The Radio Window
Earth's atmosphere is transparent to radio waves from about 10 MHz to 300 GHz, creating a broad window through which we can observe the cosmos. Radio telescopes exploit this window using parabolic dishes that focus incoming radiation onto a feed antenna. The dish's beam pattern — its angular sensitivity profile — determines both resolution and sensitivity, governed by the same diffraction physics that limits optical telescopes.
Beam Pattern Physics
The primary beam of a circular aperture follows an Airy pattern: a central lobe surrounded by concentric sidelobes of decreasing intensity. The half-power beamwidth scales as 1.22 λ/D, meaning a 25 m dish at the 21 cm hydrogen line resolves about 3.5 arcminutes. The sidelobe structure matters for rejecting interference from off-axis sources, especially terrestrial radio frequency interference (RFI).
Gain and Sensitivity
Antenna gain measures how effectively the dish concentrates sensitivity into the main beam compared to an isotropic antenna. Gain scales as the square of D/λ, so doubling the dish diameter quadruples the gain (a 6 dB improvement). Aperture efficiency η accounts for real-world losses: illumination taper, feed spillover, surface roughness, and structural blockage. Typical well-designed dishes achieve 55-65% efficiency.
From Jansky to FAST
Karl Jansky's 1932 discovery of cosmic radio emission using a rotating antenna launched radio astronomy. The field progressed from war-surplus radar dishes to purpose-built instruments: the 76 m Lovell Telescope (1957), the 305 m Arecibo dish (1963-2020), and China's 500 m FAST (2016). Each generation pushed dish size, surface accuracy, and receiver sensitivity, revealing pulsars, quasars, the CMB, and the large-scale structure of the universe.