CHINA'S ban on unlicensed ownership of satellite television dishes, due to come into effect in April, is virtually unenforceable, according to a Sri Lankan entrepreneur who is re-broadcasting BBC TV World Service News and Prime Sports in Colombo using home-made equipment. Nahil Wijesuriya, quoted in the March issue of Asia Inc. said: ''You can make a satellite dish with concrete and kitchen foil.'' In countries such as Malaysia, where satellite dishes are banned, the dish could even be disguised as an ornamental fish pond or bird bath, he added. The electronic components required could readily be obtained for about US$100 (HK$775), and the total cost was likely to be less than US$350. Mark Long, the publisher of World Satellite Almanac, an American reference text, estimated that there were between 600,000 and one million satellite dishes in China, with an audience of between 30 million and 70 million. ''Realistically, if they wanted to go round and dismantle these, they would have to have a very large police network,'' he said. Mr Wijesuriya's do-it-yourself guide said the size of a dish required depended on the strength of the satellite's signal, and the distance from the centre of its broadcast ''footprint'' to the dish. Increasingly powerful satellites would, therefore, lead to reductions in dish sizes, reducing costs and making concealment progressively easier. The DIY instructions provided in the article supply the geometric formulae required to fashion the necessary parabolic shape and locate the focal length, the precise locations of eight satellites, construction details and a shopping list of components.