With the small sampling length, a novel Direction-of-Arrival (DOA) estimation method based on a single programmable metasurface sensor is proposed in this article. Serving as a physical random sampling receiver, a dynamic metasurface generates series of random radiation patterns to sense the incident signals, which is subsequently processed by the compressive sensing (CS) orthogonal matching pursuit (OMP) algorithm to recover the DOA information. A mathematical model of DOA estimation is first built for theoretical analyses. Furthermore, numerical simulations are investigated with extensive performance analyses. In comparison with the metasuface-based beam-scanning method, metasurface-based CS for DOA estimation has superior performance with a smaller sampling length. Finally, a proof-of-concept experiment is made to verify the feasibility of the programmable metasurface for DOA estimation. Compared to other traditional estimation techniques that require multiple channels, the programmable metasurface can achieve estimation with only a single channel and small sampling length.

Single Sensor to Estimate DOA With Programmable Metasurface

This paper presents a highly compact frequency-selective surface (FSS) that has the potential to switch between the X-band (8 GHz–12 GHz) and C-band (4 GHz–8 GHz) for RF shielding applications. The proposed FSS is composed of a square conducting loop with inward-extended arms loaded with curved extensions. The symmetric geometry allows the RF shield to perform equally for transverse electric (TE), transverse magnetic (TM), and 45° polarizations. The unit cell has a dimension of 0.176 λ0 and has excellent angular stability up to 60°. The resonance mechanism was investigated using equivalent circuit models of the shield. The design of the unit element allowed incorporation of PIN diodes between adjacent elements for switching to a lower C-band spectrum at 6.6 GHz. The biasing network is on the bottom layer of the substrate to avoid effects on the shielding performance. A PIN diode configuration for the switching operation was also proposed. In simulations, the PIN diode model was incorporated to observe the switchable operation. Two prototypes were fabricated, and the switchable operation was demonstrated by etching copper strips on one fabricated prototype between adjacent unit cells (in lieu of PIN diodes) as a proof of the design prototypes. Comparisons among the results confirmed that the design offers high angular stability and excellent performance in both bands.

https://www.mdpi.com/2079-9292/10/4/476/pdf

C-Band and X-Band Switchable Frequency-Selective Surface

To satisfy the demands of wireless communication systems for tunable shielding materials, in this work, a graphene/ion gel/graphene sandwich structure is proposed, based on graphene and a solid ionic material ion gel. After modelling, preparing and testing, it was found that the structure could achieve more than 10 dB shielding effectiveness tuning in GHz and THz bands. Meanwhile, the adjusting speed of the structure was also studied, displaying effective dynamic tuning in the second order. Furthermore, the fabricated samples have the advantages of a low profile, easily conformable, and convenient processing, which are of great potential in emerging electronic devices.

Demonstration of Tunable Shielding Effectiveness in GHz and THz Bands for Flexible Graphene/Ion Gel/Graphene Film

A novel omega shaped microwave absorber with wideband negative refractive index for C-band applications

In this article, a PIN-diode-based high-intensity radiation fields (HIRF) protection method was proposed and the sensitivity to the field intensity of incident wave has been verified by a printed dipole antenna loaded with three pairs of PIN diodes at S-band. To illustrate this method, an equivalent linear circuit was raised to analyze the basic operation principle of the proposed antenna. With a low-power incident wave, the PIN diodes are in off-state and the gain of the proposed antenna remains unaffected; while it drops considerably as the PIN diodes are triggered by HIRF. Furthermore, extensive parametric studies of the structure were analyzed by full-wave simulation to investigate the key factors affecting the performance of HIRF protection and antenna characteristics. Finally, a prototype was fabricated and experiments were implemented to verify the feasibility, which shows a high isolation of 19 dB. To the best of our knowledge, the proposed antenna is the first one to achieve HIRF protection through integrated antenna technology, which features with low cost and compact structure, providing a new perspective.

PIN-Diode-Based High-Intensity Radiation Fields (HIRF) Protection of a Printed Dipole Antenna

In this letter, a flexible and frequency-reconfigurable bandstop frequency selective surface (FSS) is studied theoretically and experimentally. The FSS patterns are fabricated on an ultrathin flexible substrate through screen printing, a low-cost, high-efficiency additive manufacturing methodology. With the embedded varactors biased from 0 to 20 V, resonant frequency tuning from 3.5 to 5.7 GHz was measured. Outstanding wide angular stability under different bias voltages has also been examined in theory and measurement by changing the incidence angle from 0° to 60°. As a demonstration of the flexible FSS in the conformal application, the FSS is tightly covered on a half-cylinder with a diameter of 10 cm, which agrees well with the case when placed on a flat surface. Furthermore, the unit cell has a size of λ/15 at 3.5 GHz, featured with miniaturization. The mechanical flexibility, frequency reconfigurability, and angular stability make it meaningful in applications of flexible/conformal covering for aiming objects, including electromagnetic scattering control, electromagnetic compatibility (EMC), etc.

Flexible and Reconfigurable Frequency Selective Surface With Wide Angular Stability Fabricated With Additive Manufacturing Procedure

Flexible metamaterial absorber manufactured with a layer-by-layer screen-overprinting methodology has been theoretically and experimentally studied in this letter. The effective absorption of the absorber is in the X -band (8.3–10.9 GHz) and its working principle has been verified and analyzed. Benefiting from the elimination of the parasitic effect brought by package-free and direct-contacted planar resistors, high agreement between simulations and measurements has been achieved. The working frequency band can be easily tuned by replacing different supporting and covering layers to fit various purposes. Mechanical flexibility of the absorber enables its conformal coverage to aiming objects for radar cross-section reduction, and is experimentally confirmed on metallic cylinders with different diameters. It has been clearly demonstrated that the high-efficiency and cost-effective automatic screen-overprinting method matches perfectly for large-size flexible absorbers and their industrial applications.

Screen Overprinted Flexible Radar Absorber Composed of Planar Resistor Loaded Metamaterials

Electrically tunable metamaterial is attracting extensive attention recently for its potential in electromagnetic compatibility (EMC) and radar scattering manipulation, while still remains a great deal of challenges in its tunability. In this paper, a conductivity tunable graphene-ion gel-graphene (GIG) sandwich structure is fabricated and measured to investigate the properties of ion gel, showing its advantages over ion liquid on flexibility and tunability. Besides, a frequency and amplitude dual-tunable metamaterial absorber based on the GIG sandwich structure is proposed, modeled and optimized. From the simulated response, the absorber can achieve the tuning of absorption frequency from 5.4 GHz to 7.1 GHz and the reflection minimum from -15 dB to -45 dB. With the introduction of tunability from varactor diodes and graphene, the working frequency and absorption can be almost independently tuned. This work gives a meaningful reference for graphene based tunable metamaterial based on solid ion gel, especially in conformal and outdoor environment applications.

Dual-tunable metamaterial absorber based on solid ion gel-graphene sandwich structure

Transparent electromagnetic interference (EMI) shielding film has a wide range of applications in the optoelectronic and display systems in military, aerospace and other fields, which has attracted widespread attentions. In this paper, the electrohydrodynamic jet printing method was introduced into the additive preparation of transparent conductive films, and a batch of flexible transparent EMI shielding films with good EMI shielding effectiveness and low cost were achieved in a simple and easy way. We utilized electrohydrodynamic jet printing technique to directly print a grid-like micro structure on a flexible and transparent substrate, and a transparent EMI shielding film with a line width of 13.3 μm, period of 300 μm, and light transmittance of 73.2% was obtained. The EMI shielding effectiveness is about -18 dB in X band. In addition, the printing parameters was optimized to obtain transparent EMI shielding films that meet different performance requirements. The transparent EMI shielding film prepared by printing method requires no high-resolution mask and exposure in traditional microfabrication, and is quite low cost and convenient to adjust parameters digitally. This work proves that the additive electrohydrodynamic jet printing perfectly fits the transparent film research in stage of sample design and prototype.

Transparent electromagnetic shielding film based on additive electrohydrodynamic jet printing technology

This paper proposes a graphene-based flexible printed attenuator consisting of printed microwave transmission line and graphene/ion-gel tunable structure, which can realize the transmission of microwave signals with tunable attenuation and have flexible conformal characteristics. According to the adjustable resistivity of graphene in microwave band, we utilize ion-gel to produce electric double layer (EDL) effect to adjust the conductivity of graphene at a low bias voltage. The coplanar waveguide (CPW) transmission line is prepared on a flexible substrate by screen printing process. The implementation of this flexible printed attenuator relies on the integration of printed CPW and graphene/ion-gel functional components. The flexible attenuator has an obvious attenuation characteristic at a lower applied bias voltage (1.3V), and the adjustable range of attenuation is greater than 15 dB in the frequency range of 500 MHz - 4.8 GHz. This work demonstrates a proof-of-concept tunable attenuator with the characteristics of dynamically adjustable attenuation, low working voltage, facile fabrication and flexibility, and will provide some references for the future development of planar tunable attenuators.

Shanrong Hu

Papers

Flexible and Reconfigurable Frequency Selective Surface With Wide Angular Stability Fabricated With Additive Manufacturing Procedure

Screen Overprinted Flexible Radar Absorber Composed of Planar Resistor Loaded Metamaterials

Dual-tunable metamaterial absorber based on solid ion gel-graphene sandwich structure

Transparent electromagnetic shielding film based on additive electrohydrodynamic jet printing technology

Flexible printed microwave transmission line with tunable attenuation based on graphene/ion-gel structure