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"A hundred years ago, electrical pioneer Nikola Tesla was working on a radical new type of radio using waves that skim the surface of the earth rather than radiate into space. Tesla believed he could transmit signals across the Atlantic using these surface waves but never succeeded in his lifetime, and the idea faded into relative obscurity. Today it's back, with the promise of a new system for high-speed data transmission that would combine the benefits of wired and wireless communication. Surface waves, or electromagnetic waves, which tend to follow the contours of a surface, had been proven to exist mathematically in Tesla's time. But their practical use was debated. Because they follow the curvature of the earth, surface waves can reach a distant receiver on the ground that is beyond the horizon. \"An inexpensive instrument, not bigger than a watch, will enable its bearer to hear anywhere, on sea or land, music or song, the speech of a political leader, the address of an eminent man of science, or the sermon of an eloquent clergyman,\" Tesla wrote in 1908. Tesla's attempt at long-range radio failed, apparently because the theoretical physicists neglected a factor that meant the waves could cancel themselves out. But these days, thanks to different wavelengths and materials, scientists are overcoming those problems and creating radio transmissions that can reach over the horizon. At high frequencies, a type of surface wave called Zenneck waves can propagate along a surface. They travel better on some materials than others, but performance is best with a conductor covered in a dielectric material. As with wires, these surfaces can carry high bandwidth, are secure, do not cause interference, and require little power. But as with wireless communication, physical contact is not required. Janice Turner and colleagues at Roke Manor Research of Romsey, U.K., have developed a Zenneck wave demo unit. This can transmit high-definition video over a length of conductor covered with dielectric with a bandwidth of up to 1.5 gigabits per second. Because Zenneck waves do not extend far from the surface there is no interference with electronics and no frequency-licensing issues as there are with other radio-frequency systems. Turner says that tears or breaks in a surface do not cut the connection, making it more robust than wiring, and it's inexpensive to manufacture. One of the first applications for Roke Manor's waves is likely to be onboard communications on aircraft and satellites. For example, sensors embedded in an aircraft wing could easily communicate with a central computer via surface waves that travel along the wing and fuselage. Satellite components could send data to each other at high speed without the need for complex connectors. Ships are another likely market, because their metal walls block wireless communication. Turner's team is also looking at wearable wireless gadgets. A lapel camera or a pulse-sensing wristband could connect to a smartphone in your pocket. Such gadgets already exist , but communicate with a phone via Wi-Fi or Bluetooth. This approach has lower power requirements and higher bandwidth, Turner says. They have also had enquiries about using surface waves to recharge devices wirelessly, and this is possible—in principle. Meanwhile, surface waves are also proving valuable for long-range radar, like the new High Frequency Surface Wave Radar (HFSWR) that the defense contractor Raytheon is developing. Some of the first radar operated via surface waves, and the U.S. Navy used surface-wave radar in the 1950s, but the technology ultimately lost out to other types—in particular, the sky-wave radar in which the signal is reflected back from the ionosphere. However, normal radar has a serious limitation: It operates within line of sight, which makes objects close to the surface difficult to spot. This is why airborne radar was developed, to prevent intruders from slipping in below the radar. But maintaining continuous radar coverage from the air is expensive and requires a lot of manpower. Surface-wave radar provides an alternative, because the signal clings to the sea surface and follows the curvature of the earth. Tony Ponsford, technical director for HF Radar at Raytheon Canada, says that that latest version can track ships at about 230 miles from land. (The surface waves work best over a conductive surface, so this type of radar has a much longer range over salt water than over fresh water or land.) Raytheon is building the device for the Canadian government to help manage the country's exclusive economic zone, a region that extends to that distance out to sea. It will undergo operational evaluation later this year. Raytheon's HFSWR incorporates a number of features to operate safely in the crowded high-frequency band. If it detects another signal on the same wavelength, such as a radio transmission, it automatically switches to a different wavelength. Raytheon says its patented set of algorithms removes clutter so shipping can be picked out more easily. This type of radar can be used to track cargo vessels, watch for illegal trawling or dumping, and help with search-and-rescue operations. It can also track smugglers, as it is capable of picking up small go-fast boats. It can even detect icebergs; although obviously nonmetallic, they create a disturbance that shows up \"like a hole in the sea,\" Ponsford says. Beyond what Raytheon and Roke Manor are doing in the field, there is also some classified military work on surface waves. Some of this appears to be focusing on covert communications, using the unique properties of surface waves to send a signal that cannot be intercepted, over either land or water. Although scientists have known about them for more than a century, these are in some ways still early days for surface waves. They have so far been exploited in only very limited ways compared to other forms of radio wave, but that may be set to change. Perhaps Tesla's faith in surface waves was simply a sign that he was ahead of his time."
<https://www.popularmechanics.com/technology/infrastructure/a8778/skimming-the-surface-the-return-of-teslas-surface-waves-15322250/>