Low Frequency Magnetic Communication

Discussion in 'Wireless & RF Design' started by ryakel, Jun 10, 2014.

  1. ryakel

    Thread Starter New Member

    Jun 8, 2014
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    Hello,

    I have been working on building a crude transmitter/receiver who's signal can be measured through soil and rock. Like these "flushable transmitters"
    https://www.prototek.net/Flushables.php.

    There is a great article about low frequency magnetic communication that I have been referring to:
    http://ww1.microchip.com/downloads/en/AppNotes/00232B.pdf

    The first trial circuit I have built is similar to that built in the microchip document, except for different capacitance and inductor (I was trying to roughly match industry standard 512 hz resonance). PIC PWM controls the recommended MOSFET (TC4421), which feeds the series tank circuit. I'm confident this is working as it should, because my oscilloscope measures almost identical to what LTSpice shows for voltage across the inductor(attached .pdf).

    I can get a signal using a parallel tank circuit with identical components up to about 12 inches away, but I would like to stretch that to 10 or 15 feet if possible. The 600Ω resistance of the inductor is certainly limiting the voltage of the resonance peaks. Will it help my cause if I can get those resonant peaks to more like 200 volts like the microchip document shows? Also, so long as I maintain resonant frequency, will a larger antenna help on the receiving side? Something like a loop or ferrite rod antenna?

    I'm a mechanical engineer who enjoys tinkering, so please bear with me! Thanks in advance!
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why not try for 455 kHz? lots of tuned circuits, ceramic filters and such, used for "invisable fence" instalations.
     
  3. ryakel

    Thread Starter New Member

    Jun 8, 2014
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    Thanks alfacliff.

    So the advantage of 455 kHz is that I don't need to worry about matching the tuning of the transmitter and receiver, because there are circuits already tuned as such?

    So what might be on the inside of these dog collar receivers? Like a tiny radio I suppose? A resonant circuit, filters, amplifiers? I feel more confident in the design of the transmitter than I do in how to design a sensitive receiver I guess. More testing is required...
     
    Last edited: Jun 10, 2014
  4. sirch2

    Well-Known Member

    Jan 21, 2013
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    You might want to Google "Cave Radio" and "Mole Phone". Cavers have done quite a lot of work on this and can get through hundreds of feet of rock, admittedly using much larger antennas than could be accommodated in the unit you linked to.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the advantage for 455 khz is that parts are available, if transformers, ceramic filters and such. 455 khz has been the standard IF frequency for am radios for years. you wouldnt have to try to find parts that are rare.
     
  6. sirch2

    Well-Known Member

    Jan 21, 2013
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    Does 455khz travel far through soil/rock though? Might be easy to find parts but it's not much good if that frequency is heavily attenuated. Generally the lower the frequency the further the signal will travel in the ground. IIRC the cave radios use something around 70kHz.
     
  7. ryakel

    Thread Starter New Member

    Jun 8, 2014
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    From my research of designs on the market, the lower the frequency the better penetration of the signal (as your saying sirch2). For example, the above link to prototek website says that the 16 Hz is used to penetrate steel or iron, the 512 Hz for cast iron or non-metallic, the 8 kHz, and 223 kHz are nonmetallic only. The cave radio (good recommendation, lots of DIY sources down that path) looks typically around 3-4 kHz.

    I did run some numbers and found that the "Q" Quality factor of my circuit was like 2.7...I expect a much higher Q like the 25 or so recommended in the microchip document will help my range. The high resistance of the inductor is what what choking my first design. I'll try another set up with a higher Q when I get a couple more parts in.

    I'm considering building a loop antenna to use on the receiver end, but I'm having concerns about matching it's resonance to the transmitter, and what kind of output it might produce.

    Thanks guys.
     
  8. sirch2

    Well-Known Member

    Jan 21, 2013
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    I'd love to know how you get on, I think a loop on he receiver is the way to go. There is a lot of stuff out there on antenna impedance matching.

    There are cave location beacon designs that locate the cave by finding the points where the signal is maximum when the loop is held at 45 degrees to the horizontal. Find three or more points this way and you can fairly accurately triangulate the location of the beacon.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    I find the information that you posted very interesting. The thing I did not see is the typical circuit for the receiver. I have a good background in electronics and a lot of experience designing and building special circuits. The magnetic field radiation is used in a lot of devices. I have limited experience with them but I think if you use a series resonant circuit for the transmitter you will have greater distance radiation because the current in the series resonate circuit is greater than the parallel resonate circuit.
    I once did some transmitters that radiated audio frequencies into the earth. I developed a way to match the impedance of the audio generator to the impedance of the ground. More recently I built a magnetic field generator that radiated a audio frequency. It used a oscillator, low pass filter and a 8 watt audio amplifier to radiate the frequency. I did not check the range.
    If I can be of help I would be glad to.
     
  10. ryakel

    Thread Starter New Member

    Jun 8, 2014
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    k7elp60,

    That sounds interesting, and along similar lines as my goal. I'm not sure I understand your project though. When you say "radiated a audio frequency" do you mean it resonated at an audio frequency? Or it actually used sound waves?

    My attack plan for the second iteration is attached. I have two things I want to test. My current inductor with a new capacitor designed for a higher Q, and a new L and C resonator that attempts to maximize the attached magnetic field equation.

    I included some thoughts on a receiver using a magnetic loop antenna, but I'm kind of shooting in the dark here. Eventually I'm hoping it will output a few mv across the antenna at 10 feet or so, but I have NO idea if that's realistic at all. Any help is appreciated, and I'll keep this forum updated as I progress.

    By the way, I'm doing this because we don't know where our septic tank is (first time homebuyers...) and the septic guy charges $250 to flush a transmitter down the toilet and find it. I thought "Challenge Accepted!!". Its very interesting to learn about this stuff, I plan on more radio projects in the future, and thanks again everyone for the help. Of course if I just went into the yard with a poker, I would have found it by now probably...but this is way more elegant!
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
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    Yes I used audio frequencies. The attachment is a picture of my most recent project. The frequency is about 3KHZ and was designed to radiate some distance. A resonate circuit about 6 inches a way had over 20 volts peak to peak across the coil. I just happened to have a 100 ft roll of #26 wire that was
    resonate at those frequencies with a stocked capacitor.
     
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  12. ryakel

    Thread Starter New Member

    Jun 8, 2014
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    Hmmm that's interesting. Do you know what the voltage across the transmitting coil was? How much power was this drawing? Was the resonant coil with the 20 V P-P the same coil/capacitor combo?
     
  13. k7elp60

    Senior Member

    Nov 4, 2008
    478
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    The peak to peak voltage across the transmitting coil was 36V. The output was a series resonate circuit with a peak to peak output power of 16 watts. I used a home made oscillator circuit and a audio amplifier kit running on 12VDC. The receiving coil was a duplicate of the transmitting circuit. I usually would have not tested it, but I became somewhat interested as another project I was working on had to do the electromagnetic radiation.
     
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