L1 - Introduction to Radio Astronomy & Radio Physics

Question 1

What is the principle of emission by astronomical objects?

Answer 1

Interaction of electromagnetic wave with charged particles.

Question 2

What are the two types of astronomical objects emission?

Answer 2

Continuum emission: acceleration or deceleration (braking) of charged particles (electrons, protons).

Line emission: transition of an electron between two bound states in an atom or molecule. Can occur when an;
• Electron in hydrogen atom flips its spin (21 cm line).
• Electron changes energy state in atom (mostly hydrogen after recombination of proton and electron in plasma: recombination line).

Question 3

What are the Physical processes that produce radio emission?

Answer 3

• Thermal emission (Bremsstrahlung)
• Synchrotron emission

Question 4

Describe Thermal emission (Bremsstrahlung).

Answer 4

Produced by the random thermal motions of electrons passing by a charged particle.
At radio frequencies: the intensity, I = B(λ,T) ≈ 2kT/λ².

Question 5

Describe Synchrotron emission.

Answer 5

Deflection of electrons in a magnetic field:
• Electrons spiral around the magnetic field lines and emit a pulse whenever their motion is directed towards us.
• Superposition of contributions by all electrons in the plasma ((thermal) distribution of speeds) gives: continuous spectrum (decreases with frequency, high frequency cut off, linear polarization).

Question 6

What different types of antenna exist?

Answer 6

There are 5 different types of antenna:
• Dipoles (Narrowband: each dipole = half wavelength of detected electromagnetic radiation).
• Horn antenna (Broadband: needs to be larger than wavelength).
• Yagi-Uda antenna (Narrowband: width = half wavelength, length = anything, of detected electromagnetic radiation).
• Helix Antenna (Array) (Moderately broadband: circumference = wavelength of detected electromagnetic radiation).
• Parabolic dishes (Wideband: diameter > 10λ of detected electromagnetic radiation).