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SLAC’s Mark Kemp and his collaborators are testing a new antenna for very low frequency (VLF) radiation by sending signals to a transmitter 100 feet away. (Dawn Harmer/SLAC National Accelerator Laboratory)
A Mechanical Antenna
To generate VLF radiation, the device exploits what is known as the piezoelectric effect, which converts mechanical stress to a buildup of electrical charge.
The researchers used a rod-shaped crystal of a piezoelectric material, lithium niobate, as their antenna. When they applied an oscillating electric voltage to the rod it vibrated, alternately shrinking and expanding, and this mechanical stress triggered an oscillating electric current whose electromagnetic energy then got 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 us to efficiently excite electromagnetic waves with wavelengths that are much larger than the motions along the crystal and without large tuners, which is why this antenna is so compact,” Kemp said.
Principle of a new compact very low frequency (VLF) antenna. It consists of a rod-shaped crystal of a piezoelectric material, lithium niobate (at left). An oscillating electric voltage (red wave) applied to the bottom of the rod makes it vibrate. This mechanical stress triggers an oscillating electric current (arrows) whose electromagnetic energy then gets emitted as VLF radiation (blue waves). The device can be switched during operations to tweak the wavelength of the emitted radiation and optimize the rate at which the device can transmit data. (Greg Stewart/SLAC National Accelerator Laboratory)
The researchers also found a clever way of tweaking the wavelength of the emitted radiation, he said: “We repeatedly switch the wavelength during operation, which allows us to transmit with a large bandwidth. This is key to achieving data transfer rates of more than 100 bits per second – enough to send a simple text.”
This work was done in collaboration with SRI International and Gooch and Housego, a photonics technology firm. The project is part of the AMEBA effort, funded by the Defense Advanced Research Projects Agency (DARPA). The research team has a pending patent for their technology, filed through Stanford University.
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