This article proposes a new method to realize a low-profile broadband absorber. The unit cell of the proposed absorber consists of two layers: a lossy layer with four rotationally symmetric bent metallic strips embedded with two chip-resistors, which is modified from a doubly fed dipole antenna, and a metallic ground separated from the lossy layer by an air spacer. Three resonant modes of the metallic strip embedded with two chip-resistors are generated, and the current in the strip passes through the chip-resistors under different modes and it is finally consumed, resulting in energy dissipation. As a result, an ultrawideband absorber is realized. The designed absorber is fabricated and measured, and the measured absorption band with a fractional bandwidth of 127.9% is achieved for at least 10-dB reflectivity reduction under the normal incidence. In addition, the thickness of the designed absorber is only 0.08λL, where λL is the wavelength at the lowest operating frequency.

This paper proposes a hybrid absorptive-diffusive frequency selective radome (AD-FSR), which has a broadband transmission window with wide absorption and diffusion bands located at two sidebands, respectively. The proposed hybrid AD-FSR consists of a resistive sheet and a bandpass frequency selective surface (FSS) integrated with a coding metasurface separated by a certain distance. The wide absorption band is realized by the combined effect of the resistive sheet and the FSS integrated with a metasurface as a reflector in its lower stopband, while a broad passband is obtained when the electromagnetic (EM) wave penetrates through the resistive sheet in the passband of the FSS. In the upper side of the passband, the EM wave is diffused to reduce the radar cross-section (RCS) of the surface by arranging the unit cell of the AD-FSR and its mirror based on the phase cancellation theory. Equivalent circuit and relevant theoretical formulas are utilized to better comprehend the physical mechanism of the proposed hybrid AD-FSR. A design example is then fabricated and measured, and the experimental results show that a broad transmission band is achieved from 6.92 to 13.12 GHz with a minimum insertion loss of 0.43 dB in a wide low reflectivity (|S11|< -10 dB) band from 2.24 to 18 GHz.

A novel theory is proposed in this article to design a bandpass frequency-selective surface (FSS) with a stable angular response. Based on this theory, the performance of the structure is closely related to its tangential and normal components of permittivity and permeability. Theoretical formulas are derived to realize a high-efficiency transmission response with angular independence. To validate our design concept, an ultrawide-angle bandpass FSS with metallic strips and square patch printed on the dielectric substrate is designed, and it exhibits a characteristic impedance matching for nearly all incident angles from 0° to 80 °. Moreover, varactor diodes are mounted at the center of the square patch, and the transmission window can be tuned by changing the capacitance of the varactors from 6.07 to 6.61 GHz without affecting its ultrawide-angle characteristic. Finally, a prototype of the tunable ultrawide-angle bandpass FSS is fabricated and measured, and the experimental results are in satisfying agreement with the predicted ones.

A cognitive antenna array, which is designed by using deep reinforcement learning (DRL) is proposed in this article to adapt to the complex electromagnetic environment. Specifically, the phased array antenna is utilized as the manipulatable component to achieve the characteristic of beam steering with the help of the DRL algorithm. We begin by establishing a DRL-based framework, which is comprised of a microprogrammed control unit, power divider, digital phase shifter, and the patch antenna array. In the DRL algorithm, the desired beam steering is obtained through trial-and-error interactions with the environment, which is required to observe predefined rewards based on the current state and action. Next, the system is trained to perform beam steering, and a set of hyperparameters of the deep neural network are obtained and stored for practical usage. A good agreement is achieved between the simulated and measured radiation patterns of the planar phased array antenna, which validates the DRL-based phase distribution regulation algorithm. Finally, the algorithm is implemented in the design process of a conformal phased array antenna, and it is shown that the measured radiation performance of the array is satisfactory for different beam scan angles.

A dual-band multi-polarized aperture-shared antenna array is presented, achieving Ku-band dual linearly polarized (LP) operation and Ka-band dual circularly polarized (CP) operation. Its dual-band aperture-shared structure is fed by two groups of independent feed networks, from the side and the bottom, using the quasi-planar coaxial waveguide line (QCWL) and the hollow-waveguide (HW) feed structure, respectively. Ku-band antenna and Ka-band antenna are carried out through the same quad-ridge waveguide aperture, and have the same array spacing. Thus, the large element spacing at high-frequency band inevitably leads to high sidelobes. The parasitic elements are adopted to suppress the high sidelobes in the E-plane and H-plane, and the realized gain of the high-frequency antenna is increased accordingly. In addition, a 1/2λ short-circuited stub is designed in the QCWL-based feed network for isolation enhancement. To demonstrate, a 4×4-element Ku/Ka-band multi-polarized antenna prototype is fabricated and measured. Experimental results show that the impedance bandwidths of 19.4% (14.4–17.5 GHz) for dual LP and 7.4% (32.5–35 GHz) for dual CP are achieved. The peak gains of the dual LP are 18.4 dBi and 18 dBi respectively while those of the dual CP are 23.4 dBic and 22.9 dBic.

In this paper, a novel method for developing a dual-polarized broadband microwave absorber (MA) for use at an extremely oblique incidence angle is presented. A detailed theoretical analysis is performed and formulas are derived to obtain the required relative permittivity and permeability tensors to achieve dual-polarized absorption at an arbitrary oblique incidence angle. As a proof of concept, a broadband dual-polarized MA operating at an oblique angle of 75° is designed whose unit cell consists of a pair of rectangular patches and a rectangular loop embedded with four resistors. A prototype is fabricated and measured for verification. Both the measured and simulated results demonstrate that the proposed MA achieves more than 90% absorption within a broad frequency with a relative bandwidth of 61.2% for both transverse electric and transverse magnetic polarizations at an extremely oblique incidence angle of 75°.