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Ferrite antenna com system

Discussion in 'Electronic Design' started by Henry Kiefer, Oct 24, 2006.

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  1. Henry Kiefer

    Henry Kiefer Guest

    Hello all -

    I built a simple ferrite antenna communication system. Unfortunately it
    won't work if I set the sender more distanced than about a meter. That is
    even true with different transmitter configurations.

    Here the details:
    ferrite antenna: diameter 8mm , 50mm long
    frequency is 77.5KHz, digital modulation is AM 25%
    bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
    insulated copper wire coil 10 turns

    The transmitter is self-constructed and delivers a very good signal.

    same antenna copied, but a built-in resonating capacitor.
    ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

    The transmission works over about one meter without any shortage.

    Now the problem is that I can change the transmitter parameters but I cannot
    reach a substancial greater distance. I changed:
    - the coil wound times
    - output current to the antenna (measured across a series resistor)
    - added an antenna current sensor coil to sense the antenna current and to
    see if the ferrite antenna saturizes (NO! Very clean sinusoid)

    Googling around to find theoretical aspects of ferrite antenne got no good
    results. I spent several hours and read all I can read.

    Have someone suggestions to try or good links to read? Especially for:
    - when a ferrite or iron powder rod/bar goes in saturation?
    - optimal rod dimensions
    - optimal coil design (I suggest single layer, resonating with good Q
    capacitor, about 3 to 10 turns)
    - LNA design for such a low frequency?
    - antenna field theory in near-field.

    If you need further details please ask.

    Thanks in advance.

    Regards -
  2. John Larkin

    John Larkin Guest

    The propagation mode here is pure magnetic coupling, not a proper
    electromagnetic "radio" wave, because the antennas are so small
    compared to a wavelength. Dipole magnetic fields fall off with the
    cube of distance.

    A lot more turns on the rods, and resonating with a cap, will help
    some. Longer rods would help some, too, but 1/d^3 is a cruel function.

    How far do you need to go?

  3. Henry Kiefer

    Henry Kiefer Guest


    I remember 1/d^4 for a full EM-field here.

    The receiver is a WORKING time-code receiver. Working in distance at least
    2000km from the time-code transmitter with an EIRP of 30KW. The time-code
    transmitter have of course a VERY BIG antenna (120m height).
    So I'm a little confused of your capacitor idea. That is true?: The
    transmitter is NOT sending an electromagnetic wave but the same antenna
    system at the receiving end reads it as an full established EM field?

    How far: Hm, several km's if possible. Maybe I should go higher in
    What other small effective antennas work here?

    I think the problem is not the minimum turns because I tested it with an
    original ferrite rod - the same as in the original time-code receiver. It
    have a lot of turns, probably 100 or more. The same behaviour with 100 turns
    AND with 10 turns. No difference!

    What I understand of ferrrite antenna theory is:
    That the coil is simply an impedance transformer and bandpass (with a
    parallel capacitor for narrow-band reception) to couple the preamplifier to
    the antenna system (= ferrite rod).

    But I miss something. Maybe something with differences between transmitting
    and receiving with a ferrite antenna. The antenna is not pure reciprocal -
    because the ferrite (or iron powder) can be nonlinear!

    - Henry
  4. Rich Grise

    Rich Grise Guest

    Yes, a ferrite stick antenna works quite well for receivers, but not for
    transmitters. Try winding a few dozen turns around the whole room - i.e.,
    up the wall, across the ceiling, down the other wall, across the floor,
    and so on. Or, you could wrap a piece of 50-conductor ribbon cable, and
    make loops by soldering the ends together offset by 1. ;-)

    I don't know very much about antenna theory, but I know that the bigger
    the better. ;-)

    Something's telling me that it's theoretically possible to transmit with
    a ferrite stick, but from the kind of power you'd have to run through it,
    it would probably blow up. =:-O

    Good Luck!
  5. Roy Lewallen

    Roy Lewallen Guest

    1. Ferrite and powdered iron are entirely different materials, with
    different physical and magnetic characteristics. Powdered iron isn't a
    good choice for this application.
    2. You're not likely to drive either one into a nonlinear region when
    they're in the form of a rod because of the large air gap in the
    magnetic path.

    Roy Lewallen, W7EL
  6. In addition to what others have said, the most field you can
    generate with the ferrite rod antenna will occur when it is
    almost reaching saturation, and that takes a lot of ampere
    turns. You can deliver more ampere turns to the rod than
    your transmitter output can deliver if you resonate the coil
    with a capacitor. That way, you have the current bouncing
    back and forth through the capacitor added to the current
    from the amplifier. If the coil-capacitor Q is, say, 100,
    there will be 100 times more current through the coil than
    the transmitter is delivering. This will probably take a
    coil with a considerable mass of copper in it.

    With this approach, you might reach 10 meters.
  7. So there is a resonant circuit at the transmitter and not just a coil?

    With such low number of turns (and hence low inductance), the
    capacitor would have to be huge to resonate it at 77.5 kHz. Where do
    you get high Q capacitors with such capacitances ?

    The resonant circuit impedance levels are quite low in this
    configuration (small L/large C), how do you effectively couple power
    from the transmitter to this low impedance level at the resonant
    circuit ?

    The skin depth at this frequency is about 0.25 mm, so any wire thicker
    than 0.5 mm will not utilise the full copper wire, so some kind of
    Litz wire with separately insulated strands could be used to keep the
    coil resistance low.

    The inductance of some ferrites varies if there is some DC field
    present. This inductance change could detune the resonant circuit and
    drop the radiated power. Are you sure that the transmitter coil is not
    carrying any DC components or some even harmonic distortion, which
    would cause an unbalanced magnetic field in the ferrite rod ?
    The band noise is the dominant (compared to "white" amplifier) noise
    when listening to the band with your transmitter switched off, the
    receiver noise performance should be adequate.

    Paul OH3LWR
  8. Tim Shoppa

    Tim Shoppa Guest

    I can't really help you with ferrite antennas for transmitting, but can
    tell you that if you google around for "lowfer" and the Longwave Club
    of America you will find a lot about antenna
    designs that are suitable for this band. They will also might have
    recommendations for frequencies of operation that are legal for
    transmission in your home country (I don't even know what that is!)

    LNA isn't really applicable here because there is so so so much
    man-made and natural noise in this band.

    I'm a little surprised that your achieved range was so small from a
    ferrite rod antenna, actually. Did you really tune both antennas, in
    place and in circuit, for resonance? The resonance is so so super
    narrow that strays between design and circuit make a big difference. I
    mean, CRT screens with flybacks, and faulty flourescent lamp ballasts,
    and incadescent dimmers radiate all sorts of crap around the LF
    spectrum for blocks, and they aren't even trying to be intentional
    transmitters! And don't get me started about induction heaters and
    welding machines, those can be heard across several states!

  9. Henry Kiefer

    Henry Kiefer Guest

    But I miss something. Maybe something with differences between
    You made my day :)
    BTW: Your idea with the ribbon cable gives you a very easy made
    transformator if using clamping connectors. This works very good. I
    practiced it 10 years ago.

    I heart it several times that a ferrite stick antenna cannot work as a
    useful transmitter antenna. But why?????
    Sure, for reasonable antennas. But if the antenna is very VERY big in
    relation to wavelength it even cannot work! Read somewhere.
    I found no saturation state but I have not enough power at the moment to
    drive it very powerful. Something I try later ...

    Regards -
  10. Henry Kiefer

    Henry Kiefer Guest

    1. Ferrite and powdered iron are entirely different materials, with
    Powdered iron should work better because of the higher permeability even
    under heavy load in comparision to ferrite. I think so in theory - not
    Can you explain this more detailed Ron? What will happen with the air gap?
    The losses in the air gap radiates and that is the antenna function?

    - Henry
  11. Henry Kiefer

    Henry Kiefer Guest

    In addition to what others have said, the most field you can
    John, that is what I have seen! I resonated the antenna coil and driven it
    with it's resonance frequency. Seems that the achievable distance was a
    little more than the circuit without resonating capacitor.

    You say, that driving the ferrite rod into saturation will force it to leave
    more power into air? Why?

    - Henry
  12. Iwo Mergler

    Iwo Mergler Guest

    Efficient antennas at that frequency are effectively very long
    bits of wire. The ferrite rod is small compared to the wavelength
    and very inefficient at generating a far field.

    This is the antenna of the DCF77 transmitter (same frequency):

    Kind regards,

  13. Henry Kiefer

    Henry Kiefer Guest

    Have someone suggestions to try or good links to read? Especially for:
    I tested it as resonating circuit using the original time-code receiver
    antenna AND a second time without the capacitor.
    Maybe I got a little more power in the air with the resonating circuit, but
    it was not very distingiuable.
    I don't know the exact manufacturer of the time-code receiver ferrite
    antenna but I comparable model reads:
    see original data,5-DE.pdf
    It is not the same antenna but very similar.

    The original foil-capacitor is 682 labeled. I don't measured it but I think
    it should be 6800pF reading.

    For my second experiment I used no capacitor and turns=10.

    If I would find a PSPICE model for an ferrite antenna ...
    Hm. I thought here: just trying different turns value to achieve this. The
    coil is the impedance transformer for the ferrite rod (=antenna). I'm wrong
    The original coil is thinner than 0.3mm. If I compare it to my 0.3mm wire
    maybe it is 0.18mm. The second experiment with the 10 turns coil is 0.3mm
    enamelled copper wire.
    I will give Litz wire a try if the system as such works...
    Good question. I series blocked DC with a WIMA MKS4 1.0uF 100VDC
    high-quality capacitor. As measured the "big" capacitor is outside the
    bandwidth of the antenna.

    I don't think there is any DC component left. And yes, there is no magnet on
    my desk laying around :)
    Is there any internal rectifiation phanomen in the ferrite possible?
    How much band noise should I expect?

    - Henry
  14. Henry Kiefer

    Henry Kiefer Guest

    I can't really help you with ferrite antennas for transmitting, but can
    Thanks. I will look there.
    Maybe the time-code receiving IC is a bad design. I don't know. It's
    operating current is 500uA only. That is very small. It can receive the
    time-code over 2000km with such an antenna with an transmitter EIRP of 30KW.
    The receiving antenna is 700Hz bandwidth. I don't think this is super
    narrow. Even if we look at the time-code receiver quartz filter with a
    bandwidth of about 10Hz I can met it with my stable wave generator. It is a
    PLL-design with a clock quartz. Should be typical 10ppm. I don't have a very
    good frequency meter to verify it.

    In my second transmitter experiment I used a not-resonated driver design. So
    there are no problems with detuning the transmit antenna expected. It is
    just driven by the 77.5KHz power signal.

    CRT screen is off if I experiment. Otherwise I seen a very big CRT signal at
    the receiver...

    If the two ferrite rods will detune because of the close proximity I cannot
    control it. I don't think so.

    If you can hear induction heaters or something this is surely with a very
    big antenna and a resonable good receiver design.

    - Henry
  15. Guest

    Henry Kiefer wrote:
    and that takes a lot of ampere
    You misunderstood what I said. It was, " the most field you can
    generate with the ferrite rod antenna will occur when it is
    almost reaching saturation,"

    If you saturate the rod, the field you generate will have lotsof 3rd
    harmonic components in it, but little more of the fundamental. I was
    trying to emphasize that you will need as strong a magnitic field as
    possible aat the transmitting antenna, and just below saturation is
    that limit, when a ferrite core is involved.

    If the rod has a large lenght to diameter ratio (say , above 10) then I
    think the uptimum coil arrangement on the rod also doffers considerably
    for the transmitting and receiving cases, since the receiving case does
    not deal with saturation.

    In the receiving case, the end sections of the rod act as flux
    collectors, and only the middle thirs or so has almost all the
    collected flux passing through it, so this third is the optimum place
    for the coil. /in the transmitting case, the rod has a tendency to
    saturate at the center, first, with this arrangement, and you want
    essentially the whole rod to approach satuation at the same ampere
    turns. This will produce a field that acts as if it has been produced
    by the full length of the rod. You can achieve something close ot this
    by spreading the turns out, all over the rod, with an extra
    concentration (a second or third layer layer, perhaps) at the ends.
    Something like this (shown in cross section. View with fixed width
    font i.e. Courier, so charcters are on grid pattern):

    * = wire in cross section
    # = rod

    *** ***
    ****** ******
    ****** ******
    *** ***
  16. Henry Kiefer

    Henry Kiefer Guest

    I understand that. I added a second coil on the ferrite rod to measure the
    antenna current and set it just below the point where I saw harmonics (or
    say non-sinusial) waveform on the scope.
    That is a very interesting configuration. Never seen such a design. I read
    about a old-fashion remote controller system having a ferrite antenna
    transmitter. There someone wrote, the transmitter antenna was a mignon
    battery-shaped ferrite rod. e.g. much shorter but wider than mine. So an
    optimum ferrite transmitter antenna is maybe more like a fat battery shaped.

    - Henry
  17. The problem with loop antennas made of ribbon cable (or other
    multiconductor cable connected this way) is the stray capacitance
    between turns. The self resonance frequency (without external
    capacitor) may be below the band of interest, so you can not resonate
    such antenna with an external capacitor.

    Paul OH3LWR
  18. Henry Kiefer

    Henry Kiefer Guest

    The problem with loop antennas made of ribbon cable (or other
    I found it very useful for design of hard-driven Power MOSFET driver
    transformator till 400KHz without problems.

    - Henry
  19. Joel Kolstad

    Joel Kolstad Guest

    Ob-flippant remark: Sure you can, you just need a negative capacitor! ;-)

    I suppose if one actually built a negative capacitor out of, e.g., a gyrator,
    the noise performance would be pretty much shot? Might be OK for a
    transmitting antenna, though?
  20. Roy Lewallen

    Roy Lewallen Guest

    The effective permeability of a rod is dictated largely by the air gap
    in the magnetic path, which is a function of the length/diameter ratio
    of the rod. Powdered iron in general has very low permeability compared
    to ferrite. If you really wanted to apply a huge amount of power to a
    rod antenna, powdered iron might be a better choice because of its high
    saturation flux density. But I doubt you could get the Q of a ferrite
    rod antenna at the frequency in question, so it would be considerably
    less efficient. You'd probably end up with less power radiated than if
    you ran less power to a ferrite rod antenna, and a less efficient
    antenna would impact your received signal. You'd have to crunch some
    numbers or make measurements to find out for sure.
    The presence of even a small air gap has the effect of reducing the
    effective permeability of the core and therefore the inductance of the
    winding. It also dramatically reduces the core flux density for a given
    number of winding amp-turns. This makes it very hard to saturate.
    Inductors used for power applications commonly have a small core gap for
    this reason. A rod has a very large air gap in the path -- from one end
    of the rod, curving around outside the rod, to the other. And for many
    ferrites used at radio frequency, the material loss is high enough that
    the core would be hot enough to explode well before you reach a flux
    level anywhere close to saturation. This isn't true of all materials at
    all frequencies, of course.

    The radiation takes place from the field outside the core, i.e., in the
    air gap. If you didn't have a gap, you wouldn't have any significant

    And it's Roy, not Ron.

    Roy Lewallen, W7EL
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