Introduction to Radio Waves Vincent L. Fish source: Windows to the Universe (UCAR)‏ Image courtesy of NRAO/AUI

Waves Light is a transverse wave phenomenon Electric and magnetic fields are perpendicular to direction of travel (and each other)‏ Waves can be characterized by a few parameters: Amplitude A Wavelength λ (or frequency f, also called ν) Phase φ Wavelength and frequency are related λ · f = v (velocity of wave)‏ For light, v = c = x cm s x 10 8 m s x 10 5 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 ν h = x erg s = x 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 mm Frequency < 1 THz (10 12 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 unit wavelength) is at λ max = 2.9 mm / T[K] Room temperature 300 K: λ max = 10 μm (ν = 30 THz)‏ Cosmic microwave background 3 K: λ max = 1 mm (ν = 300 GHz)‏ Radio can see colder things than 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 usually polarized Some astronomical radio signals are polarized also

Radar

Doppler effect The apparent frequency of a sound wave is shifted when the source moves relative to the observer The same is true for light Source moving away: lower frequency redshift Source approaching: higher frequency blueshift 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 at specific frequencies – a “fingerprint” of the atom or molecule

Doppler effect on spectral lines Spectral lines give velocity information Rest frequency (wavelength) is known a priori – every atom and molecule has a fingerprint Measuring the frequency (wavelength) of emission or absorption line gives Doppler shift Doppler shift is proportional to velocity Direction of Doppler shift tells us whether the source is moving toward or away from 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 ( GHz rest frequency) observed with 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 frequently used for communications Example: AM and FM radio AM = amplitude modulation FM = frequency modulation Remember, even though a car radio produces sound waves, the information is broadcast using light waves (radio waves)‏ Phil Erickson will talk more about AM/FM source: Berserkus, Wikipedia

Standing waves Consider a wave travelling in a cavity (e.g., sound in an organ pipe, radio wave in a waveguide)‏ In general, allowed propagation modes will have one of two boundary conditions: node (zero) or anti-node (maximum displacement)‏ Reflection off surface causes waves to travel in both directions Peaks at multiples of λ/2 Source: Brews ohare, wikipedia

Phasing and interference Many radio stations have antenna arrays to allow directional broadcasting Altering power and phase at each antenna changes the beam (broadcasting pattern)‏ AM stations may have different beam patterns (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/AUIsource: rigel.org.uk