Download (J. Condon) Antenna theory and radiometers

Radio Telescopes and Radiometers
2015 Single Dish School
Jim Condon
NRAO, Charlottesville
Atacama Large Millimeter/submillimeter Array
Karl G. Jansky Very Large Array
Robert C. Byrd Green Bank Telescope
Very Long Baseline Array
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 viceversa (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
Single Dish School 2015 July 6
Dipole antenna
Power pattern
Single Dish School 2015 July 6
Reciprocity theorem
 The receiving and transmitting
patterns of an antenna are identical.
Single Dish School 2015 July 6
Ground-plane
vertical = ½ of a
half-wave dipole
Waveguide horn
Single Dish School 2015 July 6
The waveguide horn used to discover
λ = 21 cm HI emission from our Galaxy
Single Dish School 2015 July 6
Parabolic reflector:
directivity and
collecting area
Prime focus
Single Dish School 2015 July 6
Aperture
Single Dish School 2015 July 6
Illumination, field, and power patterns
Single Dish School 2015 July 6
Cassegrain subreflector
Single Dish School 2015 July 6
140-foot (43 m)
Cassegrain
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Reflector surface errors
Single Dish School 2015 July 6
100 m homology telescope in Effelsberg
Single Dish School 2015 July 6
GBT: homology plus active surface
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GBT Offset Gregorian + Prime Focus
for unblocked aperture
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Single Dish School 2015 July 6
GBT feeds and
radiometers
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Antenna output noise: voltage and power
TA = “antenna temperature”
Ae = effective area
S = flux density
Pν = power per unit frequency
k = Boltzmann’s constant
≈ 1.38 × 10−23 Joules per
Kelvin
Single Dish School 2015 July 6
The simplest radiometer
Single Dish School 2015 July 6
Square-law detector: output noise
voltage is proportional to input power
Single Dish School 2015 July 6
Integrator output noise for:
N = 50 samples
N = 200 samples
Single Dish School 2015 July 6
Differential radiometer
Single Dish School 2015 July 6
Superheterodyne receiver
Single Dish School 2015 July 6
Spectrometers and software-defined
digital back ends

Single Dish School 2015 July 6
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)
The National Radio Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.
Single Dish School 2015 July 6