Download CCC Module PH19510

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 (1791-1867)




Chemist, Physicist
Discovered
electromagnetic
induction
Changing current in a
one coil induces
current in another
linked coil.
Faraday’ Law
,
d B
  N
dt
Faraday and the £20 Note
James Clerk Maxwell (1831-1879)



Mathematician
Brought together work
of Faraday, Gauss,
Ampere
1864 presented
“Maxwell’s Equations”
Maxwell’s Equations
  H  0
No magnetic
monopoles
E
H  J 
#
t
Changing Electric fields
 Magnetic fields
H
  E  
t
Changing magnetic
fields
 Electric fields
  E  
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
  H  0
No magnetic
monopoles
E
H  J 
#
t
Changing Electric fields
 Magnetic fields
H
  E  
t
Changing magnetic
fields
 Electric fields
  E  
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
c
1
 0 0
Heinrich Rudolf Hertz


1857 – 1894
1887 Proved
Maxwell’s proposition
that Electromagnetic
Waves can travel
over distance
Hertz’s Experiment
1 – Primary circuit
Capacitor
•Current flows
through coil
• magnetic field in
core
• Interrupter Opens
•Current dies away
• magnetic field
dies away
•Interrupter Closes
•Current Flows
•(repeat forever)
Battery
Primary Coil
Interrupter
Core
Secondary
Coil
Hertz’s Experiment
2 – Secondary Circuit
Antennae
Plates





Spark Gap
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
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 (1856-1943)



1893-1895
Investigated high
frequency Currents
Produced reliable
radio Frequency
generator
‘Narrow’ frequency
band
Guglielmo Marconi (1874-1937)


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

Antenna

Coil
Variable
Capacitor
Diode
Capacitor
Ground
Earpiece


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:
Transmitted  Carrier  (1  ModulationDepth  Signal )
Amplitude Modulation #1

Carrier Waveform
0.951
1
0.5
Carrier( tim )
0
0.5
0.951
1
0
0
1 10
5
2 10
3 10
5
tim
5
4 10
5
5
5 10
5

5 10
Amplitude Modulation #2

Signal Waveform
1
1
0.5
Signal( tim )
0
0.5
1
1
0
0
1 10
5
2 10
3 10
5
tim
5
4 10
5
5
5 10
5

5 10
Amplitude Modulation #3

Transmitted Waveform
Transmitted  Carrier  (1  ModulationDepth  Signal )
1.423
1.5
Modulation Depth x 2
1
0.5
Amp( tim )
0
0.5
1
1.423 1.5
0
0
1 10
5
2 10
3 10
5
tim
5
4 10
5
5
5 10
5

5 10
Demodulation of AM Signals

1.423
1.5

1
Recovered( tim )

0.5
0
0
0
0
1 10
5
2 10
3 10
5
tim
5
4 10
5
5
5 10
5

5 10
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