This paper presents the concept of spread-spectrum selective camouflaging based on time-modulated metasurface. The spectrum spreading is realized by switching the metasurface between the reflective states of a Perfect Electrical Conductor (PEC) mirror and a Perfect Magnetic Conductor (PMC) mirror, using an array of microstrip patches connected to the ground via diode switches, according to a periodic pseudo-random noise sequence. As the spectrum spreading induces a drastic reduction of the power spectral density of the signal, the level of the scattered wave falls below the noise floor of the interrogating radar, and the object covered by the metasurface is hence perfectly camouflaged to a foe radar. Moreover, the object can be detected by a friend radar possessing the spread-spectrum demodulation key corresponding to the metasurface modulation, and this detection is robust to interfering signals. The proposed system is analyzed theoretically, and demonstrated by both simulation and experimental results.

In this paper, a 2-bit reconfigurable reflective metasurface (RRM) with multifunction for both far-field pattern and polarization performance is proposed. To facilitate the independent design of two orthogonal polarizations as well as the diode integration at the bottom, the radiation and phase shifting parts are designed separately. The top patch is designed to receive and re-radiate EM waves, which converts the wave states between free-space wave and guided wave. The bottom phase shifting structure is designed to switch four phase states with 90° interval for the reflected guided wave. An 8×8 RRM is designed and fabricated using the proposed 2-bit element. Driven by a field programmable gate array (FPGA), the RRM achieves real-time control for different functions of beam deflection, radar cross section (RCS) reduction, and polarization conversion. The measured results show that the steered beam from -30° to +30° can be obtained around 3.83 GHz with different coding matrices, and more than 10 dB RCS reduction from 3.70 GHz to 3.84 GHz can be achieved by properly distributing phase responses randomly. Moreover, a linear polarization (LP) to left-handed circular polarization (LHCP) conversion in the range 3.82 GHz-3.85 GHz and right-handed circular polarization (RHCP) conversion in the range 3.68 GHz-3.82 GHz are realized by switching all the PIN diodes ON and OFF.

We propose a novel design of a near-field transmitting focused metasurface, which transforms a linearly polarized planar incident wave into a near-field focused beam with multiple foci and multiple nulls in this paper. A generalized method of obtaining the metasurface field distribution on its aperture for specified near-field foci and nulls is firstly introduced. Then, a three-layer metasurface particle structure with close to full range reconfigurable amplitude and phase transmitting response for realizing the required field distribution is studied. By adjusting the arc length and the rotating angle of the middle-layer metasurface particle structure, a transmitting response with arbitrary amplitude and phase is achieved. Finally, a focused metasurface with two near-field foci and two near-field nulls is designed based on the proposed theory and metasurface particle structure. The designed metasurface is full-wave simulated, fabricated, and measured in the experiment. The experimental results agree well with the full-wave simulating results, showing that both the foci and nulls are well presented at the specified positions and the measured power of the near-field nulls is more than 26 dB below the power of the foci.

In this article, pseudorandom noise (PN) sequence time-modulated reflective metasurfaces are proposed to provide a low-cost, low-complexity, simple solution for target recognition. By covering the target to be recognized with a time-modulated metasurface modulated by a PN time sequence, the metasurface scatters the interrogating harmonic wave into a binary PN-modulated wave. As a result, the radar detector with the foreknowledge of the modulating PN sequence may recognize the target by applying the properties of strong autocorrelation and weak cross correlation of PN sequences and performing cross correlation between the modulating PN sequence and the picked-up signal after downshifting its frequency without actually decoding and reading the scattered information. A 1-bit reconfigurable metasurface unit cell operating at 2.4 GHz is proposed and full-wave simulated to achieve out-of-phase reflective responses by controlling the ON and OFF states of the p-i-n diode for PN time modulation. The target recognition is demonstrated by experiment in two different scenarios where only one metasurface and multiple metasurfaces are present for recognition. The experimental results show that by providing an appropriate threshold in the cross-correlation detecting results, the proposed method can recognize the targets well in both scenarios.

In this paper, a single-beam-scanning metasurface with dual circular polarization using hybrid coding mechanism is presented, which can steer the scattered wave in a large range of angles under the planar incident wave. In order to suppress the grating lobes generated by periodic 1-bit phase quantization errors, elaborately designed two-dimensional fixed phase delays (FPDs) are introduced. The 1-bit active coding is designed based on the current reversal method, which realizes two states with 180° phase difference over the frequency range of 8.5 GHz - 11 GHz by switching two PIN diodes for both left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) incidence. The passive coding is realized by rotating the metasurface unit cell based on Pancharatnam-Berry (P-B) phase principle to introduce the two-dimensional FPD. The proposed design mechanism is theoretically analyzed, full-wave simulated, and demonstrated by experiment. An 8×8 metasurface prototype is fabricated and measured. The experimental results show that steered beams from -30° to +30° without grating lobes in both x-o-z plane and y-o-z plane can be obtained around 9.5 GHz. The side lobe level (SLL) can be minimized to -8.87 dB, and the beam pointing deviation is within 3°.