converted with 100% efficiency back to electricity, that power cannot be converted to the voltage required for the phone without loss, and finally, charging the battery cannot be done without further losses.

    Using very generous ballpark figures, assume that the transmitter is powered on 50% of the time, that you can capture 10% of the power, that conversion efficiency is 80%, that charging efficiency is 90% (and ignoring all other losses, then you can get a continuous 7.2mW of energy put back into the battery. That equates to 200 hours to charge the battery (or more than a week).

    Does the phone even last a week between charges if you do *absolutely* nothing?

    The real figures would be unlikely to be even within an order of magnitude of this, so we're looking at 2 months to charge a battery.

    Oh, hang on, a 9dbi antenna? that's even worse. you calculate -5dbm, so at 1 metre, you're looking at 0.3mW or thereabouts. using the rest of my generous figures you get 0.1mW into the battery, or a full charge in about 2 years.

    1) No, it doesn't sound viable.

    2) voltage multiplication won't do anything other than lose you more power

    3) moot. (but it is high until you draw power from it, then it plummets)
    Supercap2F and KrisBlueNZ like this.
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Correct, it's not viable. Transferring power through the air at distances measured in metres without enormous losses... well, I'd be foolish to say it's impossible, but it's never been done. Tesla couldn't do it. He wasted a lot of time and money trying though. Should have stuck with the big cool sparky things!
    Don't know sorry.
    You mean one with diodes? The kind that need several hundred millivolts before they start to conduct? You can't define a meaningful input impedance value for a circuit like that.

    BTW, how many microvolts do you expect to get out of the antenna? What transformer ratio would you need to step this voltage high enough to make the diodes conduct? Transforming voltage up causes current to be transformed down, remember!
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