  Electromagnetic scattering from moving bodies, being inherently
time-dependent phenomenon, gives rise to a generation of new frequencies, which
could characterize the motion. While a standard linear path leads to a constant
Doppler shift, accelerating scatterers could generate a micro-Doppler frequency
comb. Here, a spectra produced by rotating objects, was studied and observed in
a bi-static lock in detection scheme. Internal geometry of a scatterer was
shown to determine the spectra, while the degree of structural asymmetry was
suggested to be identified via signatures in the micro-Doppler comb. In
particular, hybrid magneto-electric particles, showing an ultimate degree of
asymmetry in forward and backward scattering directions were investigated. It
was shown that the comb in the backward direction has signatures at the
fundamental rotation frequency and its odd harmonics, while in the forward
scattered field has the prevailing peak at the doubled frequency and its
multiples. Additional features in the comb were shown to be affected by the
dimensions of the particle and strength of magneto-electric coupling.
Experimental verification was performed with a printed circuit board antenna,
based on a wire and a split ring, while the structure was illuminated with at
2GHz carrier frequency. Detailed analysis of micro-Doppler combs enables remote
detection of asymmetric features of distant objects and could find use in a
span of applications, including stellar radiometry and radio identification.
