This letter proposes a nonreciprocal metasurface retroreflector, which introduces isolation between two orthogonal polarizations. The proposed metasurface is based on the transistor-loaded nonreciprocity mechanism and receiving-reradiating structure. Benefitting from this design, the nonreciprocal polarization response introduced by the backloaded transistor amplifier can provide isolation between the two orthogonal polarizations. Besides, the unit cross-polarized reflection phase can be designed arbitrarily to realize desired phase gradient. As a result, the metasurface converts vertically polarized waves to horizontally polarized waves with amplification and retroreflects at a specific incident angle. Meanwhile, absorption occurs when horizontally polarized waves are incident at the same angle. A prototype is fabricated and measured. The measured results prove that the proposed metasurface can realize retroreflection at a 30° incident angle with cross-polarization isolation of 29.42 dB at 5.44 GHz. The proposed metasurface is expected to expand applications of planar retroreflectors.

A novel scheme for designing electromagnetic functional surfaces (EMFSs) is proposed in this article. Different from traditional frequency selective surface, this scheme uses the antenna array and the back-loaded radiofrequency circuit (BLRFC) to control the frequency response of EMFSs. The antenna array and the BLRFC can be designed independently resulting in great design flexibility and functional diversity. The sharp skirt out-of-band suppression and tunable frequency response can be achieved by using this scheme. Three design levels (element level, subarray level, and array level) are introduced to illustrate the design philosophy. Several EMFSs are designed, and one of them has been fabricated and measured. Actually, the prototype is a dual-polarized absorptive frequency selective reflector (AFSR). For both TE and TM polarizations with normal incidence, the AFSR exhibits one reflection band with 3-dB bandwidth from 10.6 to 16.3 GHz, two absorption bands with the absorptivity greater than 90% from 6 to 10 GHz, and from 16.8 to 19 GHz. The AFSR has a stable angular performance up to 50°. All the design examples and fabricated prototype verify the flexibility and the superiority of the novel scheme, and a multiplicity of EMFSs can be realized by using the BLRFC technique.

A novel design based on surface-circuit metasurface (SCM) is proposed to realize polarization conversion and retro-reflection simultaneously. Unlike conventional reflective metasurfaces, the meta-atoms of the SCM consist of two parts: antennas on top surfaces and back-loaded circuits. With this configuration, the manipulation of spatial electromagnetic (EM) waves can be converted to the operation of guided waves in microwave circuits. This configuration will bring benefits like eliminating unwanted coupling effects between neighboring elements and compact size for some complicated EM manipulation tasks. In this work, a back-loaded microstrip line connecting two ports of a dual-polarized patch antenna for orthogonal polarization is utilized to achieve linear-to-linear polarization conversion and desired reflection phase delay. Retro-reflection is then realized by generating gradient phase distribution on SCM by controlling the lengths of back-loaded circuits of meta-atoms. Finally, an 8 × 8 SCM prototype with a thickness of 0.068 λ ₀ at 23.8 GHz is designed, fabricated, and measured. Both simulation and measurement results show that the SCM can simultaneously achieve high-efficient polarization conversion and retro-reflection under the incidence of -30°. It is especially useful in some applications like RF tags and point-to-point wireless communication where high retro-reflection angular selectivity is needed.

In this letter, a broadband filtering power divider (FPD) based on a quarter circular/cambered substrate integrated waveguide (QCCSIW) cavity and microstrip line (MSL) resonators is proposed. First, a second-order bandpass filter (BPF) is designed based on the arc-shaped coupling of the QCCSIW. Second, a pair of quarter-wavelength ( $\lambda $ /4) MSL resonators are integrated into the QCCSIW cavity to form a fourth-order asymmetric BPF with a pair of transmission zeros (TZs). Finally, a hybrid QCCSIW and MSL FPD with isolation is investigated. For verification, all BPFs and FPD are simulated, fabricated, and measured. The proposed FPD demonstrates a center frequency of 4.99 GHz, a 3-dB fractional bandwidth (FBW) of 37.11% (1.85 GHz), and an insertion loss (IL) of 0.8 dB. The port isolation is greater than 11 dB within the entire passband. Meanwhile, a pair of TZs are generated at 3.62 and 6.35 GHz respectively. The proposed hybrid QCCSIW and MSL FPD possesses an overall size of 1.73 $\times$ 1.74 $\lambda_{{g}}$ , featuring advantages of broad bandwidth, compact size, and superior selectivity.

In this paper, a microwave absorption/transmission switchable frequency selective surface (A/T-SFSS) with enhanced multispectral functionality in the visible and infrared spectra is developed. By simply controlling the flowing liquid medium (pure water in this work), efficient manipulation of two opposite states, absorption and transmission, can be achieved in the microwave frequency band with a wide switchable bandwidth. When functioning as a wideband absorber, the proposed A/T-SFSS exhibits an absorption rate over 90% from 6.50 GHz to 10.48 GHz. Conversely, it can be employed as a transmitter with high selectivity, featuring a second-order passband with two transmission zeros at both sides and an impressive −1 dB transmission bandwidth ranging from 7.04 GHz to 8.47 GHz. The design also demonstrates stable responses under different polarizations and incident angles. Additionally, the proposed A/T-SFSS offers flexibility in tailoring the visible and infrared spectra by actively changing the color and temperature of aqueous solutions. The influences of electromagnetic (EM) parameters and liquid temperature variations on the microwave performance are also thoroughly investigated. Finally, experimental verification through free-space tests of scattering parameters, image analyses of grayscale histogram, and infrared radiation detected by thermal imager provides compelling evidence for the potential of the developed A/T-SFSS. The results demonstrate its remarkable capability to achieve switchable microwave functionality as well as active control of visible light and infrared spectra, which significantly contributes to the field by advancing the development of multifunctional and multispectral structures.

A planar wideband circularly polarized (CP) end-fire antenna is presented for millimeter-wave (mm-wave) applications. First, the antenna consists of an open-ended substrate integrated waveguide (SIW) feeding line in the middle, a pair of stacked substrates with shorted triangular patches on both upper and lower sides, and an offset parallel strip. The shorted triangular patch is viewed as a horizontally polarized electric dipole, while the open-ended SIW is equivalent to an enhanced magnetic dipole due to the stacked substrates. Wide axial ratio (AR) performance is achieved by simultaneously exciting the two dipoles with orthogonal phase supplied by the offset parallel strip. Then, working principle and parameter analysis are explained and discussed for a further understanding. Finally, a prototype is fabricated and measured to verify the design scheme, which demonstrates wideband impedance bandwidth and AR bandwidth performance, novel and efficient radiating structure, simple and small antenna size. This indicates that the proposed antenna is a promising candidate to design wideband end-fire CP antenna in the mm-wave frequency band.