Atacama Large Millimeter/submillimeter Array

Karl G. Jansky Very Large ArrayRobert C. Byrd Green Bank Telescope

Very Long Baseline Array

Radio Telescopes and Radiometers2015 Single Dish School

Jim CondonNRAO, Charlottesville

Radio Telescopes and Antennas• An antenna is any device that

converts electromagnetic radiation traveling through space to electrical currents flowing in a wire (receiving antenna) or vice-versa (transmitting antenna).

• Radio telescopes, and only radio telescopes, contain antennas.

• Most of a typical radio telescope is not an antenna − the big dish just redirects electromagnetic radiation to the antenna part.

Single Dish School 2015 July 6

Electromagnetic radiation is produced by accelerating charged particles

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Dipole antenna Power pattern

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Reciprocity theorem

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The receiving and transmitting patterns of an antenna are identical.

Ground-plane Waveguide horn vertical = ½ of a half-wave dipole

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The waveguide horn used to discover λ = 21 cm HI emission from our Galaxy

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Parabolic reflector:directivity and collecting area

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Prime focus

Aperture

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Illumination, field, and power patterns

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Cassegrain subreflector

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140-foot (43 m)Cassegrain

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Reflector surface errors

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100 m homology telescope in Effelsberg

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GBT: homology plus active surface

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GBT Offset Gregorian + Prime Focusfor unblocked aperture

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Single Dish School 2015 July 6

GBT feeds andradiometers

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Antenna output noise: voltage and power

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TA = “antenna temperature”Ae = effective area

S = flux densityPν = power per unit frequencyk = Boltzmann’s constant ≈ 1.38 × 10−23 Joules per Kelvin

The simplest radiometer

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Square-law detector: output noise voltage is proportional to input power

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Integrator output noise for: N = 50 samples N = 200 samples

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Differential radiometer

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Superheterodyne receiver

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Spectrometers and software-defined digital back ends

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The National Radio Astronomy Observatory is a facility of the National Science Foundation

operated under cooperative agreement by Associated Universities, Inc.

To learn more about radio astronomy, Google Essential Radio Astronomy http://www.cv.nrao.edu/course/astr534/ERA.shtml or see the printed book (4 copies are on reserve)

Single Dish School 2015 July 6