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March 2, 2011

I've just gotten accustomed to decoding all the LOTRs I see on message boards, and now they hit me with LOFAR, an acronym just similar enough to slow my reading. LOFAR stands for LOw Frequency ARray, a radio telescope operating at unusually low frequencies. Radio telescopes operate principally at ultra high frequencies, not because that's where all the cosmic action is, but rather that's where it's easy to build directional antennas. I reviewed this in a previous article (Radar and Lunar Lidar, January 6, 2011). There's the further problem of the Earth's ionosphere, which not only bounces our own low frequency radio signals back to us, but shields us from such signals impinging on Earth from space.

There was one adventuresome radio astronomer who was undeterred by the problems of doing low frequency radio astronomy. This may have been because he was a self-taught radio astronomer. In fact, he was the world's second radio astronomer, the only person to follow-up on Karl Jansky's discovery of extraterrestrial radio sources.[1] This was Grote Reber (1911-2002), who was an amateur radio operator (W9GFZ) in Wheaton, Illinois, a suburb of Chicago. Jansky's discovery was in 1933, and by September, 1937, Reber had built his own 31.4 foot diameter parabolic antenna (see photograph).[2] The antenna was built out of iron, which was a cheap, conducting metal.

Grote Reber's original radio telescope antenna

Grote Reber's original radio telescope antenna (Wheaton, IL, 1937)
(Source: NRAO Archives)

Reber first started searching for signals at 3300 MHz, but he needed to pull back to 160 MHz before finding signals from the same source as Jansky.[2] He published these observations in The Astrophysical Journal.[3] After World War II there was a tremendous surge of interest in radio astronomy that was partially fueled by the availability of surplus military radar sets. In just a decade, radio astronomy became a "big science" that was driven as much by a quest for funding as science.[4] This didn't appeal to Reber, who decided to listen on low frequencies, below 30 MHz, where no one else was listening. Reber relocated to Tasmania, where there was a low level of man-made radio interference and reasonable transparency of the ionosphere in winter.

There were others looking at low frequencies, although not as low as Reber's target frequencies. One array at the Mullard Radio Astronomy Observatory (Cambridge,UK), the Cambridge Low Frequency Synthesis Telescope, is a telescope array that's done surveys at 151 MHz and 38 MHz. This array of 60 steerable Yagi antennas has a sensitivity of about 50 mJy per beam. A Jansky (Jy), named after Karl Jansky, is a non-SI unit of received spectral flux density. Its SI equivalent is 10−26 watts/m2/Hz.

Well, Big Science has continued Reber's vision with the construction of LOFAR (web site at www.lofar.org). Because of the long wavelengths associated with low frequencies, LOFAR is necessarily the largest radio telescope ever built. It's composed of thousands of antennas distributed across The Netherlands, Germany, Great Britain, France and Sweden. Its enabling technology is advanced digital signal processing, which can combine signals from these thousands of antennas, covering about a square kilometer in toto, into one interferometer. The signal processing is done by a Blue Gene/P supercomputer at the University of Groningen. Data pipes into Groningen handle several gigabits per second.

LOFAR high-band and low-band antennae

LOFAR high-band (left) and low-band (right) antennae. (Source: LOFAR web site.

LOFAR was built to observe in the range 10-240 MHz. The antennas come in two styles, low-band and high-band. as shown above. The low-band antennas are orthogonal dipoles of 45-degree slanted wires above a ground plane, and they can operate from 10-80 MHz. The high band antennas, which operate from 120-240 MHz, are bowtie antennas that are arranged in 4 x 4 grids. They have an integrated amplifier designed to suppress interference from signals in the commercial FM broadcast band.

What is LOFAR expecting to see at these low frequencies?[5]
• The farther we look towards the edge of the universe, the more the light we see is redshifted. This means that the neutral hydrogen frequency at 21.10611405413 cm (1420.40575177 MHz) is shifted into the LOFAR observation frequencies when the redshift is between 6 and 10.
Jupiter emits low frequency radio waves, so it's likely that some extrasolar planets would do the same. There may also be transient events from stellar mergers and the accretion of matter by black holes.
• Closer to home, LOFAR will detect coronal mass ejections from the Sun, a cause of damaging geomagnetic storms.
• Since LOFAR peeks through Earth's ionosphere, it will be able to monitor it as well. This is a method to detect distant gamma ray bursts.

There's also the possibility of serendipitous discoveries, a phenomenon that's been a part of science since its beginning. Pulsars were an accidental discovery, as was Jansky's discovery of extraterrestrial radio signals, so you can never tell what might be lurking at these low frequencies.

One interesting thing about LOFAR is that the object that took Reber so long to see with his radio telescope, Cygnus-A, has such a strong signal that it's a problem. Cygnus-A, and a number of other sources must be filtered from the LOFAR data.[6]


  1. For an image of Jansky's discovery trace, see the article, Don Backer, This Blog, August 5, 2010.
  2. Grote Reber page on the National Radio Astronomy Observatory web site.
  3. Grote Reber, "Cosmic Static," Astrophysical Journal, vol. 91 (1940), pp. 621ff.
  4. For an interesting account of the "business" of early radio astronomy, see Bernard Lovell's book, Story of Jodrell Bank (Oxford University Press,May 1968), 282 pages (via Amazon),
  5. LOFAR page on Wikipedia.
  6. Cygnus A at 240 MHz with LOFAR, LOFAR web site, August 30, 2010.

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