This brief demonstrates the use of metamaterial absorber (MA) to achieve high isolation between two patch antennas in a 2-element multiple-input–multiple-output (MIMO) system operating at 5.5 GHz resonant frequency useful for WiMAX application. The proposed flower shaped MA, designed on a $9\times 9$ mm2 FR-4 substrate with 1 mm thickness, exhibits near unity normalized impedance at 5.5 GHz with an absorptivity of 98.7%. A four element array of the MA is arranged in the form of a line in the middle of the two radiating patches in order to suppress the propagation of surface current between them at the operating frequency. Using the proposed flower shaped MA, an isolation of nearly 35 dB is achieved. The MIMO structure is studied in terms of return loss, isolation, overall gain, radiation pattern, envelope correlation coefficient (ECC), diversity gain (DG), total active reflection co-efficient (TARC), etc. The structure is finally fabricated and measured to show good agreement with the simulated results.

Isolation Improvement of MIMO Antenna Using a Novel Flower Shaped Metamaterial Absorber at 5.5 GHz WiMAX Band

In this research work, the design characterization of metamaterial absorber (MMA) has been analysed for a unit cell structure of shorted stubs circular rings metamaterial absorber. The simulated results show that dual resonance peaks occur at 17 GHz and 18 GHz with absorbance peak response of 99.99% and 99.83%, almost unity absorbance. The corresponding full width at half maximum (FWHM) bandwidth at these resonant frequencies are 4.90% and 5.07% respectively. A parametric study has been done to analysed the effect of variation of circular ring radius, shorted stubs length and rectangular bar length upon the resonance frequency. The analysis shows that we can achieve a lower resonant frequency with almost unity absorbance. Moreover, the resonant frequency drifts to lower value without affecting absorbance. The effect at oblique angle of incidence and polarization sensitivity is also considered for both TE and TM modes. So, this unit cell structure acts as a controllable and tunable MMA structure. The dual resonances MMA structure are becoming a prime technology for many present and future wireless communication systems such as radar cross section (RCS) reduction for stealth technology, satellite navigation systems, radar frequency identification and wireless handheld devices.

Dual resonance shorted stub circular rings metamaterial absorber

A sub-wavelength second-order band-stop frequency selective surface (FSS) is presented. The single layer FSS has a band reject operating mode for TE- and TM-polarizations incidence. Miniaturization of the FSS unit cell is achieved by coupling two resonators separated by single FR4 substrate. The unit cell size of the proposed FSS is 0.25λ0 × 0.25λ0, which λ0 is the free space wavelength at the center frequency (10 GHz). As experimental results proved, the proposed FSS has a great angle independence up to 60°. An equivalent circuit model is illuminated the operating principle of designed FSS. The validity of FSS is obtained by comparing experimental results and full-wave simulations.

A sub-wavelength polarization sensitive band-stop FSS with wide angular response for X- and Ku-bands

A new thin and wideband RCS reduction metasurface is designed and fabricated by using two different AMC unit cell tiles arranged in chessboard like configuration. One of these tiles is formed by saltire arrow unit cells, whereas the other one is formed by Jerusalem unit cells which are designed to achieve out phase difference at broad frequency range. This metasurface reduces the RCS more than 10-dB from 15.75 GHz to 41.3 GHz (90% bandwidth). The designed metasurface works properly at both TE and TM propagation modes, also. Moreover, it has stable 10-dB RCS reduction bandwidth for TM propagation mode up to 50° oblique incident angle and 30° for TE ones. These good specifications prove the designed metasurface capability compared with the state of the art references.

Wide band radar cross section reduction by thin AMC structure

In this work, a novel Circularly Polarized (CP) slot antenna is proposed for low RCS and high gain applications. The proposed antenna is designed in two phases. Initially, a metasurface is designed, which is composed of two similar artificial magnetic conductor (AMC) unit cells arranged orthogonally in chessboard-like configuration for broadband RCS reduction. Then, the CP slot antenna is designed by placing dual SRR on the backside of the slot for impedance matching and to achieve circular polarization. Detailed analysis is conducted to investigate the performance of metasurface loaded CP slot antenna. The proposed antenna shows 10 dB RCS reduction over the bandwidth of 41.3% at boresight direction compared to CP slot antenna. The maximum in-band and out-of-band RCS reduction achieved is 24 dB and 20 dB, respectively. The maximum gain of the antenna is also increased by 2.7 dB as a result of parasitic radiation of the metasurface and an improvement in overall performance of the antenna is observed by the employment of metasurface. Measured results of the fabricated prototype are in good agreement with the simulated results.

RCS reduction and gain enhancement of SRR inspired circularly polarized slot antenna using metasurface

A novel EM (electromagnetic) design of low observable antenna using active hybrid-element frequency selective surface (FSS) as ground plane is presented in this paper for stealth applications. The EM characteristics of active hybrid-element FSS structure are reconfigured from reflecting mode to transmitting mode and vice versa by switching ON and OFF the PIN diode at 10 GHz. In ON state of diode (reflection mode), the FSS structure shows extremely less transmission ( 2 ) as compared to conventional MPA. Finally, a prototype active FSS-based antenna is fabricated and measured for proof-of-the concept.

Design of low observable antenna using active hybrid-element FSS structure for stealth applications

Measurements of electron density/irregularities and ion drift velocity/neutral wind were undertaken with Electron Density and Neutral Wind (ENWi) payload on‐board an RH300 flight on 6 April 2018 as part of the Sounding Rocket Experiment (SOUREX1). The presence of a clearly structured ENWi measured electron density profile is an interesting feature observed on this day. Concurrently electron density/irregularities are measured by a Langmuir probe payload which confirmed the presence of stratifications in electron density. The neutral wind measurements by ENWi also reveal a structured pattern. The hodograph analysis of the neutral wind components reveals clockwise rotation indicating downward phase propagation characteristics associated with the gravity waves. Analysis of the requisite ion convergence at the magnetic equator for occurrence of electron density stratifications using the observed electron density structures during the post sunset hours in relation to the ion convergence due to gravity wave winds is performed according to earlier theoretical simulations. This analysis reveals that conditions favoring the formation of stratification related ion convergence are present therein. Thus the present study for first time uses actual measurements of neutral winds and electron densities from same instrument to demonstrate the role of gravity wave induced winds in producing ion convergence at the magnetic equatorial region Trivandrum in the post sunset hours.

Generation of Post Sunset E Region Electron Density Stratifications at the Magnetic Equator: An Analysis Using In Situ Measurements and Theoretical Estimations

Thattanvalappil Vasu Sruthi

Papers

Design of low observable antenna using active hybrid-element FSS structure for stealth applications

Generation of Post Sunset E Region Electron Density Stratifications at the Magnetic Equator: An Analysis Using In Situ Measurements and Theoretical Estimations