Jodrell Bank Observatory: 71MHz Searchlight Aerial

Summary

Searchlight aerial. Second World War trailer-mounted searchlight base converted in 1946 to searchlight aerial by John Atherton Clegg.

Reasons for Designation

The remains of the 71 MHz Searchlight Aerial, Jodrell Bank Observatory, converted from a Second World War trailer-mounted searchlight base in 1946 by John Atherton Clegg, is listed at Grade II for the following principal reasons:

* Historic interest: as the earliest remaining component specifically relating to Lovell’s research to survive at the Jodrell Bank Observatory, whose success in tracking meteors was fundamental to his continued development of the new science of radio astronomy here;

* Design interest: the searchlight mount was chosen for its excellent turning mechanism which enabled the adapted radar aerials to closely and accurately track meteor showers in both elevation and azimuth;

* Group value: the remains of the 71 MHz Searchlight Aerial has a strong functional link with the other main components of the Observatory, such as the Lovell Telescope and Mark II Telescope, through the discipline of radio astronomy for which the site has a long-standing international reputation.

History

The observatory at Jodrell Bank was one of the earliest planned sites for radio telescopes in the world. As such it had a pivotal role in the development of the new science of radio astronomy which was one of the first steps towards modern Astrophysics, revolutionising our understanding of the Universe. The site was first used for academic purposes in 1939 when the University of Manchester's Botany Department purchased three fields in the Cheshire countryside covering around 11 acres. The earliest use of the site for radio astronomy occurred in December 1945 when Bernard Lovell, who worked for the university’s Physics Department, moved here to escape the radio interference that occurred in Manchester city centre. His first observations used ex-army radar equipment located at the south end of the site, close to two pre-existing botany huts. Subsequently his team expanded northwards with the continuing construction of more permanent buildings, aerials and telescopes. Jodrell Bank’s status as a world-class centre of ongoing scientific research continues to this day with the construction of the global headquarters for the Square Kilometre Array project linking hundreds of telescopes and aerials in South Africa and Western Australia.

The first new member of staff to join Lovell’s research team was John Atherton Clegg, who joined him in 1946, bringing an extensive knowledge of radar aerials. Lovell suspected that meteors reflected radar signals and so at this time he also enlisted the help of amateur astronomer, JP Manning Prentice, who was then Director of the Meteor Section of the British Astronomical Association. This was a move which was to change Lovell’s post-war career from physicist to astronomer. In August 1946 Lovell, Clegg and Prentice observed the Perseid meteor shower, hoping to discover whether the echo rate of the radar equipment was related to the known meteor activity. The results were promising, but required further research to clarify the complexity of the results. Prentice suggested observing the Giacobinid meteor shower on the night of the 9–10th October 1946. Clegg realised that in order to do this an aerial was needed which could be moved continuously both in elevation and azimuth (vertical altitude and side to side to define the apparent position of an object in the sky relative to a specific observation point). He set about creating the Searchlight Aerial, using the mount from a searchlight loaned by the army for its excellent turning mechanism. This was positioned near to the Park Royal trailer, which had become permanently stuck in the mud when being moved towards the north end of the site. Clegg built an array of five 7-element Yagi aerials on the mount. The development of this equipment by Lovell and Clegg involved making substantial changes and additions to radar equipment from its war-time form. It was a great success in observing the ionised trails of the Giacobinid meteor shower, and was then shortly developed to determine the radiant points of meteors. Lovell wrote ‘The Searchlight Aerial survived for many years and became the aerial system for a variety of subsequent research programs'. Eventually, this splendid aerial system was superseded but the rusty remnants of the searchlight base remain as a reminder of an epic night in the early history of Jodrell Bank’. In 1947 Lovell and Clegg presented their findings to the Royal Astronomical Society, revealing a new revolutionary astronomical technique.

The Searchlight Aerial was used as a backdrop to a black and white photograph of Lovell and his research team thought to have been taken on the occasion of his appointment as Professor of Radio Astronomy in 1951.

The remains of the searchlight mount survives, but the aerial arrays are no longer present.

Details

Searchlight aerial. Second World War trailer-mounted searchlight base converted in 1946 to searchlight aerial by John Atherton Clegg.

MATERIALS: steel, steel scaffolding poles, concrete.

DESCIPTION: the steel trailer-mounted, wartime searchlight base is aligned approximately east-west. The wheels have been removed and the trailer chassis has been fixed down by scaffolding poles set into a concrete plinth approximately 3m x 3m (9ft 9in x 9ft 9in). On the chassis is a circular, revolving mount (unlikely to revolve now due to poor condition). Flanking the plinth there are two long, narrow, concrete channels approximately 600mm (2ft) wide and a similar depth, now overgrown and filled with water and trees. They both extend for approximately 17m (21ft 3 in) with one running in an easterly direction and one in a westerly direction. On the south side there is an arc of partially obscured hard-standing which may indicate a range of rotation of the aerials.

The two large I girders and associated bolted L-shaped girders resting on timbers across the top of the mount do not form part of the original structure.

Mapping: the location of the searchlight aerial is approximate as it is not mapped on modern digital mapping (Master Map) and the outline is unclear due to vegetation.

Sources

Books and journals
Lovell, Bernard, The Story of Jodrell Bank, (1968)
Pevsner, N, Hubbard, E, The Buildings of England: Cheshire. Kelsall: Jodrell Bank, (1971), 249

Other
Jodrell Bank Observatory Site Conservation Management Plan Site Gazetteer, December 2014 (final draft) Chris Blandford Associates.
Jodrell Bank Observatory, the University of Manchester, Conservation Management Plan, June 2016, Chris Blandford Associates

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