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Peter Cochrane's Hard Drive 1998 Signal from the underground CITIES, towns and vehicles have come to resemble pincushions, with antennas in evidence everywhere. Few people find antennas objects of beauty. Even when you understand the mathematics, physics and engineering involved, they can still be an eyesore. This is especially so with the sudden growth of home satellite, and with cellular mobile phone towers now visible everywhere. So the question arises: is there an alternative? I first became deeply involved in wireless communication and remote control before I left school. This early fascination culminated in my delving into the extremes of radio technology for land vehicles, aircraft, spacecraft, ships and submarines. In every case the singular limitation was always the antenna system. Perversely, the choice of radio frequency often saw wavelengths incompatible with the vehicle dimensions. For example, taxis and police cars using frequencies around 80MHz required antennas more than a metre long. When I came across radio communication to submerged submarines, and the provision of global time standards, I discovered that frequencies well below 20kHz were in common use and ideally demanded antennas well over 10km long. Many engineering compromises were used to minimise the cost of such systems, but among the most imaginative was underground antennas. At these frequencies the loss of radio signal in a dry trench filled with volcanic ash is small and the sub-surface angle of radiation is ideal as the propagating wave hugs the surface contours as it circumnavigates the planet. The common misconception is that antennas must be mounted on tall, isolated towers or buildings, and this is certainly true for many applications. However, domestic satellite receiver systems only need the antenna to see the satellite, which will be low on the horizon in Britain. So, if nothing is in the sight line between the satellite and the antenna, it can be hidden in a rose bush in the garden. Interestingly, mobile telephone base station antennas can now be concealed or camouflaged in the facades of buildings and structures. This is because the frequencies used are in the region of 1GHz and above, and so the wavelength is less than 30cm, and the antenna array reasonably small. Paradoxically, many buildings can benefit from an underground antenna system. Drive into the underground car park of a modern building and the chances are your call will drop out as the signal disappears. But install the base station and antenna in the basement and the signal is available immediately you drive in from street level. Even better, the signal radiates up through the building to provide service for the entire complex. A direct benefit of going for far more base sites and antennas is the ability to reduce the transmitter powers concerned while increasing the bit rates available. Because the radiated energy follows an inverse power law, doubling the number of antennas, nominally halving the distance, could see the power required drop below a quarter of that today. Should we choose to go in this direction we will require a new approach to avoid an eyesore world of antennas. Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is: |
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