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Electrically Initiated Blasting and

Electromagnetic Fields

- I.M.E. Fall Meeting 2004
- Technical Committee
- 19 October 2004

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

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.

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

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

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).

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?.

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

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.

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.

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

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.

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.

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.

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.)

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.)

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.)

The Safe Distance Equations

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.

That Concludes this Broadcast