Why does my circuit radiate?

download report

Transcript Why does my circuit radiate?

Why does my circuit radiate?
Presented by
Paul Edwards
Outline
Introduction
Storage Fields – Electric and Magnetic
Moving Charges & Radiating Fields
Antennas
Application to Circuits
Introduction
What happens when
the switch closes?
i
V
+
-
By normal circuit theory a potential
difference or voltage exists, which
causes charge, or current, to flow
around the circuit.
The amount of current which flows is
dependant on the value of the
resistor. ( V = I * R )
R
2Ω
What happens to the resistor?
It gets hot ! & Radiates heat ….
What 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 heat ?
In the resistor example……
As current flows, a.k.a free electrons, the flow is resisted by the
physical structure. The electrons collide with other particles, giving up
some of their energy. Because energy is conserved, the energy that
was 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 energy
radiation than cool objects. We use this phenomena to detect hot
objects using an infrared Camera.
The Electromagnetic Spectrum
InfraRed - Is EM energy
radiated from an object
As is Radio Frequency
energy, Light energy… and
beyond.
So… Is my simple resistor circuit
radiating?
Yes..
In fact most electrical circuits radiate. As
energy is consumed / exchanged to perform
a function, there will be losses in
the form of heat (radiation) *
( * EM waves of frequency 1012 Hz )
Radio Frequency EM Fields
Understanding EM Fields and Antennas for capturing and
radiating waves are primary objectives for an RF Engineer.
But … For the electrical engineer designing a functional
Electronic circuit, having a solid knowledge of this topic is
becoming of great importance, but is often not well
understood or neglected and labeled as “Black Magic”
The following slides are an attempt to aid your
understanding of EM Fields, Antenna Radiation and Why
some circuits behave badly !
Mathematics – don’t you just ♥ it
Amperes Law I=∫ c H·dL = ∫ s J·ds
Faradays Law V=∫ c E·dL = - ∫ s (∂B/ ∂t) ·ds
Gauss for E-fields
∫ s D·ds = 0
Gauss for M-fields
∫ s B·ds = 0
In 1873 – yep over 120 years ago….
James Clerk Maxwell (1831-1879)
Unified Electromagnetic theory – He assembled the laws of Ampere,
Faraday & Gauss together and added another term into Amperes Law.
Amperes Law Itot = ∫ c H·dL = ∫ s {J+∂D/ ∂
t}·ds
Faradays Law V=∫ c E·dL = - ∫ s (∂B/ ∂t) ·ds
Gauss for E-fields
∫ s D·ds = 0
Gauss for H-fields
∫ s B·ds = 0
DISPLACEMENT CURRENT DENSITY
( current that flows elsewhere )
“Physics, is essentially an intuitive and
concrete science. Mathematics is only
a means for expressing the laws that
govern phenomena."
Albert Einstein -
So let’s just talk Physics – and cut out this Math stuff
EM Fields
Electromagnetic fields can be described as either:
Storage Field
Stores energy in the vicinity of the
source
Field can only exist within a few λ
of a conducting structure
It will collapse once the energy
source is removed
The field can be dynamic or static
The field can exist exclusively as
an electric or magnetic field and
as a combined E & H field.
Can be called a reactive field
Near Field
Radiating Field
Propagates energy away from the
source
The field will propagate forever!
It will continue to propagate even
after the source is removed
The field can only be a dynamic
wave
The field can only exist exclusively
as a combined E & H Field .
Can be called a reactive field
Far Field
Circuits containing a storage field
Simple ideal inductor circuit
driven by an AC source.
Magnetic field
An E-field must exist to drive
charge, and constitute a
current
i
V
L
The source pumps energy into
the H field established by the
flow of current.
As the field decays it returns
energy to the circuit.
This energy cycling is
responsible for Voltage /
Current phase relationship
Reactive Fields
Bring another circuit with an
inductor in proximity to source
circuit.
Magnetic field couples to the
other inductor and induces a
current in the load circuit
(Faradays Law of EM induction)
i
V
i
R
There is a loss of energy from
the source circuit and a gain in
the load circuit, without hard
physical connection.
Here the storage field is called
a reactive field because it
“reacts” with other devices
within its field. It stores and
transfers energy.
a.k.a – a transformer
Circuits containing a storage field
Reactive Fields
Similarly the Capacitor
can store energy,
transfer energy or do
both, in the Electric
field that exists
between charged
conductors
Electrical Length
The concept of “electrical length” is needed to understand antennas and
radiating systems
Electrical length is the ratio of actual physical length to wavelength
Electrical length =


For example, a λ/2 dipole – its physical length is half a wavelength – will radiate
exactly the same amount of power regardless of physical length.
100 Hz
λ/2 = 1.5 x 106 m
(1500 km)
100 kHz
λ/2 = 1.5 x 103 m
( 1.5 km)
100MHz
λ/2 = 1.5 m
( ~ 4.5 feet)
1GHz
λ/2 = 1.5 x 10-1 m (150 mm)
Electrical Length
Electrical lengths that are even fractional multiples of wavelength
can make good Antennas – e.g. λ/2 , λ/4, λ/8… λ/20 but cannot
radiate as much power.
From a practical standpoint – it is obvious why practical radio
systems use higher frequencies. e.g Cell phone (GHz), FM Radio
(MHz)
Circuits that Radiate
Two ideal antenna examples. Loop & Dipole of electrical length λ/2.
Energy is not stored – but propagates away to infinity. This energy loss
appears like a resistance to the source. But why do they do it?
L = λ/2
L= λ/2
Static Charge
For simple static charge (electron) the electric field forms a radial
pattern from the centre of the charge. Conventionally the field lines
are outward for a positive charge and inward for a negative charge.
E-fields cause action at a distance – small at great distances.
+
∞
-
There is no magnetic field associated with a static charge
Bringing unlike charges together
Bring unlike charges together and the fields
+
-
Moving charges => moving fields
A moving charged particle (constant velocity) carries its field
wherever it goes and will always look the same as the static case
+
Accelerating charges – changing fields
As a charge accelerates its fields start to bend - but will catch up
eventually. In the meantime the field is disturbed, changed.
+
+
+
Acceleration –
d(velocity)/d(time)
Charge U’ies – “Feel the Force Luke!”
When charge is accelerated back and forth – constant U-turns.
The moving charge field exerts a force on the surrounding static
field. The field is disturbed, energy is exchanged to the
surroundings, and a moving field propagates from the source
+
Changing Electric and Magnetic Fields
Primarily we have just shown how electric field changes when a
charge is in motion. How does the magnetic field vary?
A static charge has no magnetic field, but a moving charge has
constant magnetic field, and an accelerating charge has a changing
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 circuits
Dipole – consider an AC source applied to the dipole. Charge in the
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.
Putting it all together Here is a Java Simulation showing the variation and propogation of EM
Fields due to moving charge, from an AC source.
http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html
References