Principle of a new compact very low frequency (VLF) antenna. It consists of a rod-shaped crystal of a piezoelectric material, lithium niobate (center). 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.