Building the dish
10 Mar 2011



The site




The Chilbolton Observatory opened in 1967, having taken over two years to plan, design and build. This page describes the engineering feat that made the Chilbolton dish a reality.

The site for the Chilbolton dish was chosen for its chalk base and gently rolling, arable countryside, which would allow researchers to investigate low angle radio paths, well away from unwanted radio signals.

The support tower

Concrete and steel were the main materials used in the tower that would support the massive dish. Over 2000 tonnes of concrete was poured in total. Each section had to be poured non-stop, including one unrelenting session when 750 tonnes of concrete was poured in 24 hours. At the top of the concrete tower, constructors bolted a capping ring, designed to support the ill-fated azimuth bearing.

Equipment Cabin

The main equipment cabin was clad in lightweight, corrosion-proof aluminium on the ground and lifted onto the concrete tower. The lower floor contained apparatus; the upper floor housed the motor and switch gear for the aerial drive. The cabin rotates as the dish rotates, its 80 tonnes finely balanced on the horizontal bearing.

The dish and backing structure

The dish was intended to rotate a full 90 degrees from its vertically-pointing parked position to the horizontal, so it would be capable of scanning the full horizon. Aerial rotation of the dish was to be achieved using semi-circular racks, nearly 15 feet in diameter. These were machined as one massive circular structure, which constructors then cut in half. The supporting structures for these racks alone weighed 20 tonnes each.​

The backing structure for the parabolic dish itself was ass​​​embled on the ground, then lifted to the top of the tower. Building the Chilbolton antenna used some of the most powerful cranes available. These were in short supply, so each component of the structure was designed so that it could be lifted with ease and safety with the equipment available at the time. There was no point in designing a perfect antenna that could never be built because the components could not be got off the ground.

Supporting ribs projected radially from the backing structure, to hold the 48 aluminium petals that made up the smooth curve of the dish's surface. Each petal was as lightweight as possible, made from aluminium honeycomb and covered with aluminium sheeting. Despite these efforts, the weight of the dish had to be counterbalanced by tonnes of concrete and steel ballast, so that it would tilt in a controlled way without being affected by untimely gusts of wind. To achieve this fine balance of ballast, an unlucky construction worker had to climb inside the cramped and sweaty ballast chamber and settle the ballast into the corners using a small handheld shaker.

The radio aerial

The radial aerial was fixed at the focus of the dish, at the end of 20 inch diameter poles 43 feet in length. These poles were attached as close to the edge of the dish as possible, to avoid affecting the signal.

Calibrating the antenna

The curve of the dish was vital - its profile had to be accurate to 1/10 of an inch. The focus of the dish was measured using a sturdy bit of equipment called a 'parabscan', itself over a tonne in weight. Each panel was painstakingly adjusted by hand to get the best possible fit to the ideal parabola.

Lab buildings

As well as a dish, more buildings were needed to house the control equipment, not to mention the scientists. A number of flat roofed prefabs were built a short distance from the antenna. The control room and tower were linked by an underground tunnel made from concrete sewer pipes, carrying cabling and incidentally providing a dry route for the scientists on winter days.

The finished product

In two years, the Chilbolton Observatory progressed from a scale model on an engineer's desk to the full-size, 25 metre structure that is still in use today. The main contractor was Associated Electrical Industries Ltd, and the project was directed by the Ministry for Public Buildings and Works.

Chilbolton was originally designed for research on radiowave propagation and atmospheric physics, subjects that are at the forefront of the Chilbolton Group's research programme 40 years later.

Contact: Jeffery, Judith (STFC,RAL,RALSP)