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Introduction to Radio WavesVincent L. Fish
source: Windows to the Universe (UCAR) Image courtesy of NRAO/AUI
Waves
Light is a transverse wave phenomenonElectric and magnetic fields are perpendicular to direction of travel (and each other)
Waves can be characterized by a few parameters:Amplitude AWavelength λ (or frequency f, also called ν)Phase φ
Wavelength and frequencyare related
λ · f = v (velocity of wave)
For light, v = c =2.99792458 x 1010 cm s-1
2.99792458 x 108 m s-1
2.99792458 x 105 km s-1
Energy of light
Quantum mechanics tells us that we can think of light as a particle
Energy of a photon: E = h νh = 6.626068 x 10-27 erg s = 6.626068 x 10-34 J s (Planck's constant)
We can equivalently talk about wavelength, frequency, or energy
Radio waves
Radio waves are the longest wavelength (lowest frequency, lowest energy) portion of the electromagnetic spectrum
Wavelength > 0.3 mmFrequency < 1 THz (1012 cycles s-1)
No sharp cutoff to definition of “radio” – usually defined by hardware/technique or atmosphere
Radio
1 THz1 GHz1 MHz
Blackbody radiation
Thermal emission shows a characteristic spectrum
Peak of emission (per unitwavelength) is atλ
max = 2.9 mm / T[K]
Room temperature300 K: λ
max = 10 μm
(ν = 30 THz)
Cosmic microwavebackground3 K: λ
max = 1 mm
(ν = 300 GHz)
Radio can see colder thingsthan can other wavelengths
source: Giro720, wikipedia
Rayleigh-Jeans approximation valid
“Classical” behavior
Planck's law required
Atmospheric transmission
The atmosphere is transparent to most radio waves
Water vapor is a problem at millimeter wavelengths – requires going to very dry (usually very high) site
Polarization
Polarization can be linear, circular, or elliptical
The two linear polarization modes (horizontal and vertical) are orthogonal – if you have two crossed polarizers, no light will get through
The two circular modes (left and right circular) are orthogonal
However, linear and circular modes are not orthogonal
Terrestrial radio signals are usuallypolarized
Some astronomical radio signalsare polarized also
Radar
Doppler effect
The apparent frequency of a soundwave is shifted when the sourcemoves relative to the observer
The same is true for light
Source moving away:lower frequencyredshift
Source approaching:higher frequencyblueshift
Image courtesy Windows to the Universe
Doppler effect
If v << c, ∆λ/λ ~ ∆ν/ν ~ v/c
Rule of thumb: The fractional Doppler shift in frequency (or wavelength) is the speed in relativistic units (c = 1)
source: TxAlien, Wikipedia
Applications of Doppler effect
The Doppler shift gives the line-of-sight velocity of an object
Examples:radar (reflection off moving object)exoplanet detection (through reflex motion of star)
Spectral lines
Energy levels of atoms and molecules are quantized
Energy can only be absorbed or emitted atspecific frequencies – a “fingerprint” of theatom or molecule
Doppler effect on spectral lines
Spectral lines give velocity information
Rest frequency (wavelength) is knowna priori – every atom and molecule hasa fingerprint
Measuring the frequency (wavelength)of emission or absorption line givesDoppler shift
Doppler shift is proportional to velocity
Direction of Doppler shift tells us whetherthe source is moving toward or awayfrom us
At radio frequencies, the frequency (thus, velocity) can be measured very precisely
Not moving
Moving toward receiver
Moving away from receiver
frequency
Mapping Galactic structure
Milky way has fairly well known rotation curve – far away material is Doppler shifted, and line-of-sight velocity can be used as proxy for distance
Map of galaxy in CO (115.271203 GHz rest frequency) observed with 0.0005 GHz resolution (1.3 km s-1)
CO is abundant in molecular clouds, which trace the spiral arms of the Milky Way – these stand out in the longitude/velocity plot
Dame, Hartmann, & Thaddeus (2001)
Communications
Radio waves are frequentlyused for communications
Example: AM and FM radio
AM = amplitude modulation
FM = frequency modulation
Remember, even thougha car radio produces soundwaves, the information is broadcastusing light waves (radio waves)
Phil Erickson will talk more about AM/FM
source: Berserkus, Wikipedia
Standing wavesConsider a wave travelling in a cavity(e.g., sound in an organ pipe,radio wave in a waveguide)
In general, allowed propagation modeswill have one of two boundary conditions:node (zero) oranti-node (maximum displacement)
Reflection off surface causes wavesto travel in both directions
Peaks at multiples of λ/2
Source: Brews ohare, wikipedia
Phasing and interference
Many radio stations have antenna arraysto allow directional broadcasting
Altering power and phase at each antennachanges the beam (broadcasting pattern)
AM stations may have different beampatterns (and power) for day and night
KMTI photo: smeter.net reception maps: radio-locator.com
Interferometers
Radio astronomy uses arrays of telescopes called interferometers
These are analogous to phased broadcasting arrays, except that they receive radiation
Altering the delays (phases) between telescopes changes the reception pattern
Image courtesy of NRAO/AUI source: rigel.org.uk