Why must a radio dish surface be smooth to a fraction of the wavelength?

Bumps and sag in the reflector scatter signal away from the focus, lowering efficiency. The Ruze relation shows the loss grows steeply once surface errors reach roughly a tenth of the observing wavelength, which is why millimetre-wave dishes need surfaces accurate to tens of microns.

Why do some radio telescopes use arrays of dipoles instead of dishes?

At long wavelengths a dish would have to be impractically large, and beams can instead be formed electronically by combining many simple fixed dipole antennas with the right phases. This gives a steerable, reconfigurable telescope with no moving parts, ideal for low-frequency surveys.

Radio Telescope Antennas

Radio telescope antennas are the reflectors and feeds that intercept incoming radio waves and concentrate them onto a receiver, setting the collecting area, beam shape, and frequency range of a radio telescope.

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Definition

A radio telescope antenna is the structure, typically a reflecting dish or array of elements, that captures radio-frequency radiation and couples it to a receiver, characterised by its effective collecting area, beam pattern, and operating frequency band.

Scope

This topic covers parabolic dish reflectors and their feed and subreflector arrangements, dipole and phased-array antennas for long wavelengths, beam patterns and sidelobes, aperture efficiency and surface accuracy, and the structural and pointing demands of large steerable and fixed antennas.

Core questions

How does antenna size and surface accuracy set the resolution and highest usable frequency?
What distinguishes dish reflectors from dipole and phased arrays?
What are beam patterns, gain, and sidelobes?
How is aperture efficiency defined and maximised?

Key theories

Antenna beam and the reciprocity of pattern: An antenna's response on the sky, its beam, is the Fourier transform of the aperture illumination, so larger and more uniformly illuminated apertures give narrower beams and higher resolution.
Aperture efficiency and surface accuracy: Deviations of the reflector surface from an ideal paraboloid scatter signal out of the beam, and the Ruze relation shows efficiency falls sharply once surface errors approach a tenth of the wavelength.
Phased arrays for low frequencies: At long wavelengths fixed dipole elements are combined electronically into beams, allowing flexible, steerable apertures without moving structures, as used in modern low-frequency arrays.

Clinical relevance

Antenna design fixes the sensitivity, frequency coverage, and resolution of every radio facility; the surface accuracy of large dishes determines whether a telescope can reach the millimetre and submillimetre bands where cold gas and dust radiate.

History

Reber's backyard parabola of 1937 established the steerable dish, and ever-larger dishes followed, from Jodrell Bank to the 100-metre Effelsberg and Green Bank telescopes and the fixed 305-metre Arecibo and 500-metre FAST reflectors. Phased dipole arrays have revived low-frequency radio astronomy.

Key figures

Grote Reber
John D. Kraus

Seminal works

wilson2013
kraus1986

Frequently asked questions

Why must a radio dish surface be smooth to a fraction of the wavelength?: Bumps and sag in the reflector scatter signal away from the focus, lowering efficiency. The Ruze relation shows the loss grows steeply once surface errors reach roughly a tenth of the observing wavelength, which is why millimetre-wave dishes need surfaces accurate to tens of microns.
Why do some radio telescopes use arrays of dipoles instead of dishes?: At long wavelengths a dish would have to be impractically large, and beams can instead be formed electronically by combining many simple fixed dipole antennas with the right phases. This gives a steerable, reconfigurable telescope with no moving parts, ideal for low-frequency surveys.