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Electrically Initiated Blasting and Electromagnetic Fields

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H Magnetic Field Strength Amperes/Meter. c Speed of ... J current density Ampere/Meter2. charge density Coulomb/Meter2. Maxwell's Equations ... Ampere's Law ... – PowerPoint PPT presentation

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Title: Electrically Initiated Blasting and Electromagnetic Fields


1
Electrically Initiated Blasting and
Electromagnetic Fields
  • I.M.E. Fall Meeting 2004
  • Technical Committee
  • 19 October 2004

2
Outline of Topics
  • The physics of the field around a
    current-carrying conductor
  • Background of electric, magnetic and
    electromagnetic fields
  • James Clerk Maxwell's equations of waves in free
    space
  • The wave equation and wave propagation
  • Radiating and non-radiating fields
  • Near Field and Far Field, (Fraunhofer) Effects
  • High current, low frequency power transmission
    lines
  • The reactive near-field
  • Transmitting and receiving antennas
  • Definitions
  • Units
  • Antenna Configurations
  • Blasting circuits as receiving antennas
  • Safe Distance equations and parameters affecting
    safe distance

3
The Physics of the Fields around a
Current-Carrying Conductor
  • Background
  • The term field refers to the mathematical
    description of the forces created between
    charges. There are three fields of interest from
    DC to microwave, electric, magnetic and
    electromagnetic (radiation) force fields. Fields
    are not physical things but mathematical
    descriptions of influences that fields have over
    free space that occur over a distance.
  • The Electric, (Coulomb), field results from
    uneven charge distribution. The charge
    distributions may be static or dynamic. Electric
    fields go hand-in-hand with voltage difference
    between two physical points.
  • The Magnetic field results from moving charge,
    for example, current flow in a conductor.
  • The Electromagnetic field, (radiation), results
    from accelerating charge, i.e., when charge
    changes speed or direction.
  • Antennas have all three fields associated with
    them.
  • The stationary, moving or accelerating charge of
    concern is the mass of free electrons in the
    current carrying elements of an antenna.

4
Definitions of Terms
  • Term Definition Units (MKS)
  • E Electric Field (Electric Force per
    unit Volts/Meter
  • Charge)
  • B Magnetic Field (Field of Influence Tesla (1
    Tesla104 Gauss in cgs units)
  • as a result of charge in motion)
  • H Magnetic Field Strength Amperes/Meter
  • c Speed of Light 3x108 Meter/Second
  • e0 permittivity of free space (how a
    medium 8.8542 x10-12 Coulomb2/Newton Meter2
    changes to absorb energy in an EM field)
  • µ0 permeability of free space (response of
    a 4?x10-07 Newtons/Ampere2
  • medium to a magnetic field)
  • J current density Ampere/Meter2
  • ? charge density Coulomb/Meter2

5
Maxwell's Equations
  • James Clerk Maxwell's Equations in differential
    form
  • Gauss' Law for Electricity
  • The electric flux out of any closed surface is
    proportional to the total charge enclosed within
    that surface
  • ?? E ?/e0 4? k ? where k1/4?e0
    or Coulomb's Constant
  • Gauss' Law for Magnetism
  • The net magnetic flux out of any closed surface
    is zero
  • ? ? B 0
  • Faraday's Law of Induction
  • The line integral of the electric field around a
    closed loop is equal to the negative of the rate
    of change of the magnetic flux through the area
    enclosed by the loop or is equal to the generated
    voltage in the loop.
  • ? x E - ?B/?t
  • Ampere's Law
  • In the case of a static electric field, the line
    integral of the magnetic field around a closed
    loop is proportional to the electric current
    flowing through the loop.
  • ? x B µ0 J (1/c2) ?E/?t where c2
    1/µ0e0

6
The Wave Equation and Wave Propagation in Free
Space
  • From the third of the previous four equations we
    take the curl of both sides of the equation
  • ? x (? x E ) - (?/?t) (?x B)
  • From substituting equation four of the previous
    four equations where J 0 into the above and
    solving
  • ?2E -µ0e0 ?2E/?t2 where c
    (1/µ0e0)½
  • The solution of the equation is the simple wave
    equation showing that the propagation of
    electromagnetic radiation is transverse, (TEM),
    and the electric, (and also magnetic), fields
    oscillate in a plane perpendicular to the
    direction of propagation, (and are perpendicular
    to each other).

7
Radiating and Non-Radiating Fields
  • Field Regions
  • The volume of space surrounding an antenna
    consists of two or three distinct regions
    depending on the nature of the electromagnetic
    field produced by the antenna.
  • Far Field or Fraunhoffer Region
  • The far field region of a radiating antenna is
    the region far enough from the source that only
    the radiating field components are significant.
    The electric and magnetic fields decay inversely
    with distance from the source, the energy is
    equally distributed between the electric and
    magnetic fields and the field components are
    orthogonal. Power density decays with the inverse
    square of the distance from the source. Angular
    field distribution is independent of distance
    from the antenna.
  • Radiating Near Field or Fresnel Region
  • The radiating field predominates, but the
    non-radiating fields are not insignificant. The
    outer boundary is approximated by R 2D2/?,
    where D is the largest dimension of the antenna.
    Interference between different parts the antenna
    is significant. The angular field distribution is
    dependent on distance from the antenna.
  • Reactive Near Field
  • The non-radiating electric and magnetic fields
    dominate. For electrically small antennas, the
    region is either predominately electric or
    predominately magnetic. R ?/2?.

8
Antenna Basics
  • Any conductor will radiate at any frequency. The
    purpose for the variety of different antenna
    shapes is to control the radiation pattern.
  • Insulators can also radiate electromagnetic
    energy.
  • Antenna Gain
  • The radiated power of any antenna attached to a
    transmitter of constant power output is constant.
    An isotropic antenna radiates uniformly in all
    directions or has a gain of unity. By changing
    the shape of the radiation pattern, the radiation
    can be concentrated in preferred directions,
    hence achieve antenna gains larger than unity.
  • Antenna Aperture
  • The portion of a plane surface normal to the
    direction of propagation near a radiating antenna
    through which most of the radiation passes.
  • Effective Radiated Power
  • ERP Power Input to Antenna X Antenna Gain

9
High Current, Low Frequency Power Transmission
and Distribution Lines
  • Fields surrounding 60 Hertz power transmission
    lines
  • The frequency of the alternating current sent
    through transmission and distribution lines is
    50-60 Hertz, thus the wavelength is greater than
    5000 kilometers, (c f ?), making the power line
    a poor transmitter of radiation. The near field
    extends out very far and the non-radiant electric
    and magnetic fields decay rapidly with distance
    from the power lines.
  • The significant fields are the non-radiating
    electric and magnetic fields
  • Transmission Voltage E Field _at_ 30 meters B
    Field _at_ 30 meters
  • (volts) (Volts/Meter) (milliGauss) (1µT 10
    mG)
  • 115,000 0.07 1.7
  • 230,000 0.30 7.1
  • 500,000 1.0 12.6
  • Since power lines have opposing, separated
    currents, EM fields are produced that diminish
    with the inverse square of distance.
  • The radiative component is so small, a 500
    Megawatt power line will radiate approximately 1
    milliwatt per 10 kilometer length _at_ 60 Hertz.

10
Transmitting and Receiving Antennas
  • Antenna Polarization
  • Also referred to as wave polarization, it is the
    orientation of the electric flux lines, (not the
    magnetic flux), in an electric field. The best
    transmission of RF occurs when both the receiving
    and transmitting antennas have the same
    polarization. When the receiving and transmitting
    antennas are at right angles to each other, the
    least efficient coupling is the result. Some
    antenna systems use circular or elliptical
    polarization where the electric flux lines rotate
    either in a clockwise or counterclockwise
    orientation with each wave cycle. These antennas
    are commonly used for satellite uplink or
    downlink communications. The antennas look like a
    "coil spring" with a back reflector.
  • The Antenna as a Reciprocal Device
  • Antennas receive as well as transmit
    electromagnetic energy. They work both ways with
    equal validity.

11
Units pertaining to RF Transmission
  • Parameter Units
  • Antenna Power or watts or milliwatts (mw)
  • Effective Radiated Power
  • Antenna Gain dimensionless or dBm (reference 1
    mw) or
  • dBi (reference isotropic antenna, (Gain1)
  • Beamwidth The angle to the direction of the
    main lobe of the antenna where the power is
    -3 dB down. A measure of the antenna's
    directivity.
  • Bandwidth The measure of how the frequency to
    the antenna can be varied and obtain
    acceptable performance

12
Antenna Configurations
  • Reference Antenna
  • Radiation pattern is isotropic, radiates equally
    in all directions, Gain 1, it is a reference
    for all other antenna types with a gain of dBi
    0. It is a theoretical model only and does not
    exist other than as a mathematical baseline.
  • Dipole
  • A horizontal long wire of length of some multiple
    of ?/2, generally center fed, with a gain of 2.14
    dBi. The antenna is horizontally polarized. An
    EED with its legwires separated is a dipole
    receiving antenna.
  • Monopole with Ground Plane Reflector
  • A center-fed dipole that is vertically mounted
    with the lower half removed and a ground plane
    reflector substituted in its place. Physically,
    the antenna is a vertical mast perpendicular to
    the ground with the direction of polarization
    being vertical. It is the common mobile
    transmitting antenna configuration and the common
    mobile vehicular-mounted receiving antenna. Gain
    for a (1/4)? is 5.16 dBi. The AM broadcast band
    transmitting antennas are of such a configuration
    with the antenna height being 75 meters tall for
    AM transmission.

13
Antenna Configurations (cont'd)
  • Small circular loop
  • When the loop is oriented so that the loop is
    parallel with the ground the polarization is
    horizontal with a gain from -2 to 2 dBi
  • Parabolic Dish
  • Has the same polarization as the antenna
    feedline. Gain is 20 to 30 dBi. Highly
    directional, used for a variety of applications
    at frequencies from 400 MHz to 13 GHz.
  • Yagi
  • Commonly used as an outside antenna for TV and FM
    reception. Consists of a reflector and one or
    more directors, the Yagi is highly directional
    with a gain of 5 to 15 dBi. Frequency ranges from
    50 MHz to 2 GHz.
  • Horn
  • A high gain antenna used on cellular telephone
    repeater towers and microwave relays, frequency
    range is commonly 40 GHz to 50 GHz. Gain is 5 to
    20 dBi.

14
Blasting Circuits as Receiving Antennas
  • Detonator "no-fire" power level
  • Sensitivity to induced voltages can vary greatly
    with the detonator geometry, materials, and the
    presence of elements which will dissipate energy.
    This data is obtained by testing the device using
    a statistical test method such as a Bruceton
    "up-down" test. Since RF power is not as
    efficient in heating a bridgewire than direct
    application of DC to the legwires, the DC
    "no-fire" data is a conservative approach to
    predicting safe levels of RF energy.
  • "Worst Case" Analysis
  • Franklin Applied Physics models the receiving
    antenna, (blasting circuit), as the most
    effective antenna possible. That would be a shot
    line geometry if a vertically polarized
    transmitter is nearest the "hellbox or blasting
    machine" and a person picks up one leg of the
    shot line five feet above ground forming an
    isosceles triangle with the legwire of 7.35E02
    cm perimeter and 2.32E04 cm2 loop area.
  • From prior studies, a 40 milliwatt "no-fire"
    power level was used to represent the typical 1
    ohm, 1 amp, 1 watt "all-fire" detonator. This was
    and still is the basis for the tables in IME
    SLP-20. If some other value may be the case, it
    is the responsibility of the detonator's
    manufacturer to determine the electrical
    characteristics of the device.
  • Nearby Reflective Surfaces
  • A blasting circuit may be located in the vicinity
    of a large flat metal reflective surface. A
    reflection coefficient on one is assumed for a
    conservative calculation.

15
Safe Distance Equations from RF Sources to
Blasting Circuits the Associated Parameters
  • The detonator circuit is modeled as a receiving
    antenna, (recalling the reciprocal nature of
    antennas), with the receiving antenna pattern
    pointed toward the RF source. The receiving
    antenna is located in such a way that the maximum
    amount of power is dissipated in the load,
    (detonator).
  • In the case of AM broadcast band transmission, ,
    (0.54 MHz to 1.6 MHz),
  • The receiving antenna is a "small loop", (small
    electrical size compared to the wavelength of the
    transmission). The worst case is used where
    someone picks up one leg of the shotline to an
    elevation of five feet above the ground. Using 20
    AWG shotline and the usual constants for the wire
    and a 40 milliwatt 'no-fire" current for the
    detonator, Table 1 values in SLP-20 are computed.
    Since the safe distance increases with frequency,
    the high end frequency of 1.6 MHz is used. An
    antenna gain of 10 is assumed. (Refer to Equation
    1.)

16
Safe Distance Equations from RF Sources to
Blasting Circuits the Associated Parameters
(cont'd)
  • The case of Medium to High Frequency Fixed
    Vertical Transmitters up to 50 MHz other than AM
    Broadcast Band Transmitters
  • The same equation is used that was used for AM
    sources for transmitters up to 50 MHz with an
    antenna gain of 10. This simulates an operation
    in the vicinity of a shortwave broadcasting
    antenna or medium wave amateur operations, (80,
    40, 20 meter bands). The worst case frequency is
    approximately 22.8 MHz. Corresponds to Table 2 in
    IME SLP-20. (Refer to Equation 1.)
  • The case of low-end Medium Frequency Mobile
    Transmitters from 1.7 MHz to 3.4 MHz
  • A frequency of 3.0 MHz with an antenna gain of
    1.6 is assumed. The transmitting antenna is a
    whip antenna as commonly found on mobile
    transmitters in vehicles. A reflection
    coefficient of unity is assumed in the event that
    the blasting circuit is located near a perfect
    reflecting surface. Corresponds to IME SLP-20
    Table 3,Column 2. (Refer to Equation 2.)

17
Safe Distance Equations from RF Sources to
Blasting Circuits the Associated Parameters,
(cont'd)
  • The case of High Frequency through UHF, (28 MHz
    and above)
  • For this case, the electrical dimensions of the
    receiving antenna, (blasting circuit), are large
    in comparison to wavelength of the RF
    transmission. This is the basis for IME SLP-20
    Table 3, Columns 3 through 6. This service
    includes amateur, marine, public service,
    railroad and aircraft communication. (Refer to
    Equation 3.)
  • The case of VHF TV, UHF TV, and FM Broadcasting
  • This case applies to transmitters with
    horizontally polarized radiation through a mast
    antenna of known height. This is the basis for
    the safe distance Table 4, column 2 (Channels 2
    to 6), column 3 (FM Radio), and column 4
    (Channels 7 to 13), and Table 5 (UHF TV Channels
    14 to 69, maximum ERP is 5,000,000 watts), in
    SLP-20. A mast height of 2000 feet, (610 meters),
    is assumed with an antenna gain of 1. (Refer to
    Equation 4.)

18
The Safe Distance Equations
19
Bibliography
  • "Safety Guide for the Prevention of Radio
    Frequency Hazards in the Use of Commercial
    Electric Detonators", Institute of Makers of
    Explosives, SLP 20, July 2001.
  • Stutzman, W. L., Thiele, G. A., "Antenna Theory
    and Design", 2nd edition, Wiley, 1998.
  • "Electromagnetic Radiation Theory", Franklin
    Applied Physics, April 2001.
  • "IEEE Recommended Practice for Determining Safe
    Distances from Radio Frequency Transmitting
    Antennas When using Electric Blasting Caps during
    Explosive Operations", IEEE Std C95.4?-2002.

20
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