Chaos, Communication and Consciousness Module PH19510 Lecture 6 Radio Days

Overview Theoretical Beginnings  Faraday & Maxwell First Transmission  Hertz Practical Systems  Marconi, Tesla, Braun Transmitting Information  CW  Amplitude Modulation

Michael Faraday ( ) Chemist, Physicist Discovered electromagnetic induction Changing current in a one coil induces current in another linked coil. Faraday’ Law,

Faraday and the £20 Note

James Clerk Maxwell ( ) Mathematician Brought together work of Faraday, Gauss, Ampere 1864 presented “Maxwell’s Equations”

# Maxwell’s Equations Changing magnetic fields  Electric fields Changing Electric fields  Magnetic fields No magnetic monopoles Electric Charges  Electric Fields

Divergence  · Scalar quantity Measure of ‘outgoingness’ of field at a point Flow across boundary of infinitesimally small sphere at point. Expansion  +ve divergence Contraction  -ve divergence

Curl  × Vector  Rate of rotation of a field  Points along axis of rotation  Right hand rule Example  Uniform rotation  Constant curl  Points into page

# Maxwell’s Equations Changing magnetic fields  Electric fields Changing Electric fields  Magnetic fields No magnetic monopoles Electric Charges  Electric Fields

Consequence of Maxwell’s Equations Electric and Magnetic Forces are linked Wave nature of electromagnetism Showed waves moved at speed of light Suggested light was form of electromagnetic wave 1873 Suggested propogation of EM waves

The Electromagnetic Spectrum

Heinrich Rudolf Hertz 1857 – Proved Maxwell’s proposition that Electromagnetic Waves can travel over distance

Hertz’s Experiment 1 – Primary circuit Battery Primary Coil Core Interrupter Capacitor Current flows through coil  magnetic field in core  Interrupter Opens Current dies away  magnetic field dies away  Interrupter Closes Current Flows (repeat forever)

Hertz’s Experiment 2 – Secondary Circuit Many turns in secondary coil Changing field in primary coil & core  Very high voltage in secondary (10-20kV) Sparks Radiating Electromagnetic field from antennae plates Secondary Coil Spark Gap Antennae Plates

Hertz’s Experiment 3 – Receiver Electromagnetic field induced in receiver loop (Tiny) sparks appear in gap

Need to make communications system Spark transmitter produces broad band output  Splash across many frequencies  Due to rapid edge of spark Tiny sparks only visible in dark

The coherer Tube filled with silver/nickel filings Particles stick together (cohere) under influence of electric field Allows large current to flow Unstick particles mechanically (hammer) Primitive amplifier

Nikola Tesla ( ) Investigated high frequency Currents Produced reliable radio Frequency generator ‘Narrow’ frequency band

Guglielmo Marconi ( ) 1896 Demonstrates wireless telegraphy 1901 Tuned Radio  Resonance  Coherer Extensive use in ship/shore comms

The Cat’s Whiskers Karl Braun  1874 Discovered point contact semiconductor junction  1898 Cat’s Whisker  Simple Rectifier  Lead Sulphide  Phosphor Bronze spring contact

CW Modulation Continuous Wave Radio Frequency Carrier Turn carrier On & Off Transmit information by length & timing of On/Off periods Morse code Simple Digital

Simple Radio Reciever Voltage induced in antenna by EM wave Variable Capacitor & primary coil select required freq. Diode & capacitor remove RF Signal heard in earpiece

Amplitude Modulation - AM Transmit analogue signals – speech/music Same as Frequency Division Multiplexing Change amplitude of carrier depending on signal Multiplication process:

Amplitude Modulation #1 Carrier Waveform

Amplitude Modulation #2 Signal Waveform

Amplitude Modulation #3 Transmitted Waveform Modulation Depth x 2

Amplitude Modulation #4 Look at transmitted in terms of frequency f 0 is carrier Multiplication results in f 0 +f s and f 0 -f s Frequency f0f0f0f0 f 0 - f s f 0 + f s

Amplitude Modulation #5 Real signal is complex mix of waveforms Produces two sidebands Mustn’t splash into adjacent channel Frequency f0f0f0f0 Upper Sideband Lower Sideband

Demodulation of AM Signals Rectify signal with diode Remove carrier with low-pass filter Signal remains

Overview Theoretical Beginnings  Faraday & Maxwell First Transmission  Hertz Practical Systems  Marconi, Tesla, Braun Transmitting Information  CW  Amplitude Modulation