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loop antennas

Discussion in 'Electronic Design' started by garyr, Jun 13, 2012.

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  1. garyr

    garyr Guest

    I posted this a few days ago but inadvertently attached it to an old thread
    so nobody saw it, at least there were no replies. I'm hoping for better
    results this time.


    This site http://www.frontiernet.net/~jadale/Loop.htm states that: "A
    properly designed Loop primarily responds to the magnetic component of the
    radio wave. Note that noise resides primarily in the electrical
    component..."

    Whereas this site shows that that is not the case:
    http://vk1od.net/antenna/shieldedloop.

    So what is the advantage, if any, of a shielded loop antanna?

    Consider three receivers:

    1) Shielded loop antenna, receiver with differential input (center-tapped
    transformer or instrumentation amp). The two ends of the inner conductor
    of the antenna connected to the differential inputs and the shield
    connected to ground.

    2) Same as above but without the shield.

    3) Unshielded loop antenna, receiver with single-ended input.
    One end of the loop connected to the receiver input and the other to
    ground.

    Assuming equal gain and bandwidth, would there be any
    difference in the sensitivity or noise level at the output of the three
    receivers?
     
  2. Tim Williams

    Tim Williams Guest

    Not necessarily. That's true either for plane waves, or the more general
    long wave case. Near fields can have widely varying ratios of E and M,
    other than Zo. For example, Tesla coils generate a heck of a lot of volts,
    but the magnetic field doesn't extend very far, so they make terrible
    antennas.

    A loop for low frequencies is only going to intercept a small, local portion
    of a wave, and more importantly, the fields that are interfering are most
    likely near fields. Best example is electric induction from 60Hz line
    noise, which is typically unbalanced (0-120).

    Tim
     
  3. WoolyBully

    WoolyBully Guest

    Go back to huge, sharp EM spikes and Morse code. Probably pick it up
    around the world.

    Pretty annoyingly slow data rate though. The bomb would be hitting
    right about the time you get the first line of the warning message
    completed.

    I even have a name for it.

    BullyIT®
     
  4. miso

    miso Guest

    Wellbrook claims their antennas favor the magnetic part of the EM wave.
    All I can say is they work great and they cost too much money. I have
    the original ALA100. You add your own wire loop. It doesn't need to be
    shielded.

    You can DF with them at low frequencies (beacons for example). At
    higher frequencies they are more omni. They are quite directional in the
    BDB.

    The old Kiwa loop (truly overpriced) wasn't shielded either. Since Kiwa
    stopped making them, the price went crazy.

    The Wellbrook works much better in the comparison test I made, but Kiwa
    has it's fans.
     
  5. Joerg

    Joerg Guest

    And then one day the antenna is gone, on account of that nice shiny
    copper pipe ...
     
  6. Guest

    You got a problem with Darwin?
     
  7. garyr

    garyr Guest

    Yes. Many thanks to all respondents.
     
  8. josephkk

    josephkk Guest

    I then presume that you were using them as broadband antenna. My
    experience is with AM BCB receivers where they were resonated for
    selectivity and image rejection.

    ?-)
     
  9. garyr

    garyr Guest

    Coax is good: http://www.febo.com/time-freq/wwvb/antenna/
     
  10. Fred Abse

    Fred Abse Guest

    Not strictly true. The E field and H field are in phase, else
    no net energy would flow. They are *spatially* at right angles, and
    mutually at right angles to the direction of energy propagation.

    A simple dipole antenna responds to E field only, hence it could be said to
    "shield" (actually discriminate) against the magnetic component. Align it
    to the H field, you get nada (unless the E field is circularly polarized,
    in which case you can't).

    Fields in quadrature, where the energy returns each half-cycle,
    are what give rise to the "near field" component, which does not radiate
    energy. Simple induction.
     
  11. Fred Abse

    Fred Abse Guest

    Get a Tek 'scope with LCD display. "Universal Test Signal" right there.
     
  12. Tim Williams

    Tim Williams Guest

    Except that there's quite a bit of current in the middle of the dipole, and
    if that part is aligned across the H field, you get magnetic induction.

    E&M being what it is, this occurs at the same conditions the E field line up
    in. Samey samey. You can't say it's picking up one or the other because
    it's always doing both or neither.

    Tim
     
  13. Jamie

    Jamie Guest

    yup, I was going to comment about that earlier,. It seems that you
    can't have one with out the other. It's like Evil and Good!


    Jamie
     
  14. Tim Williams

    Tim Williams Guest

    Except E and M is always Good and Good :)

    Tim
     
  15. Guest

    Except when you're doing compliance testing. ;-)
     
  16. Guest

    The real reason for using small "magnetic" loops on low frequencies is
    due to how they behave with some nearby interference sources.

    In the far field from the interface source, both the E and H fields
    drop relative to 1/r and power density relative to 1/r² and the free
    space impedance is 377 ohms.

    However, in the near field (less than 1 wavelength or less than 1/6
    wavelengths depending of document) from the interference source, the
    situation is much more complex and the impedance varies greatly within
    this region. Going closer to the source, the magnetic field is
    proportional to 1/r² and very close to the source the electric field
    is proportional to /r³.

    On VHF/UHF, the near field extends to centimeters or decimeters and
    are of interest e.g. for parasitic element design in an Yagi antenna.

    However, on MF/LF/VLF etc. the near field from the interference source
    extends to tens or hundreds of meters, thus within the house and
    nearby streets.

    When using an E-field antenna, the distant preferred signal is
    received with approximately constant strength. However, when moving
    the receiving antenna closer to the local interference source, the
    interface voltage increases relatively to 1/r³ and is summed to the
    wanted signal, quickly reducing the SNR to useless.

    However, when using M-field antenna, the interference is increasing
    relative to 1/r², thus, the SNR is better _deep_ in the near filed
    than with an equivalent E-field antenna. For LF we are talking about
    interference sources within the house.

    ----

    Compare this with a small LF/MF E-field antenna, e.g. a 1 m vertical
    whip connected to a high impedance (1 Mohm) built in preamplifier.

    The extremely short whip is very reactive, thus the high impedance
    amplifier is needed to convert it to something like 50/75 ohms, in
    order to be used with coaxial cables and ordinary receiver inputs.

    Within a room with electric wiring in the walls, there can be quite
    high capacitances from the wiring to objects within the room, at least
    1-10 pF, possibly even more. Of course, coupling an oscilloscope probe
    with 1 Mohm input to some floating metallic object on the table, will
    produce a large 50/60 Hz hum display on the screen.

    These days with lot of high frequency noise in the mains wiring, the
    stray capacitance to the E-antenna whip would be in the same order of
    magnitude in the LF band as the amplifier input impedance, thus
    forming a 50:50 voltage divider and hence coupling a lot of electric
    noise to the E-antenna, thus making it more or less useless for indoor
    reception.

    The shielded magnetic loop is not affected by this capacitively
    coupled noise.

    Of course any _significant_ high frequency _current_ would connect to
    the magnetic loop through induction, but the high frequency noise
    current in domestic wiring is typically not very large. The situation
    is of course different near large VFDs driving big motors.
    A metal structure will pick some of the local interference source
    E-field, unless it is perfectly balanced ("antenna effect"). The
    shield helps keeping out the E-field interference, when only the
    M-field is wanted.
    All (V)LF and MF transmitter stations are vertically polarized. Since
    it would be impractical to erect 1/4 wave towers on LF, top hat
    capacitance loading is often used. In practice, several smaller towers
    are used, suspending an elevated network mesh of wires between them
    and the mesh is then connected with a vertical wire to the
    transmitter. The actual vertically polarization radiation occurs from
    this vertical line.
     
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