July 22 is a big day in Amsterdam: the long awaited new North/South metro line will finally open to the public. Over 120.000 passengers are expected to travel underneath the historic city center on this line daily, and naturally, they will want to be online during their journey! How does a mobile network get a signal to a cellphone travelling through narrow underground tunnels?
It does so by using leaky cables: long lengths of coaxial cables running through the tunnels. Normally, coaxial cables are insulated quite well to avoid any disruption of the signal they’re carrying. A leaky cable, however, has small holes in the outer conductor at regular intervals along its length, allowing part of the signal to radiate out of the cable. The leaky cable carries data, but is also used as a sort of really stretched out antenna, offering a 2G/3G/4G connection to mobile devices. The technology has been used previously in other cities’ underground railways and also in mines.
I’m hoping to catch a ride on the new line soon, and when I do, I’ll be paying extra attention to my cellphone’s reception.