Very low frequency (VLF) radio signals can travel beyond the horizon and penetrate deep into land and water; they are used for such purposes as navigation and communications with aircraft and submarines. But generating waves that large is often impractical because it requires a large antenna: To be reasonably efficient, the antenna must be at least one-tenth of the wavelength. For VLF waves, which are 3–30 kHz, that would be more than a kilometer. So although VLF signals are routinely generated, their use is limited because the antennas lack mobility. Electrically small antennas—those whose length is much less than the signal’s wavelength—can also transmit VLF signals, but they do so less efficiently than their larger counterparts.
Mark Kemp of SLAC and his collaborators are trying to get the best of both worlds. Their 9.6-cm-long transmitter prototype, shown in the photo, is much smaller than the wavelength of the signal it generates, but it is more efficient than a small metal antenna. The key to the researchers’ success was using lithium niobate, a piezoelectric material with a large electromechanical coupling. Their antenna doesn’t carry current the same way as a metal one; instead, an RF input signal at one end causes the material to expand and contract because lithium and niobium ions in the crystal’s unit cell shift, which polarizes the material. At the antenna’s resonant frequency, a high voltage develops between the antenna’s ends. The alternating input signal generates a time-varying electric field, and the antenna radiates as an electric dipole.
Metal antennas are often impedance matched to their associated circuits to maximize their signal’s power, but those circuits can be prohibitively energy consuming, particularly for electrically small antennas. A piezoelectric antenna doesn’t need such circuitry: Whereas a metal antenna’s equivalent circuit is just a resistor, the equivalent circuit for a piezoelectric antenna is an RLC circuit, and the values for the capacitance and inductance can be tuned using the antenna’s mechanical properties.
Although they haven’t yet measured the far field directly, Kemp and coworkers used measurements of losses in their system and theoretical models for dipole antennas to determine that their radiation efficiency is 300 times that of a similarly small metal antenna.