The ionosphere and radio wave
propagation
P. J. Erickson
Atmospheric Sciences Group
MIT Haystack Observatory
RET Workshop
Haystack Observatory
July 11, 2011
Electromagnetic Waves
• Electromagnetic spectrum has a huge
range of wavelengths or frequencies
• Wavelengths and frequencies are
related by the propagation speed of the
EM wave (= c in vacuum)
• Visible is just a narrow range - familiar
because humans have built-in sensors
• Radio waves are electromagnetic waves
- NOT sound waves - and span a huge
range of wavelengths : sub-mm to
kilometers long
• AM radio = 1 MHz = 300 meter
wavelength
• FM radio = 100 MHz = 3 meter
wavelengths
• Digital TV ranges from meter to 10 cm
scale wavelengths
To receive a radio wave, you
need an antenna and some
electronics …
Antennas come in many sizes,
depending on the wavelength
you wish to receive:
2450 MHz antenna
(wavelength: 12.2 cm / 4.82 in)
802.11b/g wireless internet
To receive a radio wave, you
need an antenna and some
electronics …
Antennas come in many sizes,
depending on the wavelength
you wish to receive:
VHF / UHF antenna
(wavelength at 300 MHz: 1 meter / 33.6 in)
Television, FM radio
To receive a radio wave, you
need an antenna and some
electronics …
Antennas come in many sizes,
depending on the wavelength
you wish to receive:
HF antenna
(wavelength at 15 MHz: 20 meters / 65.5 feet)
Shortwave radio
To receive a radio wave, you
need an antenna and some
electronics …
Antennas come in many sizes,
depending on the wavelength
you wish to receive:
MF antenna
(wavelength at 1030 kHz: 291 meters / 955 feet)
AM radio
To receive a radio wave, you
need an antenna and some
electronics …
Antennas come in many sizes,
depending on the wavelength
you wish to receive:
VLF antenna
(wavelength at 40 kHz: 7,494 meters / 4.65 miles)
Naturally generated waves from the upper atmosphere;
submarine communications
Sometimes
antennas can be very big….
CUPRI portable radar
50 MHz frequency
(wavelength:
6 meters; 19.67 feet)
SuperDARN HF auroral radar – 3 to 15 MHz frequency
(wavelength at 10 MHz: 29.9 meters; 98.2 feet)
Millstone Hill Observatory
• 150 foot steerable antenna
• 220 foot zenith antenna
Making upper
atmosphere
measurements using
radio waves (in a radar
configuration) since
1960..
Frequency = 440 MHz
(wavelength: 68 cm;
26.8 inches)
Arecibo radio telescope / ionospheric radar – 430 MHz frequency
(wavelength: 69.7 cm; 27.4 in); 300 m (1000 ft) reflector
Radio waves (EM waves) carry information
(of human or other origin)
Audio modulations (AM, FM)
Digital TV frequency spectrum
Information is transmitted by using a range of frequencies around a center frequency
The medium of propagation may act differently on these ranges of frequencies (dispersion)
Radio wave travel can be altered by the medium
Snell’s law predicts directions of reflection and refraction for a medium
with a non-zero index of refraction
Refracted wave
Change in index of refraction
Reflected wave
Incident wave
Sin i / Sin r = h’ / h
h = sqrt(m e)
m = magnetic permeability
e = dielectric constant
Analogy: optical refraction at air/water interface
Millstone Hill Radar
Radio waves refract .. Just like light
Transcontinental propagation affected
“Twinkling” causes data loss
Ionosphere: the charged portion of a planet’s upper atmosphere.
Composed of plasma.
Plasma: partially ionized gas in which electrons are free rather than bound to an atom or molecule.
Electromagnetic forces are therefore important (in addition to gravity and atomic forces).
The vast majority of the universe is in a plasma state.
The ionosphere is a continuous plasma medium which interacts with radio waves
(has a non-zero index of refraction)
Its index of refraction depends most importantly on ionospheric electron density
Marconi
Over-the-horizon communications are
made possible by “sky wave” propagation
paths due to refraction effects
The ionosphere has vertical as well as horizontal structure
Seasonal, day/night, solar cycle changes...
Variations in the ionosphere from day to night (and from space
weather) result in different indexes of refraction, and therefore
different propagation paths
(N. Atkins, Lyndon State College)
In particular, AM radio propagation is vastly different from day to night