Rectenna system for power scavenging

Discussion in 'Wireless & RF Design' started by cttan, Dec 15, 2009.

  1. cttan

    Thread Starter New Member

    Dec 13, 2009
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    I am building a rectenna system to convert the strayed energy at ~2.45GHz into DC.

    I build a equiangular spiral antenna and connect a schottky diode to it.Building the antenna, I printed the equiangular spiral shape on the PCB and etched it. I found that even if I remove the antenna, the diode itself can pickup signal.

    any advice on whats wrong with my antenna?
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    What is the source of the 2.45 Ghz R.F.?
     
  3. beenthere

    Retired Moderator

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    They sure can. Do you have any means of determining if the diode is responding to 2.45 GHz, or some local radio station?
     
  4. KL7AJ

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    The only "natural" source of 2.45 GHz I know of is leaky microwave ovens. This is supposed to be a useful power source????


    Eric
     
  5. beenthere

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    Best left to the OP to answer.

    I do recall may years ago hearing a loud, clear transmission "(Station identifier), radio check, over" come out of my stereo amp. One of the diodes got busy rectifying and made it a receiver for 3 seconds. It obviously originated at the 32nd Street Naval Base in San Diego, as I wasn't too far off at the time.
     
  6. cttan

    Thread Starter New Member

    Dec 13, 2009
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    Yes I use the leakage from a microwave oven as the source.

    No... I do not really have a way to determine if the diode is responding to 2.45GHz. However I read through the specification sheet of the diode.

    Lately I just got the optimum archimedean antenna size by MATLAB simulation. The optimum size I got is about 50cm. Isn't that absurd?
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    The other source would be a wireless router which is a common home appliance. The real problem is that it is not possible scavange any meaningful amount of RF power from the surroundings and turn it into something usefull. Your antenna/diode combination will pick up microvolt/nanoamp signals for femtowatts of DC power, which will be completely dissipated by any wire smaller than about 0000 AWG.

    But hope springs eternal so keep spending money on charlatans and magazine adds. One of these days you will wise up and not fall for such claptrap.
     
  8. Wendy

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    Actually it is a signal strength meter, which is a commercial project. While not extremely hazardous, microwaves in this area aren't particularly good for you, so they build detectors for the microwaves. The manufacturers go to great efforts to keep microwave in, and not let them out, but doors do wear out.

    Reminds me of the of the 1998 Darwin entry (turned out to be a myth). Same frequency range, this time of year.

    http://darwinawards.com/legends/legends1998-11.html
     
  9. maxpower097

    Well-Known Member

    Feb 20, 2009
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    Funny, reminds me of my old physics teacher who insisted the microwave was first discovered when a army man was working on some antenna's and got cooked. He said, they say that he noticed the hershey bar in his pocket melted but in reality he got cooked himself. Not sure if this is true at all but he was a pretty smart guy.
     
  10. Papabravo

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    The "Army guys" were probably "Air Force guys" who worked on the DEW line in Greenland(Thule) and other bases in the Arctic. DEW is an acronym for Defense Early Warning. They were RADAR installations whose purpose was to detect Russian ICBMs as they came over the pole.

    Those guys did notice that standing in the beam gave them a warm feeling. I don't think anybody got cooked, but a few had problems impregnating their wives after they returned home. At least that was the story they put out at the time.
     
  11. Wendy

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  12. maxpower097

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    hmmmm it takes a microwave on high about 30-45 seconds to melt a bar sized piece of chocolate. I would have to think the actual person who had it in their pocket would have suffered damage. Its not exactly unheard of for companies that work for the military to cover up boo boo's like this. Just remember the Radium Girls.
     
  13. beenthere

    Retired Moderator

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    There's 700 - 1200 watt magnetrons for microwave ovens, and then there are ones for radar applications. 10,000 watts is none too big. Air search radars run in the millions of watts, but not off magnetrons.

    I've heard tales of the Enterprise's SPS-32 phased array radars having enough lobe emissions that areas of the flight deck got marked off indicating where there was too much RF to be safe.
     
  14. Papabravo

    Expert

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    You did mean the aircraft carrier(CVN-65) and not the starship (NCC-1701) Right?
     
  15. Wendy

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    Actually it isn't uncommon for low level RF radiation to permeate a work floor. People working on the actual transmitters are exposed more than you think. They are working on these units, they are opened up. Low level is relative, if you are dealing with a 5 KW transmitter 5 W isn't that much.

    I remember working for Rockwell Collins someone noticed people working on the system assy floor, where the final products were tested, weren't having any kids. Turns out one of the effects of RF radiation of this frequency (same as microwave ovens I might add) is temporary sterility. After negotiations with the union they allowed younger guys whom this was an issue to transfer to other sections (it was a big plant).

    If you choose this as a field you need to be aware of the risks, don't assume everything is roses because you can't see anything wrong.

    If you are testing a 50KW transmitter you can assume you are getting your share of watts.
     
  16. beenthere

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    Of course. The aircraft carrier has to use radar - the starship crew can just read ahead in the script.
     
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