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The word: Radio-quiet reserve
29 September 2007
From issue 2623 of New Scientist magazine, page 52

Lions were pretty much the last thing the radio astronomers wanted to encounter. The researchers were on the lookout for sites in the southern African veld where stray radio signals were at their weakest, but as they crossed the border from South Africa into Botswana, they found a pride of lions already in residence at a remote site that they wanted to measure.

Lions notwithstanding, the remoteness of parts of southern Africa is one reason why the region has been shortlisted as a potential site for a huge new radio telescope called the Square Kilometre Array. The SKA will have thousands of antennas with a total collecting area of 1 square kilometre. Half of the antennas will be concentrated at the core, a region about 5 kilometres across. The others will be erected up to 3000 kilometres away. The massive telescope will be used to study, among other things, the evolution of galaxies, the nature of dark energy and the origin and evolution of cosmic magnetism.

In 2008, the SKA steering committee will choose between southern Africa and a rival location in the Australian outback. Its decision will depend partly on who can guarantee the best "radio-quiet reserve".

Just as optical telescopes are built far away from light pollution, radio telescopes work best when there is little or no interference from radio signals of human origin. The proliferation of radio, television and cellular networks has meant that such regions have become few and far between, however. To minimise interference the world's biggest radio telescopes, such as the National Radio Astronomy Observatory in Green Bank, West Virginia, are protected by regulations that limit radio transmissions within a central radio-quiet zone. For the NRAO, this zone covers about 33,000 square kilometres.

The SKA aims to do rather better than that. If it is built in southern Africa, the telescope's core will lie within the Karoo (pictured), a semi-arid, sparsely populated region in South Africa's Northern Cape province. South Africa's legislators are working to create a central zone about 400 kilometres in diameter in which all wireless transmissions would have to meet strict requirements. Low-frequency transmissions (which travel farther than high-frequency ones) would be regulated in a larger zone of about 800 kilometres in diameter, creating a total controlled area of about 500,000 square kilometres. This doesn't mean all transmissions will be banned. Mobile phone antennas, for example, can be oriented so that they transmit signals towards towns and roads but away from the telescope's core.

If built in Africa, some of the SKA's antennas will be in Namibia, Botswana, Mozambique, Madagascar, Mauritius, Kenya, and even as far afield as Ghana. The exact location of these remote antennas will not be critical, however, making it easier to find optimal radio environments for them. Staying clear of lions should not be too hard.

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