  Electromagnetic scattering in accelerating reference frames inspires a
variety of phenomena, requiring employment of general relativity for their
description. While the quasi-stationary field analysis could be applied to
slowly-accelerating bodies as a first-order approximation, the scattering
problem remains fundamentally nonlinear in boundary conditions, giving rise to
multiple frequency generation (micro-Doppler shifts). Here a frequency comb,
generated by an axially rotating subwavelength (cm-range) wire and split ring
resonator (SRR), is analyzed theoretically and observed experimentally by
illuminating the system with a 2GHz carrier wave. Highly accurate lock in
detection scheme enables factorization of the carrier and observation of more
than ten peaks in a comb. The Hallen integral equation is employed for deriving
the currents induced on the scatterer at rest and a set of coordinate
transformations, connecting laboratory and rotating frames, is applied in order
to predict the spectral positions and amplitudes of the frequency comb peaks.
Unique spectral signature of micro-Doppler shifts could enable resolving an
internal structure of the scatterers and mapping their accelerations in space,
which is valuable for a variety of applications spanning from targets
identification to stellar radiometry.
