EPJ Appl. Metamat. 6, 20 (2019)© Y. Liu et al., published by EDP Sciences, 2019https://doi.org/10.1051/epjam/2019020

Available online at:epjam.edp-open.org

RESEARCH ARTICLE

U-slot patch antenna with low RCS based on a metaferritesubstrateYujie Liu*, Philip Beal, Henry Giddens, and Yang Hao

Department of Electronic Engineering and Computer Science, Queen Mary University of London, London, UK

* e-mail:

This is anO

Received: 26 February 2019 / Accepted: 21 October 2019

Abstract.Metamaterial ferrites or metaferrites are artificial magnetic materials which mimic the properties offerrites at a certain frequency operation. Antenna engineers are therefore able to design and create artificialsubstrates which replicate the electrical properties of ferrites without actually using any in the construction.This is advantageous as ferrites can offer performance improvements to microstrip antennas, such as sizereduction and wideband impedance matching. In this paper, a metaferrite substrate designed by the use of agenetic algorithm is presented. The metaferrite was optimized in order to obtain the magnetic responses at9GHz, for its use as the substrate of a microstrip antenna. As an example, a U-slot patch antenna based on themetaferrite is demonstrated, which can achieve stable radiation and 14 dB radar cross section (RCS) reductionperformance in the measurement.

Keywords: Ferrite metamaterial / metaferrite / genetic algorithm / RCS reduction / patch antenna

1 Introduction

Loading antennas with magnetic materials is well knowntechnique that can enable engineers to design widebandantennas while maintaining a relatively low profile [1].Many previous researchers have focused on loadingantennas with ferrite materials in order to minimize theantenna dimensions and increase the radiation efficiency[2,3]. The antenna presented in [4] investigates theinfluence of the magnetic materials on the performanceof dielectric resonator antenna (DRA) and it confirms thatmaintaining a certain ratio of mr to er is criticallyimportant to control the proximity of adjacent co-polarmodes and create the DRA with a wide bandwidth ofoperation. However, naturally occurring magnetic mate-rials may actually lose their magnetic properties up tomicrowave or millimeter-wave frequency, limiting theirapplications to higher frequencies. Therefore, many effortshave been spent over the years on the study ofmetamaterials, especially those with artificial magneticproperties [5]. Some studies have shown how metamate-rials formed of dielectric substrates with periodicmetallization can be designed to behave as artificialmagnetic or magnetodielectric materials with requiredeffective permittivity and permeability at a particularfrequency [6]. For example, by using the spiral resonator

yujie.liu@qmul.ac.uk

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[7] or metasolenoid [8] as “meta-atoms”, an artificialmagnetic material can be obtained with high permeabilitybeyond microwave frequency which is easy to fabricate bylow-cost photo-etching technology.

Many of these designs are achieved by the use of theGenetic Algorithm (GA) optimization procedure [9].Furthermore, they can be also interpreted as materialshaving high impedance surfaces for antenna operation [10].In [11], an approach is used to obtain required magneticresponse at the specified operation frequency or frequenciesby synthesizing the high impedance surface resistance andreactance. Since these two values are strongly related to theeffective permeability of the equivalent material, therefore,it can be perceived as a valid approach to design“metamaterial ferrite” or “metaferrite” by optimizing thesurface impedance of a bulk material. In addition whenconsidering both permittivity and permeability, thesekinds of artificial materials also have the performance ofimpedance matching, which can be utilized as absorberswithout integrating any lossy elements, as presented in[12].

In this paper, a metaferrite material is designed andoptimized to achieve a high effective permeability of 3based on the GA technique at the working frequency of9GHz. A U-slot patch antenna positioned above thismetaferrite substrate is proposed with the 14 dB RCSreduction at the normal incidence and its operationfrequency band covers from 8.5GHz to 9.1GHz whileS11<−9 dB.

monsAttribution License (https://creativecommons.org/licenses/by/4.0),in any medium, provided the original work is properly cited.

Fig. 1. Cross sectional diagram of the meteferrite patch antennaand its equivalent magnetic material with effective meff = eeff.

Fig. 2. Flow chart of the genetic algorithm optimization relatedto the CST studio for the metaferrite design.

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2 Design procedure of the metaferritematerial

Figure 1 shows a diagram of the microstrip patch antennapositioned on the metaferrite substrate, which consists oftwo dielectric slabs enclosing a metasurface. The stack canbe considered as a single substrate of material with equaleffective permittivity and permeability, as described in[12]. The bottom layer has a thickness of t1 and e1permittivity and the upper layer has a thickness of t2 andrelative permittivity of e2. With the same PEC ground, theequivalent composite structure is a homogenous metefer-rite material with the thickness of h= t1+ t2.

As explained in [12], the surface impedance of thecomposite dielectric structure can be represented asZcomposite. Since the surface impedance is given by [13],

Zsurface ¼ Zstanhðjv ffiffiffiffiffiffiffiffiffie0m0

pnshÞ; ð1Þ

where Zs ¼ Z0

ffiffiffiffiffiffiffiffiffiffiffiffimr=er

p, and Z0 is the characteristic

impedance of the vacuum and is equal to 377V, by settingthe Zcomposite equal to the surface impedance Zsurface of theequivalent metaferrite material, the effective refractiveindex of the equivalent metaferrite substrate can beextracted and shown to be equal to

ns ¼ mr ¼ er ¼ 1

jb0htanh�1 Zcomposite

Z0

� �ð2Þ

Zcomposite ¼ Z0 � 1þ S11

1� S11ð3Þ

Zcomposite can be obtained by calculating the reflectioncoefficient S11 through the equation (3). In this case, theeffective value of meff = eeff can be designed by modifyingthe surface impedance of the composite structure, byoptimising the metasurface structure.

Here, in order to achieve the desired magnetic responseof the composite substrate, the genetic algorithm (GA), awell-known nature-inspired global optimization technique,was applied to design this metasurface. As illustrated in theflowchart of the GA’s process in Figure 2, the algorithmuses a combination of MATLAB code, which comprises thelearning algorithm itself and commercial simulationsoftware (CSTmicrowave studio). The generated structureis built and analyzed through CST for calculating the S11parameters of the metaferrite unit cell, with periodicboundaries terminating the simulation space in the x and y

directions. The results are passed back to the GA’s fitnessfunction for evaluating. Here the fitness function was:

FF ¼ �½ðm0rðfÞ � 3Þ2 þ ðm00

r ðfÞ � 1Þ� ð4Þwhere m0

r and m00r are the real and imaginary values of the

equivalent metaferrite material’s effective permittivity,derived from its surface impedance (Zcomposite) by equa-tions (2) and (3). The fitness function was evaluated at afrequency of 9GHz.

The unit cell of the composite structure here is dividedinto 18� 18 pixels with 1mm intervals, such the metallizedpixel would be encoded as 1 and the un-metallized pixel isencoded as 0. Each pixel in the unit cell is represented by asingle bit in the GA chromosome. In order to simplify theselection process, the program uses a 9-bit data string thatis then extended to 45 cells via an L-system algorithm [14].This forms the basis of the structure and provides a means

Fig. 3. (a) Optimised unit cell structure of the metaferritesurface. The black spots represent the metallized pixels which areencoded as 1, and the blank ones are unmetallized pixels which areencoded as 0. (b) Simulated real and imaginary components ofmeff = eeff of finalised metaferrite substrate with e1= 4.3,t1= 0.8mm, e2= 6, t2= 1.52mm.

Y. Liu et al.: EPJ Appl. Metamat. 6, 20 (2019) 3

of exerting some control upon the final shape. Theeightfold-symmetric pattern in the unit cell reduces thecomplexity of the design and ensures the structure has apolarization independent response, as it meant that a largersurface area could be created using only a small variablesection and minimize the computational time.

The final optimized pattern of the unit cell for themetaferrite material design is shown in Figure 3a. Thispattern can be easily fabricated by using the photo-etching technology on the Rogers TMM6 substrate withthe thickness of t2 = 1.52mm and the permittivity ofe2 = 6. By adding a 0.8mm blank FR4 substrate with aconducted ground to the bottom, seen from Figure 1, theequivalent effective permeability and permittivitycan be obtained through the methodology mentionedabove.

Figure 3b depicts the simulated real and imaginarycomponent of the equivalent metaferrite material. It canbe seen that this composite structure has the highestmagnetic responses at about 9.1GHz, and its correspond-ing effective permeability is 3.9. The fitness function herewas to achieve the metaferrite substrate with effectivematerial properties of er=mr=3-j1 for the RCS reductionapplication at the frequency of 9GHz, which wassuccessfully achieved. This proposed composite structurecan work as a frequency dependent metaferrite materialduring the frequency band from 8GHz to 9.2GHz sincetheir effective permeability and permittivity are bothabove 1.

3 Metaferrite materials for RCS reductionof U-slot microstrip patch antenna

The U-slot patch antenna is a well-known broadbandmicrostrip antenna and has been studied to apply for thehigh frequency and high speed communication sinceproposed in 1995 by Huynh and Lee [15]. By adding thisU-shaped slot to the rectangular patch, the currents on thepatch surface are modified and different resonant modesare excited to achieve wideband performance. The design

procedure has been presented in detail in [16]. Based on themetaferrite obtained above, we first design the U-slot patchantenna on the top of this effective magnetic substrate togain the RCS reduction while maintain the stable radiationand low profile characteristics. The initial parameters ofthe U-shaped slot are calculated according to [16], but theystill need to be optimized since the magnetic substrate hereis frequency dependent. Figure 4a illustrates the 3D view ofthe proposed antenna and the U-shaped slot dimensions.The final optimized parameters for the impedancematching of this metaferrite based antenna are as follows:W=L=180mm, Ws=Ls=33mm, St=1.1mm, Lt=15.53, Wt=7.76. This antenna is coaxially fed and itsfeeding point is located in the center of the U-shaped slot.As seen from the same figure, the U-slot center is 1.82mmaway from the original point in the �y direction. Thephotographs of the fabricated metasurface structure andthe U-slot patch antenna are presented in Figure 4b, andthey are both manufactured on the same substrate ofRogers TMM6 at the bottom and top sides, respectively.The total thickness of the antenna is the same with thethickness of the metaferrite material, which h= t1+ t2 is2.32mm.

Figure 5 shows the comparison between the simulatedand the measured results of the reflection coefficient S11.The simulated S11 obtained by the full-wave HFSSsimulation is below �9 dB from 8.5GHz to 9.1GHz, whichcovers the same frequency band with the metaferritematerial. It indicates that the proposed antenna is workingwell with the equivalent magnetic substrate. During themeasurement, the resonant frequencies shift in the S11 shiftupwards, which is caused by the air gap in the practicalantenna between the metasurface and the blank FR4substrate (the air gap has an effect of lowering the effectivematerial properties of the metaferrite). The radiationpatterns at 8.65GHz, 8.8GHz and 9GHz can be seen inFigure 6. The differences between the simulated andmeasured are also likely due tomanufacturing errors, as theair gap between those two substrates is inevitable inpractical structure and also hard to quantify for thesimulation.

In order to measure the RCS performance of theproposed antenna, a plane wave was launched from astandard horn antenna at the bottom onto the surface ofthe proposed antenna in the anechoic characterizationchamber, as shown in Figure 7a. The RCS response can beextracted by subtracting the reflection coefficient of a samesize PEC reflector positioned in the same place as theantenna. Time-gating was used to remove any furtherreflections from the horn antenna that were not accountedfor during the calibration of the cables. In this case, theRCS responses of the metaferrite based U-slot patchantenna have been measured and compared with thesimulated result. As illustrated in Figure 7b, the simulatedresult indicates that the antenna has a �9 dB RCSreduction at around 9GHz, and the measured RCSreduction is shifted to higher frequency of 9.35GHz dueto the same reason of the air gap fabricated issuementionedabove. Besides that, the RCS reduction has beensignificantly increased while comparing to the same

Fig. 5. Simulated and measured reflection coefficient S11 of theU-slot patch antenna with metaferrite material.

Fig. 4. (a) 3D view of the U-slot patch antenna based on the equivalent metaferrite substrate. The photograph of (b) the matesurfaceon the bottom of the Rogers TMM6 substrate, (c) U-slot patch antenna on the top.

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antenna without the optimized metasurface in betweenthose two substrates, which means that the low RCSperformance for the antenna can be obtained by applyingthe metaferrite substrate.

4 Conclusion

In this paper, we have discussed how the metaferrite can beutilized for antenna applications. The metaferrite wasdesigned through genetic algorithm (GA) optimization. AU-slot patch antenna positioned on this effective magneticsubstrate to achieve the additional RCS reductionperformance while maintaining the wideband and stableradiation properties, which may be useful for military/stealth applications when low profile antennas need to bepositioned outward facing surfaces. Future optimizeddesigns will focus on achieving a wide-band RCS reductionwhilst maintaining good radiation performance of planarantennas.

Fig. 6. Normalized measured and simulated radiation pattern of the metaferrite based U-slot patch antenna with E- and H-plane atthe frequencies of (a) 8.65GHz, (b) 8.8GHz and (c) 9GHz.

Y. Liu et al.: EPJ Appl. Metamat. 6, 20 (2019) 5

Fig. 7. (a) Diagram of themeasurement setup and the fabricatedmetaferrite based U-slot patch antenna, (b) the Simulated andmeasured RCS reduction results of the metaferrite antenna andthe comparison between the same patch antenna with theconventional dielectric substrate.

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Cite this article as: Yujie Liu, Philip Beal, Henry Giddens, Yang Hao, U-slot patch antenna with low RCS based on a metaferritesubstrate, EPJ Appl. Metamat. 6, 20 (2019)