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Antenna inductance

Discussion in 'General Electronics Discussion' started by Kardo22, Oct 10, 2014.

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  1. Kardo22

    Kardo22

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    Mar 7, 2014
    Hi
    I had some antennas (rectangular shape, it was 58 rotations, about 4,3x8,7 cm dimensions). They didn't work and I found they all had wrong inductance that was all over the space (the max/min was about 3). I returned them and got new ones that had inductance that didn't vary more than 5%. They had the same dimensions and number of rotations.
    Only thing that was different was the corners, The new, correct ones had a very nice round corner. The bad ones had a ugly bent corner (had no corner radius, it was bent so that some of the wires were even deformed).
    How does this effect the inductance?

    My measurements (rough, the dimension was uH):
    Correct antennas: 600 +-5%
    Bad ones: 150-520


    Thanks

    Edited measurements
     
    Last edited: Oct 10, 2014
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    dunno .... got a pic to show us what you are referring to ?

    Dave
     
  3. Kardo22

    Kardo22

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    Mar 7, 2014
    Ok, same pictures
    Corect:
    upload_2014-10-10_12-19-11.png

    Bad:
    upload_2014-10-10_12-19-43.png
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What are they for? They look like 125kHz RFID tag reader antennas to me.

    The shape of the corners are likely to be insignificant.
     
  5. Kardo22

    Kardo22

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    Mar 7, 2014
    Yes, card readers.
    But the corners are the only difference between the good and bad ones that I can see.
     
  6. Thirantha

    Thirantha

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    Aug 31, 2014
    Corners are insignificant. The factors affecting the inductance of a coil as follows.

    1. NUMBER OF WIRE WRAPS, OR "TURNS" IN THE COIL:
    All other factors being equal, a greater number of turns of wire in the coil results in greater inductance; fewer turns of wire in the coil results in less inductance.

    Explanation: More turns of wire means that the coil will generate a greater amount of magnetic field force (measured in amp-turns!), for a given amount of coil current.

    [​IMG]



    2. COIL AREA: All other factors being equal, greater coil area (as measured looking lengthwise through the coil, at the cross-section of the core) results in greater inductance; less coil area results in less inductance.

    Explanation: Greater coil area presents less opposition to the formation of magnetic field flux, for a given amount of field force (amp-turns).

    [​IMG]



    3. COIL LENGTH: All other factors being equal, the longer the coil's length, the less inductance; the shorter the coil's length, the greater the inductance.

    Explanation: A longer path for the magnetic field flux to take results in more opposition to the formation of that flux for any given amount of field force (amp-turns).

    [​IMG]



    4. CORE MATERIAL: All other factors being equal, the greater the magnetic permeability of the core which the coil is wrapped around, the greater the inductance; the less the permeability of the core, the less the inductance.

    Explanation: A core material with greater magnetic permeability results in greater magnetic field flux for any given amount of field force (amp-turns).

    [​IMG]
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Arouse1973 likes this.
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    It is unfortunate you sent the bad ones back. Hard to troubleshoot the problem when the problem isn't at hand. I suspect there may have been some shorted turns in the bad antennas, maybe because the insulation deteriorated over time or wasn't very good to begin with. Did you purchase new original stock from a reputable manufacturer? How many of these antennas do you need? I would have hand-wound my own if only a few are needed.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, it's possible that some of the enamel insulation was damaged in the corners. The shape of the corners itself is of little concern, but as mentioned above, shorted turns are a huge issue.

    I spotted this as a card reader coil because I re-wound one onto a ferrite rod for a little application I had. I ensured both had the same inductance and it worked a treat. However I didn't do any tests to see how changes of inductance affected performance.

    It may not have been the case, but you do need to be careful measuring inductance because the way you have your leads draped, how long they are, what they pass over or near, where the coil is placed, and even where your hands are can all affect the measurement.
     
    hevans1944 likes this.
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Measuring Inductance can be a real pain compared to measuring capacitance. All the things you mentioned can affect the measurement, especially for low inductance values less than 100 μH.

    To the OP, what exactly did you use to measure inductance? My multimeter only does capacitance, just fine down to a few picofarads or up to a few millifarads. I can zero-out the test lead capacitance for reasonable confidence that a small capacitance is measured accurately. The last inductance measurement I made with any reasonable confidence was either with a grid-dip meter using a known-value capacitor for resonance, or with a Heathkit RLC bridge nulled using its "Magic Eye" tube. Both were analog measurements with a resolution of perhaps two significant digits.

    El Cheapo digital RLC meters have not been very reliable in my experience, but maybe that is just my bad luck. I would like to find one at a reasonable price that measures down to one microhenry, or less, for RF work in Ham radio with multiple test frequencies. Good ones from, for example, Agilent are a out my reach for now.

    Maybe I will build a "modern" grid-dip meter with a digital frequency readout and variable frequency from 1 MHz to 42 MHz, which covers all the HF amateur radio bands. Details are here, and another one for 100 kHz to 1 GHz here.

    I could buy or build a vector network analyzer (VNA) which measures complex impedance, but that may be a bit of overkill for measuring only inductors. There are a few factory-built VNAs sold for the hobby trade at somewhat reasonable prices. Sometimes a used professional VNA becomes available on E-bay for only a semi-outrageous price. And there is a popular DIY VNA with professionally made circuit boards available. Biggest problems with that is placing and soldering the itsy bitsy teeny weening surface mounted parts and obtaining a special low phase-noise oscillator. Those who are into building a VNA sometimes band together to make a group purchase, but I haven't explored that avenue yet.

    Well, I guess I "jumped the shark" again with this post. Sorry 'bout that. I should maybe start another thread.
     
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