Propagation of Electromagnetic Waves

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Propagation of Electromagnetic Waves

• Rua Stob
• Warnell School of Forest Resources
• 28 January 2004

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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

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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.

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Single-stage transmitter
Capacitors
Resistors
Power Source
Crystal
Graphic by (Land 1999)
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Single-stage transmitter
Inductors
Transistor
Graphic by (Land 1999)
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Single-stage transmitter
Tank Circuit
Graphic by (Land 1999)
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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)
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Two-stage transmitter

• Unlike one-stage design, pulse generation is more
  separated from radio frequency production

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Multivibrator pulse designs

• Pulse is generated separately from radio
  frequency so neither interfere with each other

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Comparing transmitter types
Adapted from Wildlife Radio-telemetry Standards
for Components of British Columbia's Biodiversity
No. 5 (1998)
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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)
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The Electromagnetic Spectrum
Lawrence Berkeley National Laboratory 2003
Radio telemetry
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Frequency bands of the world
142,148-151,173, 230 MHz
216 MHz
40, 150-151, 165, 216-220 MHz
150-151, 165 MHz
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Why these frequencies?

• Government allocation
• Antenna efficiency
• _at_ 142-230 MHz
• Small receiver antenna
• Small (efficient) transmitter antenna
• Signal absorption vs. antenna size

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How waves travel through the environment

• Low frequencies travel best through water

• Higher frequencies have a slight advantage in
  moist tropical forests

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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

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Diffraction

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

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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

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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.

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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

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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/