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Propagation of Electromagnetic Waves

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Tank circuits, transmitter designs, and antennas. What to signal to produce ... Capacitor - Stores charge (like a battery) ... Multivibrator pulse designs ... – PowerPoint PPT presentation

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Title: Propagation of Electromagnetic Waves


1
Propagation of Electromagnetic Waves
  • Rua Stob
  • Warnell School of Forest Resources
  • 28 January 2004

2
Overview
  • How to produce a signal
  • Tank circuits, transmitter designs, and antennas
  • What to signal to produce
  • Electromagnetic spectrum and telemetry frequency
  • Interaction with objects and the atmosphere

3
First a few simple definitions
  • Capacitor - Stores charge (like a battery)
  • Inductor Device that can create and store an
    electric field
  • Resistor Component with a predetermined
    resistance
  • Transistor - A transistor regulates current or
    voltage flow and acts as a switch or gate for
    electronic signals
  • Tank Circuit - Parallel resonant circuit with
    only a coil and a capacitor. Both store energy
    for part of each cycle.

4
Single-stage transmitter
Capacitors
Resistors
Power Source
Crystal
Graphic by (Land 1999)
5
Single-stage transmitter
Inductors
Transistor
Graphic by (Land 1999)
6
Single-stage transmitter
Tank Circuit
Graphic by (Land 1999)
7
How does it work?
  • Circuit resonates with crystal to give off a
    radio frequency and charge a large capacitor to
    generate signal pulses
  • Charging the pulse capacitor at rate determined
    by the resistor creates a pulse
  • The tank circuit then broadcasts the signal from
    the antenna

Description modified from (Kenward 2001)
8
Two-stage transmitter
  • Unlike one-stage design, pulse generation is more
    separated from radio frequency production

9
Multivibrator pulse designs
  • Pulse is generated separately from radio
    frequency so neither interfere with each other

10
Comparing transmitter types
Adapted from Wildlife Radio-telemetry Standards
for Components of British Columbia's Biodiversity
No. 5 (1998)
11
Antenna types (for transmitters)
  • Loop antenna
  • - Used with collars
  • - Shorter range but useful for species that
    would chew or pull on a whip.
  • Whip antenna
  • Most commonly used
  • More uniform signal over greater distances

Adapted from Wildlife Radio-telemetry Standards
for Components of British Columbia's Biodiversity
No. 5 (1998)
12
The Electromagnetic Spectrum
Lawrence Berkeley National Laboratory 2003
Radio telemetry
13
Frequency bands of the world
142,148-151,173, 230 MHz
216 MHz
40, 150-151, 165, 216-220 MHz
150-151, 165 MHz
14
Why these frequencies?
  • Government allocation
  • Antenna efficiency
  • _at_ 142-230 MHz
  • Small receiver antenna
  • Small (efficient) transmitter antenna
  • Signal absorption vs. antenna size

15
How waves travel through the environment
  • Low frequencies travel best through water
  • Higher frequencies have a slight advantage in
    moist tropical forests

16
Waves on the move
  • All electromagnetic waves travel at the speed of
    light in a vacuum
  • Because waves travel at very close to this speed
    through the atmosphere (300x106m/s)
  • Wavelength (m) 300 / Frequency (MHz)
  • Wavelength affects how strongly a signal is
    diffracted in the environment

17
Diffraction
  • The effects of diffraction are strongest when the
    wave length is similar to object or gap it is
    passing through

18
Bouncing off and traveling through
  • Reflection
  • The angle of incidence the angle of reflection
  • Refraction
  • When the wave enters another medium its speed,
    direction and wavelength all change

19
Why does all this matter?
  • In radio-telemetry to get a (relatively) accurate
    location you need line of sight (a clear path
    between the transmitter and the antenna)
  • Understanding how the transmitter interacts with
    the environment is essential in recording
    accurate locations and estimating error.

20
Summary
  • How to produce a signal
  • Tank circuits, transmitter designs, and antennas
  • What to signal to produce
  • Electromagnetic spectrum and telemetry frequency
    (40-230MHz)
  • Interaction with objects and the atmosphere
  • Diffraction, reflection, and refraction

21
Selected sources
  • Kenwood, R. E. (2001). Basic Equipment. In A
    Manual for Wildlife Radio Tagging. 16-40.
    Academic press, London.
  • Land, B.L. (1999). Miniature telemetry
    transmitter. Projects in Neurobiology and
    Behavior. http//www.nbb.cornell.edu/neurobio/land
    /PROJECTS/TRANSMIT/
  • Lawrence Berkeley National Laboratory (2003). The
    Electromagnetic Spectrum. http//www.lbl.gov/Micro
    Worlds/ALSTool/EMSpec/EMSpec2.html
  • Ministry of Environment, Lands, and Parks
    Resources Inventory Branch (1998). Wildlife
    Radio-telemetry Standards for Components of
    British Columbia's Biodiversity No. 5.
    http//srmwww.gov.bc.ca/risc/pubs/tebiodiv/wildlif
    eradio/
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