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 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 ) What happens to the resistor? It gets hot ! & Radiates heat …. + - i VR2ΩR2Ω What happens when the switch closes?

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 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 ( ) Unified Electromagnetic theory – He assembled the laws of Ampere, Faraday & Gauss together and added another term into Amperes Law. Amperes Law I tot = ∫ 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 - Albert Einstein - So let’s just talk Physics – and cut out this Math stuff

EM Fields 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 Electromagnetic fields can be described as either:

Circuits containing a storage field Magnetic field Simple ideal inductor circuit driven by an AC source. An E-field must exist to drive charge, and constitute a current 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 i VL

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) (Faradays Law of EM induction) 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 R i V i

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 Hzλ/2 = 1.5 x 10 6 m (1500 km) 100 kHzλ/2 = 1.5 x 10 3 m ( 1.5 km) 100MHzλ/2 = 1.5 m ( ~ 4.5 feet) 1GHzλ/2 = 1.5 x 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

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 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.

References