Active frequency selective surfaces for beam switching applications

In this paper, an octahedron electronically beam-scanning antenna using a novel slot active frequency selective surface (AFSS) is presented. The antenna consists of eight panels of octagonal slot AFSS array and an omnidirectional feed antenna located in the center. The beam-scanning of the antenna is obtained by switching the states of the PIN diodes to reconfigure the AFSS as EM transparent or reflective to the incident electromagnetic waves. By controlling the DC bias voltages to PIN diodes which are grouped by columns, the directive beam of the antenna can be switched to cover the entire azimuth plane. A prototype is fabricated and measured. Simulation and measurement results of the antenna are presented to verify the design concept. It is shown that the designed octahedron electronically beam-scanning antenna can cover a full 360°horizontal angular range. The presented design shows obvious advantages in terms of low power consumption, low cost and easy fabrication.

A novel antenna with both frequency and pattern reconfigurability is presented. The reconfigurability is achieved by integrating an active frequency selective surface (AFSS) with feed antenna. The smart FSS comprises a printed slot array loaded by varactors. A novel dc biasing arrangement is proposed to feed the slots vertically so that the unwanted effects caused by bias lines are minimized. A monopole antenna is designed to illuminate the AFSS. The resulting structure can operate in a frequency tuning range of 30%. By reconfiguring the different sections of active FSS cylinder into a transparent or reflector mode, the omnidirectional pattern of the source antenna can be converted to a directive beam. As an illustration, half of the AFSS cylinder is successively biased, enabling the beam switching to cover the entire horizontal plane over a range of frequencies. An antenna prototype was fabricated and measured. Experimental results demonstrate the capability of providing useful gain levels and good impedance matching from 1.7 to 2.3 GHz. The antenna offers a low-cost, low-power solution for wireless systems that require frequency and beam reconfigurable antennas. The proposed design consumes about 1000 times less dc power than the equivalent narrowband beam-switching antenna design using p-i-n diode-loaded AFSS.

A reconfigurable antenna with frequency agility and pattern-switching ability is presented in this paper. The proposed design comprises a planar and compact frequency reconfigurable monopole antenna and an active frequency selective surface (AFSS) with reconfigurable transmission and reflection properties in dual bands. The AFSS screen is divided into four sections and each section is composed of four FSS elements with only one PIN diode in each unit cell. The monopole antenna, which is situated at the center of the box-shaped AFSS to illuminate , can reconfigure its resonating frequency between 2.5 and 3.6 GHz with the help of only one PIN diode. With the help of this transformation ability of AFSS behavior electronically, the pattern can be steered across the entire X–Z plane in eight steps at two different operating frequencies independently. This antenna can be very useful in various wireless applications such as drones, small base stations, etc.

A new conception for a reconfigurable antenna based on an active Frequency Selective Surface (FSS) is presented. The antenna system consists of a 3D cylindrical and a dipole antenna. This design can be achieved reconfigurable beams along the entire azimuth plane. The FSS unit-cell is designed in order to control the transmission characteristics of EM waves by controlling switching devices. Thus, to explain the design concept, the reflection and transmission coefficients of the 3D FSS are provided. Reconfigurable feature of the antenna is studied and demonstrated at 1.8 GHz. This smart antenna can be for wireless communication applications.

A tunable polarization-insensitive absorber based on an active frequency-selective surface is proposed for electromagnetic (EM) wave absorption in the L and S bands. The frequency-selective surface consists of a diamond-shaped loop that is interconnected by varactor diodes. The proposed design is significant because of its inclusion of wideband tuning and its ability to easily change from single-band tunable absorption to dual-band absorption. The simulation results demonstrate that the absorption response (> 80%) can vary from 1.62 GHz to 3.66 GHz under the reverse bias voltage, thus creating a tunable range of 125.9%. The absorption region (> 90%) spans from 1.86 GHz to 3.31 GHz and generates a 78% tunable range. The single-band tunable absorber is characterised by an equivalent circuit model to obtain the circuit parameters and predict the absorption frequency. Furthermore, a dual-band absorbing structure is obtained by adding a metasurface directly to the single-layer structure, which can easily be changed from single-band absorption to dual-band absorption by separately controlling the bias voltages of the upper and lower unit cells. The two fabricated prototypes were measured using free-space technology, and the experimental results closely mirror the simulation results. Results show that the proposed structure achieves the dynamic control of EM wave absorption.

Resonant frequency tunable range is an important character for active frequency selective surfaces (AFSSs), especially when building electronically steerable antennas based on AFSS. In this communication, a broadband tunable bandpass AFSS is presented. The tunable ability is achieved by the varactors mounted on the surface. The proposed design has zigzag resonant current flows through multiple unit cells. Thus, the magnetic field coupling between unit cells is enhanced and the resonant frequency tunable range is extended. The measured resonant frequency tunable range is from 935 MHz to 2.405 GHz. Another advantage of this design is that no additional bias grid is required. Furthermore, this design has stable response under different angles of incidence because it has subwavelength dimensions.

This study proposes an ultra-wideband radar cross section (RCS) reduction technique based on reflection phase interference cancellation and tunable absorption. The operational frequency band of the combined structure is enhanced by uniting two overlapped frequency ranges effectively. As proof of concept, a demonstration structure is designed with a stacked chessboard and an active frequency selective surface (AFSS) absorber. The upper chessboard constructed of a square loop and circular patch unit cells can realize destructive phase interference performance at higher frequencies, while the bottom AFSS absorber achieves tunable absorption at a lower frequency range. Three-dimensional scattering patterns at low and high frequencies are analyzed to verify these two different RCS reduction mechanisms. The combined fabrication structure can obtain an ultra-wideband RCS reduction of more than 10 dB from 4.4 to 36.5 GHz, compared to a metallic plate of the same size, indicating good agreement between full-wave simulation and measurements.

A new type of active frequency selective surface (AFSS) is proposed to realise a voltage controlled bi-state (transparent and reflecting) response at the specified frequencies. The bi-state switching is achieved by combining a passive array of interleaved spiral slots in conducting screens and active dipole arrays with integrated pin diodes at the opposite sides of a thin dielectric substrate. Simulation results show that such active surfaces have high isolation between the transparency and reflection states, while retaining the merits of substantially sub-wavelength response of the unit cell and large fractional bandwidths (FBWs) inherent to the original passive interwoven spiral arrays. Potential applications include reconfigurable and controllable electromagnetic architecture of buildings.

One of the key element in performance measurement for a neighbor discovery algorithm with switched beam antennas is how long it takes to discover the neighbors given that nodes do not have any prior knowledge about their neighbors. An impression we may have in neighbor discovery is that algorithms based on directional antennas takes longer time for a node to discover all its neighbors than those entirely based on omnidirectional antennas. We propose two neighbor discovery schemes, one is based on time slotting and the other on the well known CSMA/CA unslotted version. We evaluate the time that the sink needs to discover its neighbors when it is equipped with a switched beam antenna, and when it uses an omnidirectional antenna. Through our simulation results, we show that when neighbor discovery algorithms based on directional antennas are correctly designed, they achieve the same order of performance in terms of time needed by the sink to discover all its neighbors as those based on omnidirectional antennas. Our simulation results also show that when using a switched beam antenna and CSMA/CA algorithm for medium access at the sink, the discovery delay is higher.

A single‐layer, bandpass, active FSS is presented. It shows good angle of incidence stability for TE incidence, at the operating frequency of 2.45 GHz. It is based on circular loop aperture with each unit cell having four PIN diodes. A novel method for dc biasing is used. About 12 dB average variation in transmission loss, between ON and OFF states, has been experimentally achieved at 2.45 GHz. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2149–2151, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23615

Achieving a high‐gain omnidirectional patch antenna with low profile is a challenging task. On the basis of improving antennas by metamaterials, a broadband near‐zero‐index metamaterial (NZIM) is designed to enhance the horizontal gain of the omnidirectional patch antenna. The zero‐index metamaterial is loaded on the substrate of three antennas with different working bands (4.99–5.50 GHz). The results show that the horizontal gain of antennas is enhanced by more than 0.6 dB at various frequencies. The roundness of antennas is also optimised. Therefore, using NZIM can increase the horizontal gain with a low profile ( λ /30) structure size. The low profile, high‐gain omnidirectional patch antenna has great potential in the conformal field.

A novel beam switching antenna design is proposed in this work. This antenna uses a dipole as the fed and a cylindrical metal enclosure with a stack of circular slits is put around dipole. An array of RF switches made of pin diodes are placed across the slits. The antenna beam can be adjusted to the desired direction by changing on-off states of the switches, which in turn reconfigures the slot array orientation. The beam switching feature of the proposed antenna makes it an attractive low cost alternative to wireless communication systems that need to implement smart antennas in their base stations.

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An investigation of novel techniques in the designing of a low profile, horizontally polarized omnidirectional (HPO) patch antenna with omnidirectionality in the H-plane using a defected ground structure is presented in this study. The HPO patch antenna is designed by sequentially etching four rectangular and four open diagonal slots symmetrically on the patch and the antenna's ground plane. It creates four quarterwavelength open stubs that produce an in-phase circulating current on both the ground and the patch. Narrow bandwidth is achieved by using the four quarter-wavelength open stubs and by printing sequential slots only on the ground. Another set of four quarter-wavelength shorted stubs is achieved by sequentially etching four rectangular and open diagonal slots on the patch to resonate with another frequency and to increase the bandwidth by merging these two resonance frequencies. Combining these two resonance frequencies covers the applications of 2.39–2.52 GHz WiFi bands. The overall dimension of the proposed antenna structure is 0.32 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ×0.32 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ×0.006 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> with 5.51% bandwidth and moderate gain of 2.2dBi and roundness within 1 dB in the azimuth plane.

A novel design of switched beam antenna (SBA) system based on Fabry-Perot cavity leaky-wave antenna (FPC LWA) is designed and fabricated for base station operating in the unlicensed ISM central frequency band at 5.8 GHz of the wireless local area network (WLAN) standard. The proposed SBA is designed with hexagonal shape of FPC LWA Arrays in order to get 360° of coverage. The single element of FPC LWA array is composed of a patch antenna and covered by a Partially Reflective Surface (PRS), which is composed of a Metal Strip Grating and printed on a high permittivity Superstrate. First, the Transmission Line Model of FPC LWA is introduced to analyse and calculate the far-field components in E- and H planes by using the Transverse Equivalent Network. This approach is then compared with other full wave’s commercial software such as Ansoft HFSS and CST Microwave Studio. Second, a parametric study is performed to evaluate the effect of the angle formed by the two successive FPC LWA on the radiation efficiency of the activate sector. To examine the performance of the proposed SBA, experimental prototype was fabricated and measured. As a result, multiple orthogonal beams (six beams) of 10 dBi of gain with low Side Lobes Level and 360° of coverage are produced. This SBA structure is suitable for WLAN communication systems.