Why does my circuit radiate?Why does my circuit radiate?

Presented byPresented by

Paul EdwardsPaul Edwards

OutlineOutline

IntroductionIntroduction

Storage Fields – Electric and MagneticStorage Fields – Electric and Magnetic

Moving Charges & Radiating Fields Moving Charges & Radiating Fields

AntennasAntennas

Application to Circuits Application to Circuits

IntroductionIntroduction

By normal circuit theory a potentialBy normal circuit theory a potential

difference or voltage exists, whichdifference or voltage exists, which

causes charge, or current, to flowcauses charge, or current, to flow

around the circuit.around the circuit.

The amount of current which flows isThe amount of current which flows is

dependant on the value of thedependant on the value of the

resistor. ( resistor. ( V = I * R )V = I * R )

What happens to the resistor? What happens to the resistor?

It gets hot ! & Radiates heat ….It gets hot ! & Radiates heat ….

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2Ω

What happens when What happens when the switch closes?the switch closes?

What is heatWhat is heat ??

Heat is energy.

Heat energy travels in waves like other forms of energy, and can change the matter it comes into contact with.

The heat energy we actually measure or feel can be either radiated into or conducted through matter and “excite” the material structure causing

What is heatWhat is heat ??In the resistor example……

As current flows, a.k.a free electrons, the flow is resisted by thephysical structure. The electrons collide with other particles, giving upsome of their energy. Because energy is conserved, the energy thatwas moving the electrons forward is converted to heat energy.

What is the physical nature of Heat Energy?

Infrared: we often think of this as being the same thing as 'heat'

Hotter, more energetic objects create a higher level of energyradiation than cool objects. We use this phenomena to detect hotobjects using an infrared Camera.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

InfraRed - Is EM energy InfraRed - Is EM energy radiated from an objectradiated from an object

As is Radio Frequency As is Radio Frequency energy, Light energy… and energy, Light energy… and beyond. beyond.

So… Is my simple resistor circuit So… Is my simple resistor circuit radiating?radiating?

Yes..Yes..

In fact most electrical circuits radiate. AsIn fact most electrical circuits radiate. Asenergy is consumed / exchanged to performenergy is consumed / exchanged to performa function, there will be losses ina function, there will be losses inthe form of heat (radiation) * the form of heat (radiation) *

( * EM waves of frequency 10( * EM waves of frequency 101212 Hz ) Hz )

Radio Frequency EM FieldsRadio Frequency EM Fields

Understanding EM Fields and Antennas for capturing andUnderstanding EM Fields and Antennas for capturing andradiating waves are primary objectives for an RF Engineer.radiating waves are primary objectives for an RF Engineer.

But … For the electrical engineer designing a functionalBut … For the electrical engineer designing a functionalElectronic circuit, having a solid knowledge of this topic isElectronic circuit, having a solid knowledge of this topic isbecoming of great importance, but is often not wellbecoming of great importance, but is often not wellunderstood or neglected and labeled as “Black Magic”understood or neglected and labeled as “Black Magic”

The following slides are an attempt to aid yourThe following slides are an attempt to aid yourunderstanding of EM Fields, Antenna Radiation and Whyunderstanding of EM Fields, Antenna Radiation and Whysome circuits behave badly ! some circuits behave badly !

Mathematics – don’t you just Mathematics – don’t you just ♥♥ it it

Amperes Law I=Amperes Law I=∫∫ c c H·H·ddL = L = ∫∫ s s J·J·dds s

Faradays Law V=Faradays Law V=∫∫ c c E·E·ddL = - L = - ∫∫ s s (∂B/ ∂t) ·(∂B/ ∂t) ·dds s

Gauss for E-fields Gauss for E-fields ∫∫ s s D·D·dds = 0s = 0

Gauss for M-fields Gauss for M-fields ∫∫ s s B·B·dds = 0s = 0

In 1873 – yep over 120 years ago….In 1873 – yep over 120 years ago….

James Clerk Maxwell (1831-1879)James Clerk Maxwell (1831-1879)

Unified Electromagnetic theory – Unified Electromagnetic theory – He assembled the laws of Ampere, He assembled the laws of Ampere, Faraday & Gauss together and added another term into Amperes Law. Faraday & Gauss together and added another term into Amperes Law.

Amperes Law IAmperes Law Itottot = ∫ = ∫ c c H·H·ddL = ∫ L = ∫ s s {J+∂D/ ∂ t}·{J+∂D/ ∂ t}·dds s

Faradays Law V=∫ Faradays Law V=∫ c c E·E·ddL = - ∫ L = - ∫ s s (∂B/ ∂t) ·(∂B/ ∂t) ·dds s

Gauss for E-fields ∫ Gauss for E-fields ∫ s s D·D·dds = 0s = 0

Gauss for H-fields ∫ Gauss for H-fields ∫ s s B·B·dds = 0s = 0

DISPLACEMENT CURRENT DENSITY

( current that flows elsewhere )

““Physics, is essentially an Physics, is essentially an intuitive and concrete science. intuitive and concrete science. Mathematics is only a means Mathematics is only a means for expressing the laws that for expressing the laws that govern phenomena." govern phenomena."

Albert Einstein - Albert Einstein -

So let’s just talk Physics – and cut out this Math stuff

EM FieldsEM Fields

Storage FieldStorage Field

Stores energy in the vicinity of the Stores energy in the vicinity of the sourcesourceField can only exist within a few Field can only exist within a few λλ of a conducting structureof a conducting structureIt will collapse once the energy It will collapse once the energy source is removedsource is removedThe field can be dynamic or staticThe field can be dynamic or staticThe field can exist exclusively as The field can exist exclusively as an electric or magnetic field and an electric or magnetic field and as a combined E & H field.as a combined E & H field.Can be called a reactive fieldCan be called a reactive fieldNear FieldNear Field

Radiating FieldRadiating Field

Propagates energy away from the Propagates energy away from the source source The field will propagate forever!The field will propagate forever!

It will continue to propagate even It will continue to propagate even after the source is removedafter the source is removedThe field can only be a dynamic The field can only be a dynamic wavewaveThe field can only exist exclusively The field can only exist exclusively as a combined E & H Field .as a combined E & H Field .Can be called a reactive fieldCan be called a reactive fieldFar FieldFar Field

Electromagnetic fields can be described as either:Electromagnetic fields can be described as either:

Circuits containing a storage fieldCircuits containing a storage field

Magnetic fieldMagnetic fieldSimple ideal inductor circuit Simple ideal inductor circuit driven by an AC source.driven by an AC source.

An E-field must exist to drive An E-field must exist to drive charge, and constitute a charge, and constitute a currentcurrent

The source pumps energy into The source pumps energy into the H field established by the the H field established by the flow of current.flow of current.

As the field decays it returns As the field decays it returns energy to the circuit.energy to the circuit.

This energy cycling is This energy cycling is responsible for Voltage / responsible for Voltage / Current phase relationshipCurrent phase relationship

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Reactive FieldsReactive Fields

Bring another circuit with an Bring another circuit with an inductor in proximity to source inductor in proximity to source circuit.circuit.

Magnetic field couples to the Magnetic field couples to the other inductor and induces a other inductor and induces a current in the load circuit current in the load circuit

(Faradays Law of EM induction)(Faradays Law of EM induction)

There is a loss of energy from There is a loss of energy from the source circuit and a gain in the source circuit and a gain in the load circuit, without hard the load circuit, without hard physical connection.physical connection.

Here the storage field is called Here the storage field is called a reactive field because it a reactive field because it “reacts” with other devices “reacts” with other devices within its field. It stores and within its field. It stores and transfers energy. transfers energy.

a.k.a – a transformera.k.a – a transformer

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Circuits containing a storage fieldCircuits containing a storage field

Reactive FieldsReactive Fields

Similarly the Capacitor Similarly the Capacitor can store energy, can store energy, transfer energy or do transfer energy or do both, in the Electric both, in the Electric field that exists field that exists between charged between charged conductorsconductors

Electrical LengthElectrical Length

The concept of “electrical length” is needed to understand antennas and The concept of “electrical length” is needed to understand antennas and radiating systemsradiating systems

Electrical length is the ratio of actual physical length to wavelengthElectrical length is the ratio of actual physical length to wavelength

Electrical length = Electrical length =

For example, a For example, a λλ/2 /2 dipole – its physical length is half a wavelength – will radiatedipole – its physical length is half a wavelength – will radiate

exactly the same amount of power regardless of physical length.exactly the same amount of power regardless of physical length.

100 Hz100 Hz λλ/2 = 1.5 x 10/2 = 1.5 x 1066 m (1500 km) m (1500 km)

100 kHz100 kHz λλ/2 = 1.5 x 10/2 = 1.5 x 1033 m ( 1.5 km) m ( 1.5 km)

100MHz100MHz λλ/2 = 1.5 m ( ~ 4.5 feet)/2 = 1.5 m ( ~ 4.5 feet)

1GHz1GHz λλ/2 = 1.5 x 10/2 = 1.5 x 10-1-1 m (150 mm) m (150 mm)

Electrical LengthElectrical Length

Electrical lengths that are even fractional multiples of wavelength Electrical lengths that are even fractional multiples of wavelength can make good Antennas – e.g. can make good Antennas – e.g. λλ/2 , /2 , λλ/4, /4, λλ/8… /8… λλ/20 but cannot /20 but cannot radiate as much power.radiate as much power.

From a practical standpoint – it is obvious why practical radio From a practical standpoint – it is obvious why practical radio systems use higher frequencies. e.g Cell phone (GHz), FM systems use higher frequencies. e.g Cell phone (GHz), FM Radio (MHz) Radio (MHz)

Circuits that RadiateCircuits that Radiate

Two ideal antenna examples. Loop & Dipole of electrical length Two ideal antenna examples. Loop & Dipole of electrical length λλ/2./2.

Energy is not stored – but propagates away to infinity. This energy lossEnergy is not stored – but propagates away to infinity. This energy loss

appears like a resistance to the source. But why do they do it?appears like a resistance to the source. But why do they do it?

L= λ/2L = λ/2

Static ChargeStatic ChargeFor simple static charge (electron) the electric field forms a radial For simple static charge (electron) the electric field forms a radial pattern from the centre of the charge. Conventionally the field lines pattern from the centre of the charge. Conventionally the field lines are outward for a positive charge and inward for a negative charge.are outward for a positive charge and inward for a negative charge.

E-fields cause action at a distance – small at great distances.E-fields cause action at a distance – small at great distances.

+ -∞

There is no magnetic field associated with a static charge

Bringing unlike charges togetherBringing unlike charges together

Bring unlike charges together and the fields Bring unlike charges together and the fields

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Moving charges => moving fieldsMoving charges => moving fields

A moving charged particle (constant velocity) carries its field A moving charged particle (constant velocity) carries its field wherever it goes and will always look the same as the static casewherever it goes and will always look the same as the static case

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Accelerating charges – changing fieldsAccelerating charges – changing fields As a charge accelerates its fields start to bend - but will catch up As a charge accelerates its fields start to bend - but will catch up eventually. In the meantime the field is disturbed, changed.eventually. In the meantime the field is disturbed, changed.

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Acceleration – d(velocity)/d(time)

Charge U’ies – “Feel the Force Luke!” Charge U’ies – “Feel the Force Luke!”

When charge is accelerated back and forth – constant U-turns.When charge is accelerated back and forth – constant U-turns.

The moving charge field exerts a force on the surrounding static The moving charge field exerts a force on the surrounding static field. The field is disturbed, energy is exchanged to the field. The field is disturbed, energy is exchanged to the surroundings, and a moving field propagates from the source surroundings, and a moving field propagates from the source

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Changing Electric and Magnetic FieldsChanging Electric and Magnetic Fields

Primarily we have just shown how electric field changes when a Primarily we have just shown how electric field changes when a charge is in motion. How does the magnetic field vary?charge is in motion. How does the magnetic field vary?A static charge has no magnetic field, but a moving charge has A static charge has no magnetic field, but a moving charge has constant magnetic field, and an accelerating charge has a changing constant magnetic field, and an accelerating charge has a changing magnetic field.magnetic field.

As the charge accelerates/decelerates back and forth, the magnitude of magnetic field changes. Maximum at maximum velocity, zero at rest.

Ideal Antenna circuitsIdeal Antenna circuits

Dipole – consider an AC source applied to the dipole. Charge in the Dipole – consider an AC source applied to the dipole. Charge in the dipole will be accelerated back and forth along the dipole. There will dipole will be accelerated back and forth along the dipole. There will be a different charge distribution at any point at any instant of time.be a different charge distribution at any point at any instant of time.

Putting it all together - Putting it all together -

Here is a Java Simulation showing the variation and propogation of EM Here is a Java Simulation showing the variation and propogation of EM Fields due to moving charge, from an AC source.Fields due to moving charge, from an AC source.

http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html

ReferencesReferences