RADIO ASTRONOMY
Before 1931, to study astronomy meant to study the
objects visible in the night sky. Indeed, most
people probably still think that’s what astronomers
do—wait until dark and look at the sky using
their naked eyes, binoculars, and optical telescopes,
small and large. Before 1931, we had no
idea that there was any other way to observe the
universe beyond our atmosphere.
JANSKY’s EXPERIMENT
As often happens in science, major discoveries happen while
someone was looking for something else. Karl G. Jansky (1905-1950)
worked as a radio engineer at the Bell Telephone Laboratories in
Holmdel, New Jersey. In 1931, he was assigned to study radio
frequency interference from thunderstorms in order to help Bell design
an antenna that would minimize static when beaming radio-telephone
signals across the ocean. He built an unusual radio array that looked
more like a wooden merry-go-round than like any modern-day antenna,
much less a radio telescope. It was tuned to respond to radiation at a
wavelength of 14.6 meters (20 Mhz) and rotated in a complete circle on
old Ford tires every 20 minutes. The antenna was connected to a
receiver and the antenna’s output was recorded on a strip-chart
recorder.
He was able to attribute some of the interference for noise produced by
nearby thunderstorms, and some of it to far away thunderstorms, but some
of it he couldn’t place. He called it “a steady hiss type static of unknown
origin.” As his antenna rotated, he found that the direction from which this
unknown static originated changed gradually, going through almost a
complete circle in 24 hours. He was no astronomer, and it took him a
while to surmise that the static must be of extraterrestrial origin, since it
seemed correlated with the rotation of Earth. At first he thought the source
was the sun. However, he observed that the radiation peaked 4 minutes
earlier each day. He knew that Earth, in one complete orbit around the sun,
necessarily makes one more revolution on its axis with respect to the sun
than the approximately 365 revolutions Earth has made about its own axis.
Thus, with respect to the stars, a year is actually one day longer than the
number of sunrises or sunsets observed on Earth. So, the rotation period
with respect to the stars (known to astronomers as a sidereal day) is about 4
minutes shorter than a solar day (the rotation period of Earth with respect to
the sun). Jansky therefore concluded the source of this radiation must be
much farther away than the sun. With further investigation, he identified
the source as the Milky Way and, in 1933, he published his findings.
Grote Reber’s Prototype Radio Telescope
Despite the implications of Jansky’s work, both on the design of radio
receivers, as well as for radio astronomy, no one paid much attention at first.
Then, in 1937, Grote Reber, another radio engineer, picked up on Jansky’s
discoveries and built the prototype for the modern radio telescope in his back
yard in Wheaton, Illinois. He started out looking for radiation at shorter
wavelengths, thinking these wavelengths would be stronger and easier to
detect. He didn’t have much success, however, and ended up modifying his
antenna to detect radiation at a wavelength of 1.8 meters (about the height of
a human), where he found strong emissions along the plane of the Milky Way.
(This frequency is about 160 Mhz)
Reber continued his investigations during the early 40s, and in 1944 published the
first radio frequency sky maps. Up until the end of World War II, he was the lone
radio astronomer in the world. Meanwhile, British radar operators during the war had
detected radio emissions from the Sun. After the war, radio astronomy developed
rapidly, and has become of vital importance in our observation and study of the
universe.
Today … Why Study Radio Astronomy?
• Radio astronomy gives us a view of the
universe that is not possible using our eyes
and visible light.
• Electromagnetic effects are complex and
frequency dependent.
• Radio waves can sometimes penetrate gas
and dust and allow views that otherwise
would be obscured.
Radio astronomy gives us a view of the universe that is not possible using
our eyes and visible light.
• Typical nighttime view of a starry night:
Radio astronomy gives us a view of the universe that is not possible using
our eyes and visible light.
• Radio view at 460 Mhz
Electromagnetic effects are complex and frequency
dependent.
• Our view of the universe would be extremely limited using visible
light alone
TV Channel 2
(Red light)
Channel 3
Channel 4
(Yellow light)
(Gr/Blue light)
TV Channel 5
(Violet light)
We would no nothing of the AM broadcast band, short-wave, the rest of
the TV band, FM, public service, radar, microwave, wireless, cellular,
etc. since we are only peering at a limited range of the electromagnetic
spectrum.
Radio waves can sometimes penetrate gas and dust
and allow views that otherwise would be obscured
• The notable example, here, was the discovery of a
massive Black Hole at the center of our own
galaxy that is completely invisible in the visible
light region (due to obscuring dust)
• This object has been mapped and resolved at
several different radio Frequencies, and
subsequent radio discoveries have led to the
conclusion the Black Holes exist at the heart of
other galaxies as well.
Amateur Radio Astronomy
• Amateurs in Radio Astronomy can do quite
a bit of useful work
• Even with very modest equipment
• Simple short-wave receiving systems are all
that is required, for example, to capture
Solar Flares, and detect radiation from
Jupiter, and the Center of our own Galaxy.
SIMPLE RADIO TELESCOPE BLOCK DIAGRAM
• This diagram depicts a simple Total Power Radiometer that
can be used to capture Solar flares, Jupiter noise and detect
the Galactic Continuum Radiation:
Dipole antenna
………. Length / side (feet) = 234 / F (Mhz)
Coax transmission line
Detector and filter
(detail next slide)
Interface
ht ~20 ft above gnd
AM or SSB
Radio receiver
Manual gain
Headphone output
Tuned to a quiet frequency ~ 20Mhz
Chart recorder
(or DC voltmeter)
Detector/ Filter Detail
• Pictorial of a simple Peak Envelope Detector for a
Total Power Radio Telescope
Input from headphone jack or speaker
500
ohms
CR1 (1N914)
DC Output to chart recorder
+
10 K
R1
C1
1000 uF
R2
Recordings from Similar equipment
Solar Flare ~ 20Mhz (Relative Amplitude Calibration)
Jupiter Noise
Blue ~ 18 Mhz
Red ~ 20 Mhz
Gallactic Center Scan at UHF
What makes a Radio Telescope Different than
a Optical Telescope?
Optical Telescope responds to energy as
excited Photons
Radio Telescope responds to energy that
relates to Temperature of Lower Frequency
Electromagnetic Waves
System Sensitivity
The minimum theoretical Radiometer detectable temperature
(Tmin) is:
Tmin =
Te
——————
( (B )
where: Te is the equivalent Receiver Noise
temperature* :Te = (F - 1) T0, i.e.
F = Receiver Noise Factor, and T0
is standard Temperature (290 ° K)
B = Receiver bandwidth in Hz and
 = the integration Time Constant
after detection
* Noise factor (F) is related to Noise Figure (NF), i.e., F = 10 (NF / 10)
Noise Figure is expressed in dB, a logarithmic quantity expressing ratios of power
Sensitivity example
Receiver Noise Figure = 5 dB
Bandwidth = 10 Khz (10000 Hz)
 = 10 seconds
What is Tmin for this System?
1) First calculate F and Te:
F = 10 (NF/10) = 10 (5/10) = 3.16
Te = (F - 1) T0 = (3.16 -1) (290) = 626.4 ° K
2) Next Calculate Tmin:
Tmin = Te /  (B ) = 626.4 /  (10000 x 10)
Tmin = 1.98 ° K
Could my simple total power radiometer
achieve this level of sensitivity in practice?
Probably not …..
It would be necessary to stabilize the gain of
this receiver to better than 1 part in 330* to
effect this level of performance. This would
be a significant design challenge and would
require special signal processing techniques
that usually are not included in a general
purpose Short wave Receiver.
* Even more stabilization is required in extremely sensitive Receiver systems
Some of these special techniques would include:
1)
Noise Injection
where a calibrated amount of noise is injected into thesystem
front end and then subtracted out of the output to remove gain
variations
2)
Dicke switching
where the input to the receiver is alternately switched back and
forth between the antenna and a termination at “room
temperature”. The output is then differentially processed to
extract the antenna signal alone
3)
Phase switching
where the phase of the signal is chopped between 0 and 180
degrees, then multiplied, which extracts the original antenna
information
It is not necessary to employ any of these special techniques for
Relative power measurements where precise calibration
and stability are not required
Such is the case for the Solar and Jupiter cases that were already
mentioned.
What about resolution?
Resolution of a telescope is the ability to separate
discrete sources that are nearly co-located in space
It is a function of aperture area, just as in an Optical
Telescope, and, in general, the larger the antenna,
the greater the resolution.
It is interesting to point out that the largest single
Radio Telescope with unprocessed data has less
resolution than the human eye.
Do I need high resolution?
Again, no, not initially for the types of Radio
Astronomy that we discussed.
Resolution is important for precise source
locating or for mapping the radio sky.
Large antenna arrays are required for these
activities with special electronic circuits to
distribute the power properly between the
antenna elements.
Types of Radio Telescopes
In general there are 2 distinct types of Radio Telescopes
(Radiometers):
1)
Total Power ….. Where all of the radio energy is
confined within a single antenna beam
2)
Interferometer ….. Where the radio energy is split into
a Multi-Lobed Pattern between the elements of an
antenna array in order to increase the telescope
resolution
Interferometer Block Diagram
4 Antenna Array Interferometer
n
4 way
Power Combiner
n
Receiver
n
Output
Recorder
The individual antennas of the Interferometer are spaced
several wavelengths () apart to increase the resolution
over a single antenna
where:
 = c0 / f
c0 = 3 x 10 8 meters /sec, and
f = center frequency of the Receiver in Hz
Beamwidth of individual antenna
Composite Beamwidth of Interferometer array
where Beamwidth is defined as the location
on the antenna lobes where the power falls
to 1/2 of the value on boresite
Is Antenna tracking required?
Antenna tracking is not required if the instrument is
used as a Meridian Transit System
In this case the antenna is fixed (usually South) and the
source “drifts” through the antenna beam due to
earth’s rotation.
What does the output of a Radiometer
represent?
Noise Energy … usually expressed in terms of
Antenna Temperature
or
Flux Units which are related to Antenna
Temperature and Effective Antenna Area
These quantities are similar to the “Magnitude” units that
are familiar to optical astronomers.
Recall that Noise Power in a system is equal to:
P = K Ta B
where:
P = received excess noise power in watts
K = Boltzmann’s Constant (1.38 x10 -23 Joules /  K)
Ta = Absolute Temperature in  K and
B = Receiver Bandwidth in Hz
Solving for Ta:
Ta = P / (K B)
Ta is measured as excess noise output from
the Receiver and is then compared to the
level obtained from a standard noise source
for accurate calibration.
For strictly “relative” measurements,
accurate calibration is not required.
Flux Units can be derived from Antenna Temperature
The units of Flux (S) are watts / meter2 / Hz and are
expressed by:
Flux (S) = K Ta / Ae
where:
Ae is the effective antenna area and is given by:
Ae = G 2 / (4  )
where:
G is the Directive Gain of the antenna array
1 Flux Unit = 10 -26 watts / m2 / Hz and is defined as
1 Jansky
You got my interest, but describe
some advanced topics?
1)
2)
3)
4)
5)
Multi-channel receivers …..
where events are captured on multiple frequencies then correlated
in amplitude and phase
Digital Signal Processing
where the receiver output is first digitized, then injected and
analyzed in a computer system. Numerical analysis can then be
applied as required
Pulsar periodicity measurements which can then be correlated
with optical views
Hydrogen line studies (1420.1Mhz)
To map the location and quantity of ionized hydrogen within the
non-visible universe
And of course ……...
SETI
Search For Extraterrestrial Intelligence
Large antennas, multi-channel receivers, digital storage,
and a radio quiet location are required for SETI research.
Nonetheless, many radio astronomy enthusiasts are
involved in this interesting activity, including some of the
professional radio observatories when there is down time
on the telescopes.
More information?
Web sites:
www.wy2u.com
http://www2.jpl.nasa.gov/radioastronomy/
http://www.geocities.com/CapitolHill/1090/ws1_4.html
http://www.draco.scsu.edu/radioastro.html