SLAC develops a new compact antenna for communication in case of radios failure

A new type of pocket antenna, developed by the SLAC National Accelerator Laboratory of the Ministry of Energy, could allow mobile communication in situations where conventional radios do not work, for example under the auspices. water, in the soil and over very long distances in the air.

The device emits very low frequency radiation (VLF) with wavelengths of several tens to hundreds of kilometers. These waves travel long distances beyond the horizon and can enter environments that may block radio waves with shorter wavelengths. While today's most powerful VLF technology requires gigantic transmitters, this antenna is only 4 inches tall and can therefore be used for tasks requiring high mobility, including rescue and defense missions.

"Our device is also hundreds of times more efficient and can transmit data faster than previous devices of comparable size," said Mark Kemp, of SLAC, the project's lead investigator. "Its performance pushes the limits of what is technically possible and allow portable VLF applications, such as sending short text messages in difficult situations,".

The team led by SLAC reported on its results today to Nature Communications.

A major challenge

In modern telecommunications, radio waves carry information by air for radio broadcasts, radar and navigation systems and other applications. But shorter wavelength radio waves have their limits: the signal they transmit becomes weak over very long distances, can not cross the water and is easily blocked by layers of rock.

In contrast, the longer wave length of VLF radiation allows it to travel hundreds of feet in the ground and in the water, and thousands of kilometers beyond the horizon in the # 39; air.

However, VLF technology also presents major challenges. An antenna is more efficient when its size is comparable to the wavelength it emits; The long wavelength of the VLF requires huge antenna arrays extending for miles. Smaller VLF transmitters are much less efficient and can weigh hundreds of pounds, limiting their intended use as mobile devices. Another challenge is the low bandwidth of VLF communication, which limits the amount of data it can transmit.

The new antenna has been designed with these issues in mind. Its compact size could make it possible to build transmitters weighing only a few kilograms. In tests that sent signals from the transmitter to a receiver some 100 meters away, the researchers showed that their device was producing VLF radiation 300 times more efficiently than the previous compact antennas and was transmitting data with bandwidth almost 100 times higher.

"The technology has many interesting applications," said Kemp. "Our device is optimized for long-distance communications by air, and our research is focused on the scientific underpinnings of the method to find ways to further improve its capabilities."

A mechanical antenna

To generate VLF radiation, the device uses the so-called piezoelectric effect, which converts mechanical stresses into an accumulation of electrical charge.

The researchers used a rod-shaped crystal of piezoelectric material, lithium niobate, as an antenna. When they apply an oscillating electric voltage to the rod, the rod vibrates, contracts and expands alternately, and this mechanical stress triggers an oscillating electric current whose electromagnetic energy is then emitted in the form of VLF radiation. .

The electric current comes from electric charges that go up and down the rod. In conventional antennas, these movements are almost the same size as the wave length of the radiation they produce and more compact designs usually require tuner units larger than the antenna itself. . The new approach, on the other hand, "allows us to effectively excite electromagnetic waves with much longer wavelengths than movements along the crystal and without large tuners, which is why this antenna is so compact, "said Kemp.

The researchers also found a clever way to change the wavelength of the emitted radiation, he said: "We repeatedly switch the wavelength during operation, which allows us to transmit with a wide bandwidth bits per second – enough to send simple text. "