Pocket-sized VLF antenna keeps comms open where conventional radios fail
The US Department of Energy's SLAC National Accelerator Laboratory has developed a compact antenna that can transmit radio signals not only over long distances through the air, but also underwater, and even through solid earth. The new four-inch (10-cm) antenna exploits the piezoelectric effect to generate Very Low Frequency (VLF) waves that normally require antennae that are miles long.
If you ever travel to the Pacific Northwest town of Arlington, WA and look toward the Cascade mountains on a clear day, you'll see 12 huge red and white towers set on either side of Jim Creek Valley. Their purpose is to hold up a cats cradle of cables, each spanning one to 1.6 mi (1.7 to 2.6 km) from one slope to the other.
Built in 1953, this is Jim Creek Naval Radio Station and is designed to allow the US Navy to send one-way messages to submerged submarines using VLF radio waves at 24.8 kHz. This gigantic antenna is necessary because in order to send and receive radio waves the antenna needs to be comparable in size to their wavelength. In VLF, these are miles long.
Needless to say, this makes the setup not exactly portable, but it fulfills a very important function. Conventional radio transmitters work very well over short distances, but not over long ones, and they are easily blocked by rock or water. However, VLF radio waves are reflected by the Earth's ionosphere and can travel through the Earth and even into the deep ocean.
VLF communication doesn't have much bandwidth. For submarine communications, it doesn't send so much full messages as a "gong" signal telling the captain to go to periscope depth to use shortwave or satellite communications. But it is a way to stay in touch with nuclear submarines while keeping them well out of sight of hostile sensors.
The SLAC antenna allows for a much more compact transmitter that weighs only a few pounds, yet is 300 times more efficient than other compact antenna designs at a test range of 100 ft (30 m), and with 100 times greater bandwidth.
To achieve this, SLAC says that the new antenna uses the piezoelectric effect. A rod-shaped crystal of a piezoelectric material, lithium niobate, is subjected to an oscillating electric voltage, which causes it to vibrate and send out VLF waves.
This allows it to not only send out very large waves, but to do so without large tuners. By switching the wavelengths, data transfer rates can be boosted to more than 100 bps.
"Our device is also hundreds of times more efficient and can transmit data faster than previous devices of comparable size," says SLAC's Mark Kemp, the project's principal investigator. "Its performance pushes the limits of what's technologically possible and puts portable VLF applications, like sending short text messages in challenging situations, within reach."
The research was published in Nature Communications.