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## Electrically Initiated Blasting and Electromagnetic Fields

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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
• 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
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
• 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
and the electric, (and also magnetic), fields
oscillate in a plane perpendicular to the
direction of propagation, (and are perpendicular
to each other).

7
• 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
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.
• 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)
• 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
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
• 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.