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Need help....Coil formula for ferrite rod

Discussion in 'Electronic Design' started by George, Sep 14, 2006.

  1. George

    George Guest

    I have a ferrite rod out of a stereo L= 4.7" (119.9mm) X W= .39"
    (9.95mm) I would like to make a loopstick antenna to detect lightning
    strikes. Ideal tuning would be 40Khz, but that might take a mile of
    wire. I am looking at around 400Khz. I need to wind a coil using
    ..014" (.34mm) magnetic wire. All I can find is formulas for aircoils.
    Is there a formula or software for ferrite core coils or does it
    matter?
    I have not picked a cap yet for the tank circuit.
    Thanks
     
  2. I don't understand why you want to tune the antenna. Lightning is an
    impulse effect and is best captured with a broad band antenna. If the
    antenna is untuned, the number of turns is not at all critical. you
    can pass the signal through an audio amplifier and listen to the
    lightning.

    By the way, there are some nice, negative resistance opamp low
    frequency receiver circuits available on the ULFELF, and VLF
    discussion group files sections at Yahoo groups. Both these groups
    deal with this sort of signal, as well as lots of other low frequency
    energy sources.


    By the way, windings over the middle third of the rod make best use of
    flux passing through the rod. The outer thirds are essentially flux
    concentrators (flux arriving from all around the rod), but all the
    collected flux passes through the center section. It is very handy to
    find a small spool that slides over the rod and just wind it about
    full. Hobby and craft stores often sell small lots of colorful wire
    on such small spools.
     
  3. George

    George Guest

    Future plan is to make a identical pair of (orthogonal) cross tuned
    rods and a whip for the 180 degree shift for direction detection,
    Just getting started
     
  4. Perhaps this page will have some information you find useful:
    http://www.amidoncorp.com/aai_ferriterods.htm
     
  5. George

    George Guest

    I did a search on Yahoo, found a project link to just what I wanted,
    but as usuall another blank page. 100Khz to 450Khz. I looked at the
    Url, may take me a day to figure out the formula. Would have thought
    someone would have written a small program with fill in the blanks. I
    may have to look for a longer ferrite rod, 12-15 inch.
    Thanks for the URL
     
  6. Most antenna rods are optimized for medium wave band, not below .5
    MHz. You can assemble a cheap, low frequency rod, by gluing together
    low frequency ferrite beads made for EMI suppression. Go to Digikey,
    search for [bead], then select [Ferrite Cores for Cables & Wiring,
    EMI(108 items)] and then, under Series, select LFB. One of my
    favorites is the 5/8" diameter LFB159079-000.

    If you lap the ends with a bit of silicon carbide sand paper on a
    sheet of glass, you will get a tiny improvement in effective
    permeability. I have done lots of testing with an 11 inch rod made of
    these beads, but the URL I pointed you towards indicates that the rod
    should keep improving up to around 20 inches long, where diminishing
    returns sets in.
     
  7. Guest

    If I had the urge to build something along these lines, I'd work on
    those lightning "about to strike" boxes. I never saw one myself, but I
    understand there are devices that measure the potential of the air to
    indicate a strike is likely.

    The problem I see with building just one sensor for lightning strikes
    is you really can't do a decent job detecting the distance. Now
    multiple sensors and triangulation would be a different story. In fact,
    it would be....
    http://www.sord.nv.doe.gov/Lightning/Lightning-sord.htm
     
  8. Jim Thompson

    Jim Thompson Guest

    See...

    http://www.sovereign-publications.com/global.htm

    I did design work for Lightning Location and Protection (LLP), Tucson,
    in the early '80's.

    ...Jim Thompson
     
  9. A big aircoil might be better anyway - think a metre square. Those are used
    as boosters for radios with ferrite rods.
     
  10. Guest

    I've seen the article--it's in the Amateur Scientist CD-ROM compiled
    by Shawn Carlson.

    Not being home I can't give the publication date, but do recall they
    recommended steel reinforcing bar -- "rebar" -- for the core, and
    something like 100,000 turns of fine wire, using this apparatus to
    detect very low frequency changes in the Earth's magnetic field, as
    well as responding to lightning, IIRC. The upgraded core material
    (preferred, but more expensive) was mu-metal, with suggested suppliers
    listed.

    HTH,
    James Arthur
     
  11. Oh, that is utter rubbish! It's like the more extreme views of
    audiophools.

    If you have a magnetic core in the form of a rod, long compared with
    its diameter, its own permeability only has a minor effect on its
    inductance, because of the huge 'air-gap' in the magnetic circuit. And
    asking for 1000000 turns is just making things needlessly difficult (and
    expensive, of course).
     
  12. George

    George Guest

    Checked out the beads, $20.00 for two 11 inch stacked rods.
    What size coil did you wrap around the middle of the stacked beads?
    What kind of output did you get from the beads? Did you use an amp
    after the output?
    BIG question. What do you think Boltek is using that is so small in
    there receiver?
    http://www.boltek.com/stracker.htm
    Thanks












     
  13. George

    George Guest

    Checked out the beads, $20.00 for two 11 inch stacked rods.
    What size coil did you wrap around the middle of the stacked beads?
    What kind of output did you get from the beads? Did you use an amp
    after the output?
    BIG question. What do you think Boltek is using that is so small in
    there receiver?
    http://www.boltek.com/stracker.htm
    Thanks












     
  14. For rule-of-thumb, the rod permeability is of the order length/diameter.
    Since this rarely exceeds 20 or so, then this controls the permeability,
    since the material permeability is of the order 200 or so for most
    ferrite rods used for antennas.

    So just use your air-core formula, and multiply by l/d for an initial
    guess on the inductance.

    --
    Regards,

    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
     
  15. Lots of variations from a few turns to hundreds. Some of the test
    data is presented in the files section of the
    ferriterodantennaexperimenters Yahoo group.
    I just tested rod and coil combinations for Q with an antique Q meter.
    Don't know, but I think they are over charging for it.
     
  16. Bruce Varley

    Bruce Varley Guest

    Back several years ago, Scientific American Amateur Scientist presented a
    design for just what you're looking for. IIRC they recommended strip
    laminations, quite a bit longer than the normal ferrite rod. They gave an
    address for a company that sells specially treated material with top grade
    magnetic characteristics. They gave full details of the windings and
    electronics, alrhough like a lot of their efforts at the latter, it's not
    too hard to improve on it.

    Your city library should have back issues. It was one of the last amateur
    scientist articles, that may help you track it down.
     
  17. Guest

    Of course. I believe the rod was 1-meter-ish long, but don't recall
    the thickness.
    It may have "only" been 50,000 turns, but not 1,000,000! Either way,
    a lot of wire, and quite a bit of stray capacitance, choking off the
    usable bandwidth.

    The "low noise" amplifier for this pickup was something like an
    OP-07, inverting, with a 1k input and 100k feedback resistor. Oh well
    ! (I'll post the particulars when I get home to access it.)

    Even so, its builders reported having all sorts of fun with it, which
    is what matters most, yes?

    Best,
    James Arthur
     
  18. Bruce Varley

    Bruce Varley Guest

    Back several years ago, Scientific American Amateur Scientist presented a
    design for just what you're looking for. IIRC they recommended strip
    laminations, quite a bit longer than the normal ferrite rod. They gave an
    address for a company that sells specially treated material with top grade
    magnetic characteristics. They gave full details of the windings and
    electronics, alrhough like a lot of their efforts at the latter, it's not
    too hard to improve on it.

    Your city library should have back issues. It was one of the last amateur
    scientist articles, that may help you track it down.
     
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