  Antennas typically have emission/radiation efficiencies bounded by
A/(lambda)^2 (A < lambda^2) where A is the emitting area and lambda is the
wavelength of the emitted wavelength. That makes it challenging to miniaturize
antennas to extreme sub-wavelength dimensions. One way to overcome this
challenge is to actuate an antenna not at the resonance of the emitted wave,
but at the resonance of a different excitation that has a much shorter
wavelength at the same frequency. We have actuated an electromagnetic (EM)
antenna with a surface acoustic wave (SAW) whose wavelength is about five
orders of magnitude smaller than the EM wavelength at the same frequency. This
allowed us to implement an extreme sub-wavelength EM antenna, radiating an EM
wave of wavelength lambda = 2 m, whose emitting area is ~10^-8 m2 (A/lambda^2 =
2.5 10^-9), and whose measured radiation efficiency exceeded the A/(lambda)^2
limit by over 10^5. The antenna consisted of magnetostrictive nanomagnets
deposited on a piezoelectric substrate. A SAW launched in the substrate with an
alternating electrical voltage periodically strained the nanomagnets and
rotated their magnetizations owing to the Villari effect. The oscillating
magnetizations emitted EM waves at the frequency of the SAW. These extreme
sub-wavelength antennas, that radiate with efficiencies a few orders of
magnitude larger than the A/(lambda)^2 limit, allow drastic miniaturization of
communication systems.
