Research on Luneburg lens reflector with switchable working state

In this article, a transmission and reflection states switchable Luneburg lens (LL) reflector based on active frequency selective surface (AFSS), namely LL-AFSS reflector, is proposed. The reflector is composed of a miniaturized AFSS reflector, which can change the working state, and a LL composed of six layers and five media from inside to outside. By changing the bias voltage of the PIN diode loaded on the AFSS reflector, the transmission and reflection working states of the LL reflector can be freely switched in the frequency range of 3.5–5 GHz. When the PIN diodes are in ON state, the incident electromagnetic wave will not be reflected through the LL reflector. When the PIN diodes are in OFF state, the radar cross section (RCS) generated by the LL reflector to the incident electromagnetic wave reaches about 15 dB within the angle range of −30 ≤ φ ≤ 30°. The electromagnetic numerical simulation and experimental sample verification of LL-AFSS reflector are carried out. The results show that the proposed design method is feasible and the RCS changes of transmission and reflection characteristics of LL reflector are obvious.

A dual-function switchable and frequency tunable active frequency selective surface (AFSS) is proposed in this article. The proposed AFSS consists of a dielectric substrate and metal patches on the top and bottom surfaces of the substrate, while varactor diodes and PIN diodes are soldered on the top and bottom layers, respectively. A simple bias network is designed to facilitate the function adjustment of the AFSS. By controlling the OFF and ON states of the PIN diode, the AFSS can switch between filtering and wave absorbing functions. Furthermore, by adjusting the bias voltage of the top varactor diode, the AFSS can achieve continuous frequency tunability. Simulation and experimental results confirm the dual-function switchable and frequency tunable performance of the proposed AFSS.

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 reconfigurable active frequency selective surface (AFSS) with ultra-wideband (UWB) characteristics is presented in this article. The proposed AFSS is a periodic array structure which consists of three metal layers and two dielectric layers. PIN diodes are arranged on the top and bottom metal layers which can rebuild the function of AFSS. The transmission bandwidth of the OFF-state AFSS is 8-12.5 GHz with a fractional bandwidth of 44%, and the transmission coefficient of the ON-state AFSS is lower than -10 dB from 2 to 18 GHz. Additionally, the AFSS has dual polarization characteristics, and it is angular stable and polarization-insensitive for two polarizations. The simulation results show the proposed AFSS is a valuable candidate for radome applications.

A polarisation-independent switchable absorber/reflector based on active frequency selective surface (AFSS) has been reported. The FSS design consists of diagonally connected circular loops on which semiconductor switches (PIN diodes) have been mounted. By controlling the bias voltage of the PIN diodes, the structure shows perfect reflection in ON state while exhibits single band absorption during OFF state. The proposed design is four-fold symmetric, thereby realising polarisation-insensitive behaviour, unlike the earlier reported AFSS designs. Furthermore, a prototype of the proposed structure has been fabricated where the PIN diodes have been biased through a novel biasing technique. The measured result also shows good agreement with simulated response under normal incidence.

A novel active frequency selective surface (AFSS) exhibiting a multifunctional characteristic, switching and frequency tuning, is present in this paper. The proposed structure is made up of periodic metallic patterns printed on the opposite sides of a dielectric substrate. Two different active components (varactors and PIN diodes) are mounted on the top and bottom layers, respectively. The switching characteristics between transmission and absorption modes can be obtained by controlling the biasing condition (OFF/ON state) of the PIN diodes on the bottom layer, where both modes having frequency tuning capability by regulating the voltage of varactor diodes on the top layer. Besides, full-wave simulation has been investigated to study the characteristic of the proposed designed. The structure has been fabricated to verify with the simulation results. In transmission mode, the proposed AFSS realizes tunable bandpass responses from 3.70 to 4.27 GHz with 1.96-4.30 dB insertion loss. In absorption mode, the absorption frequency can be tuned from 4.28 to 5.12 GHz with 78.77-90.78% absorptivity.

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.

In this paper, a beam switching antenna with a novel active frequency selective surface (AFSS) is proposed at 5.2 GHz. The antenna is composed of an omnidirectional patch antenna and AFSS. The AFSS is made up of 6 columns, each with 2 unit cells, which are placed vertically around a patch antenna to form a hexagonal wall. To steer beam of the antenna, high frequency pin diodes are precisely inserted into the AFSS to reconfigure electromagnetic (EM) response of surfaces. The reflection and transmission characteristics of the proposed AFSS unit cell are investigated. The simulation results show that the transmission and reflection bands of AFSS unit cell can be changed by setting diodes on and off. This new AFSS can be used to implement a sweeping radiation pattern at all azimuth angles. The antenna can sweep the entire azimuth plane in six steps and effectively operate over a bandwidth of more than 500 MHz.

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

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.

Conventional active frequency selective surfaces (AFSS) are designed with a DC biasing circuit to apply DC biasing voltage across active components. An innovative method to design AFSS is proposed in this paper to use an induced voltage across both ends of the surface loading components as the biasing signal to control FSS's transmission properties. The result from simulation proved that, a pass band exists on the AFSS at 3.5GHz when it is exposed to an electric field at a smaller intensity; at the electric field intensity (EFI) over 70 V/m, the insertion loss of the electromagnetic wave (EMW) increases at 3.5 GHz; in the case that the incident microwave's field intensity reaches 5000 V/m, the insertion loss is over 20 dB. This interesting phenomenon make the new AFSS has the ability of HPM protection, which mean that a pass band exists when EFI is weak enough, along with increasing incident field intensity, the pass-band finally turns into a stop-band because of the break though of the diodes.

In this communication, a novel filtering antenna is proposed by utilizing active frequency selective surface (FSS), which can simultaneously achieve filtering and beam steering function. The FSS unit is composed of a metallic rectangular ring and a patch, with a pair of microwave varactor diodes inserted in between along incident electric field polarization direction. Transmission phase of the emitted wave can be tuned by changing the bias voltage applied to the varactor diodes. Through different configurations of the bias voltages, we can obtain the gradient phase distribution of the emitted wave along E- and H-plane. This active FSS is then fabricated and utilized as a radome above a conventional horn antenna to demonstrate its ability of steering the beam radiated from the horn. The experimental results agree well with the simulated ones, which show that the horn antenna with the active FSS can realize beam steering in both E- and H-plane in a range of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\pm 30^{\circ}$</tex></formula> at 5.3 GHz with a bandwidth of 180 MHz.

Reflectivity-Amplitude Modulation of A Radar Absorber Based on Active Frequency Selective Surface

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.

In this letter, a broadband polarization-independent tunable absorber is proposed based on active frequency selective surface (AFSS). The AFSS consists of periodic patterns of four-patch array with four p-i-n diodes loaded among them, and a simple bias network is designed behind the ground plane through metalized via holes, which can effectively reduce the influence on the absorbing performance. By changing the bias voltage applied on the p-i-n diodes, the reflectivity of our absorber can be dynamically adjusted from near-unity reflection to less than -10 dB absorptivity under normal incidence over a wide frequency band ranging from 8.5 to 18.2 GHz. Due to the symmetrical structure of the AFSS, our absorber still has polarization insensitivity. We fabricated this AFSS absorber sample, and its measured results agree well with the simulated ones.