A pocket-sized antenna was developed that could enable mobile communication in situations where conventional radios don’t work such as underwater, through the ground, and over very long distances through air. The device emits very low frequency (VLF) radiation with wavelengths of tens to hundreds of miles. These waves travel long distances beyond the horizon and can penetrate environments that would block radio waves with shorter wavelengths. While today’s most powerful VLF technology requires gigantic emitters, this antenna is only four inches tall, so it could potentially be used for tasks that demand high mobility including rescue and defense missions.
In modern telecommunications, radio waves transport information through air for radio broadcasts, radar, navigation systems, and other applications. But shorter-wavelength radio waves have their limits — the signal they transmit becomes weak over very long distances, it can’t travel through water, and it is easily blocked by layers of rock. In contrast, the longer wavelength of VLF radiation allows it to travel hundreds of feet through ground and water and thousands of miles beyond the horizon through air.
VLF technology also comes with major challenges. An antenna is most efficient when its size is comparable to the wavelength it emits; VLF’s long wavelength requires enormous antenna arrays that stretch 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’s compact size could make it possible to build transmitters that weigh only a few pounds. In tests that sent signals from the transmitter to a receiver 100 feet away, the device produced VLF radiation 300 times more efficiently than previous compact antennas and transmitted data with almost 100 times greater bandwidth.
To generate VLF radiation, the device exploits the piezoelectric effect, which converts mechanical stress to a buildup of electrical charge. A rod-shaped crystal of a piezoelectric material, lithium niobate, is used as the antenna. When an oscillating electric voltage is applied to the rod, it vibrates, alternately shrinking and expanding. This mechanical stress triggers an oscillating electric current whose electromagnetic energy is then emitted as VLF radiation.
The electric current stems from electric charges moving up and down the rod. In conventional antennas, these motions are close to the same size as the wavelength of the radiation they produce and more compact designs typically require tuning units that are larger than the antenna itself. The new approach, on the other hand, allows researchers to efficiently excite electromagnetic waves with wavelengths that are much larger than the motions along the crystal and without large tuners, which is why the antenna is so compact.