Looking at the Radio Universe. By Geoff Varrall, RTT Online


On a sunny Friday in early June, the Cambridge Wireless Heritage Special Interest Group set off to explore the outer edges of the known Universe, or more prosaically went on a tour of the Mullard Radio Astronomy Observatory nestling alongside the A603 beyond Barton.

Radio astronomy is the science of using radar to study the stars and other extra-terrestrial things that emit radiation including the interstellar and inter galactic medium and the dust clouds of the Milky Way, the nursery for the formation of new stars and planets

Radio astronomy was invented by accident by Karl Jansky in the 1930’s while investigating static on 30 MHz radio links used by the Bell Telephone Company in the US.

There was a great leap forward after the Second World War when a generation of radar engineers including Bernard Lovell in Manchester (Jodrell Bank Observatory) and Martin Ryle in Cambridge (The Mullard Radio Astronomy Observatory) repurposed radar antenna and receiver system including the 8 metre diameter German Wurzburg radar to look at signals from space.

This prompted a whole new generation of deep space radio observation techniques including Martin Ryle’s use of multiple pairs of parabolic reflectors mounted on rails on an East to West axis, the One Mile and 5 Kilometre arrays, a large aperture antenna radio telescope with resolution determined by the spacing distance.

The East/West axis allowed the antennas to sweep across a segment of sky hence the description of the device as an ‘earth rotation interferometer’. The arrays were and are used to make high resolution maps of radio galaxies (large scale galaxies viewed at radio rather than optical wavelengths) and quasars (quasi-stellar small scale compact objects of high brightness including neutron stars).

After an upgrade in the 1980’s the array was renamed the Ryle Telescope and used to measure Cosmic Microwave Background radiation – the emission signature of the Big Bang 13.7 billion years ago and it’s immediate (370,000 year) aftermath.

One of Martin Ryle’s colleagues, Antony Hewish helped by a research student Jocelyn Bell built a very different antenna consisting of 2048 dipoles 3.7 metres in length spread across 4.5 acres working at 81 MHz (3.7 metre wavelength)

In 1968 this instrument was the first instrument to detect a pulsar, a compact neutron star producing bursts of radiation due to its rapid rotational spin.

The most recent telescope to be commissioned on the Cambridge site is the Arcminute Microkelvin Imager (AMI)

This is two separately correlated arrays of receivers operating in the 12 to 18 GHz band with a small array of ten 3.7 metre parabolic dishes in a compact configuration able to resolve angular scales of 2 to 16 arc minutes linked to a large array formed by a compact configuration of eight of the 12.7 metre Ryle Telescope dishes in the Ryle array including two off set dishes to create a north south baseline to cover angular scales of 0.5-5 arc minutes.

MRAO resized

Picture taken by Wireless Heritage SIG Champion Stirling Essex

The combination of the two systems works on the basis of the small array detecting shadows that galaxy clusters have imprinted on the cosmic radio background with the large array providing correction for contaminating radio sources.

The overall bandwidth is six GHz divided into eight broadband 750 MHz channels. Independently of red shift this combined array should be able to see clusters that are impossible or hard to detect optically, for example clusters hidden behind dust clouds.

This should take us back to the period between 370000 years and one billion years including pre galaxy structures as they begin to coalesce under the influence of gravitational wave energy.

About the SIG

The Wireless Heritage SIG studies examples of radio technology history that can be shown to have direct relevance to present and future decision making in the radio communications industry. Their next event is 75 Years of Radar at the RAF Air Defence Museum in Horning, Norfolk on 16 October.

Our thanks to the MRAO for hosting this Cambridge Wireless tour and for permission to use photographs of the Ryle and AMI arrays.


About The Author

Geoff Varrall is Director at RTT Programmes

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