Provided herein are methods and systems for quasi-harmonic tags (qHT) including an electromagnetic resonator, an input mesh including an input notch filter having a resonant frequency of .sub.input, an output mesh including an output notch filter having a resonant frequency of .sub.output; and an antenna, wherein the qHT is configured to emit a comb output signal responsive to an input signal having an input frequency (op twice a resonance frequency of at least one of the electromagnetic resonators.

Techniques for characterizing a vocal tract by using radar measurements. A radar marker that is arranged in or on the vocal tract is used.

A device includes a receive antenna configured to receive a radar signal. The device includes a return signal generator configured to generate a return signal based on the received radar signal. The return signal includes at least a first component at a first frequency offset from a carrier frequency of the received radar signal and a second component at a second frequency offset from the carrier frequency. The device also includes a transmit antenna configured to transmit the return signal.

A system for communication can include a frequency selective surface having a tunable radar cross section configured to receive an electromagnetic signal. The frequency selective surface can include an isolation layer, a plurality of conductive elements disposed on a first side of the isolation layer, and a plurality of tunable components, each coupled with a respective conductive element. A control system can be configured to adjust a bias voltage applied to the tunable components to modulate the radar cross section of the frequency selective surface and encode binary data in a reflected electromagnetic signal. A radar system can be configured to transmit the electromagnetic signal and receive the reflected electromagnetic signal. The frequency selective surface can operate within a frequency band corresponding to an operational frequency of the radar system to enable adaptive, reflected-signal communication.