electronics

Article Broadband Reflective Rotator Built on Single Substrate

Xiaofan Yang 1, Tao Qi 2, Yonghu Zeng 1,*, Xiaoming Liu 2,3,* , Gan Lu 2,3 and Qing Cai 4

1 State Key Laboratory of Complex Electromagnetic Environment Effects on Electronic and Information System, Luoyang 471003, China; [email protected] 2 School of Physics and Electronic Information, Anhui Normal University, Wuhu 241002, China; [email protected] (T.Q.); [email protected] (G.L.) 3 Anhui Provincial Engineering Laboratory on Information Fusion and Control of Intelligent Robot, Wuhu 241002, China 4 Shanghai Institute of Measurement and Testing Technology, Shanghai 201203, China; [email protected] * Correspondence: [email protected] (Y.Z.); [email protected] (X.L.)

Abstract: A broadband built on single substrate is presented in this work. The device is designed for operation in the K and Ka bands. A slant array is used to achieve polarization rotation by 90◦ in a reflective manner. Broadband has been obtained, with the operation frequency range covering 15–45 GHz for 3 dB criteria, which is almost 100% fractional bandwidth. In addition, the insertion loss is less than 0.3 dB over a moderate broad incident angle from 0◦–20◦. Furthermore, the polarization conversion ratio can be as high as 0.95. By using a bi-static method, the fabricated prototype is measured, and the measured results demonstrate satisfactory agreement with the simulation ones. In comparison with other reflective designs in the literature, this design provides   good bandwidth as well as polarization conversion ratio. Miniaturization can be investigated to increase the angular stability. Citation: Yang, X.; Qi, T.; Zeng, Y.; Liu, X.; Lu, G.; Cai, Q. Broadband Keywords: polarization rotator; broadband; angular stable; reflective; frequency selective surface; Reflective Polarization Rotator Built slant array on Single Substrate. Electronics 2021, 10, 916. https://doi.org/ 10.3390/electronics10080916

Academic Editors: Augustine 1. Introduction O. Nwajana and Jiafeng Zhou Polarization manipulation is a key function in a wide range of electronic systems. Polarization rotators are therefore employed in communication [1], radar detection [2] and Received: 10 March 2021 radio astronomy [3]. A polarization rotator changes the polarization state of the incident Accepted: 9 April 2021 wave into its orthogonal polarization direction, or rotates the linear polarization by 90◦ [4]. Published: 12 April 2021 Such functionality can be realized by various techniques, such as rotator [5], roof-top mirror [6], and periodical structure [7]. Among these techniques, the periodical Publisher’s Note: MDPI stays neutral structure is one of the most popular methods owing to its flexibility in unit cell design and with regard to jurisdictional claims in good performance [7–32]. published maps and institutional affil- Two types of polarization rotator can be made using periodical structures, i.e., the iations. transmission type [7–15] and the reflection type [16–26]. The transmission type requires the incident wave travel through the whole structure to realize 90◦ rotation. For the reflection type, the incident wave is reflected back to the same side with 90◦ polarization rotation. It is found that polarization rotators working in the transmission type usually exhibit higher Copyright: © 2021 by the authors. insertion loss and narrower band, in comparison with the reflection type. In light of this Licensee MDPI, Basel, Switzerland. fact, the reflection type is preferred for low loss and wideband operation, and extensive This article is an open access article investigation into this area has been conducted. distributed under the terms and A split-ring and patch array was used to realize polarization rotation [16]. Such design conditions of the Creative Commons was a typical way for creating 180◦ phase difference so that the resultant polarization could Attribution (CC BY) license (https:// be rotated [33]. This structure, however, exhibited limited bandwidth of 57.5%. A very creativecommons.org/licenses/by/ similar design used split rectangular ring (SRR) enclosed with a cross dipole was reported 4.0/).

Electronics 2021, 10, 916. https://doi.org/10.3390/electronics10080916 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 12

Electronics 2021, 10, 916 2 of 11 very similar design used split rectangular ring (SRR) enclosed with a cross dipole was reported in [17]. The main drawback of this design is low polarization conversion ratio (PCR). Double-SRR structure was also investigated [18], however, the PCR was not im- provedin [17]. considerably. The main drawback Another ofmodified this design design is wa lows based polarization on dual-slit conversion loop enclosed ratio (PCR). with aDouble-SRR cross dipole, structure which did was not also produce investigated better PCR [18 ],or however, wider bandwidth the PCR [19]. was Double-L not improved unit cellconsiderably. was designed Another and modifiedtwo cascaded design dielectric was based layers on dual-slitin [20]. The loop L-shaped enclosed withstrips a were cross dipole, which did not produce better PCR or wider bandwidth [19]. Double-L unit cell was embedded in the dielectric layers to provide broadband operation. However, the fabrica- designed and two cascaded dielectric layers in [20]. The L-shaped strips were embedded tion will be a bit more complicated. A very interesting design using perforated array was in the dielectric layers to provide broadband operation. However, the fabrication will be a reported [21]. The bandwidth can be as broad as 110% with satisfactory PCR. The main bit more complicated. A very interesting design using perforated array was reported [21]. difficulty is that fabrication on dielectric with a large number of via holes. Based on the The bandwidth can be as broad as 110% with satisfactory PCR. The main difficulty is split circular ring, a switchable reflective design was realized [22]. The bandwidth of this that fabrication on dielectric with a large number of via holes. Based on the split circular structure is around 35%, much narrower than other designs. A slant fractal array was em- ring, a switchable reflective design was realized [22]. The bandwidth of this structure is ployed to produce wideband polarization rotation with PCR as high as 0.9 [24]. This de- around 35%, much narrower than other designs. A slant fractal array was employed to sign is very instructive to create multi resonance in the passband. Three dimensional produce wideband polarization rotation with PCR as high as 0.9 [24]. This design is very structures were also developed [25,26]. Although, these 3D designs are very interesting, instructive to create multi resonance in the passband. Three dimensional structures were the fabrication is difficult and no significant improvement has been observed in terms of also developed [25,26]. Although, these 3D designs are very interesting, the fabrication is bandwidthdifficult and and no PCR. significant There improvementare many designs has beenthat observedhave been in reported, terms of and bandwidth the authors and shallPCR. apologize There are to many these designs contributors that have for beennot be reported,ing able andto include the authors these shallworks apologize in this pa- to per.these contributors for not being able to include these works in this paper. ItIt is is therefore therefore worthy worthy of of further further investig investigationation to to develop develop easy easy fabrication fabrication and and wide- wide- bandband polarization polarization rotator. rotator. In In this this paper, paper, a reflective a reflective slant slant array array is proposed is proposed to achieve to achieve po- larizationpolarization rotation. rotation. The The unit unit cell cellis a isslant a slant strip, strip, where where the ends the ends are loaded are loaded withwith stubs stubs and theand middle the middle is loaded is loaded with patch. with patch. This is This done is to done create to createmulti resonance multi resonance in the inpass the band, pass asband, well as as well to improve as to improve the in-band the in-band insertion insertion loss. Such loss. a Such structure a structure is built is on built a single on a single sub- strate,substrate, where where the thefront front layer layer is patterned is patterned with with unit unit cells cells and and the the bottom bottom layer layer is is metal ground.ground. Broadband Broadband has has been been realized, realized, and and good good angular angular stability stability is is also observed. It It is demonstrateddemonstrated that that such such structure structure has very low insertion loss and very high PCR. TheThe remaining remaining parts parts are are organized organized as as follows. follows. Section Section 2 describes2 describes the the design, design, optimi- opti- zationmization and and parametric parametric study; study; Section Section 3 3presen presentsts measurement measurement and and results; results; and Section4 4 concludesconcludes this this work. work.

2.2. Structure Structure and and Analysis Analysis ForFor a a train train of of x-polarized-polarized wave, wave, when when incident incident on on the the periodical periodical array, array, the output wavewave will will be y-polarized.-polarized. The The unit unit cell cell of of the prop proposedosed wideband reflective reflective polarization rotatorrotator isis shownshown in in Figure Figure1. To1. achieveTo achieve polarization polarization rotation, rotation, the periodical the periodical array isarray slantly is slantlyarranged, arranged, and the and unit the cell unit consists cell consists of a capital of a capital I shape I shape anda and patch a patch at the at middle. the middle. The ThePTFE PTFE substrate substrate is 1.5 is mm 1.5 thick.mm thick. Additionally, Additional thely, dielectric the dielectric constant constant of the of substrate the substrate is 2.65. isThe 2.65. back The layer back is layer a reflection is a reflection metal plane. metal The plane. parameters The parameters of the final of designthe final are design shown are in shownTable1 .in Table 1.

Figure 1. An illustration of the polarization rotator. Left panel: a demonstration of x-polarized wave incident on the periodical structure with emergence y-polarized wave; right panel: the unit cell of the polarization rotator.

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Electronics 2021, 10, 916 3 of 11 Figure 1. An illustration of the polarization rotator. Left panel: a demonstration of x-polarized wave incident on the peri- odical structure with emergence y-polarized wave; right panel: the unit cell of the polarization rotator.

TableTable 1. The 1. The key key parameters parameters for for the the unit unit cell. cell.

ParametersParametersp ap ba b w w gg hh l l Value/mmValue/mm3.9 1.7 3.9 1.11.7 1.1 0.12 0.12 1 1 1.51.5 3.8 3.8

ForFor a reflection a reflection type type polarization polarization rotator, rotator, th thee reflection reflection coefficients coefficients are are used used to to char- charac- acterizeterize the conversion. If thethe incidentincident wavewave isisx x-polarized,-polarized, the the reflected reflected wave wave may may have have two twocomponents, components, the thex-component x-component and and the they-component. y-component. These These components components are characterizedare charac- terizedusing usingRxx and Rxx Randyx, representingRyx, representing the ratiosthe ratios coupled coupled to x -componentto x-component and and the ythe-component y-com- ponentfrom from the incident the incidentx-polarized x-polarized wave. wave. The simulationThe simulation was conductedwas conducted by using by using commercial com- mercialsoftware software HFSS. HFSS. By employing By employing periodical periodical boundary, boundary, only theonly unit the cell unit is cell needed is needed to predict to predictthe response the response of the of the polarization polarization rotator. rotator. The The simulatedsimulated results results are are plotted plotted in inFigure Figure 2, 2, wherewhere three three resonant resonant frequencies frequencies (18.2 (18.2 GHz, GHz, 2727 GHz, GHz, 39.4 39.4 GHz) GHz) are are clearly clearly observed. observed. The The −3 −dB3 dBbandwidth bandwidth reads reads as 30 as 30GHz GHz (from (from 15 15GHz GHz to to45 45GHz), GHz), showing showing a 100% a 100% fractional fractional bandwidth.bandwidth. The The insertion insertion loss loss is very is very low, low, less less than than 0.3 0.3 dB dB in inthe the central central region region of ofthe the pass pass band.band. It has It has to tobe bementioned mentioned that, that, since since the the structure structure is issymmetrical symmetrical for for x-polarizedx-polarized and and y-polarizedy-polarized waves, waves, Rxy Risxy identicalis identical to R toyx andRyx Randxx isR identicalxx is identical to Ryy. toIn Rthisyy. connection, In this connection, only Ryxonly and Ryxxx areand plotted.Rxx are The plotted. parametric The parametric study in coming study inpart coming also shows part alsothat showsthe variation that the in variationthese coefficients in these is coefficients negligible iswithin negligible the fabrication within the accuracy. fabrication Therefore, accuracy. the Therefore, above ob- the servationsabove observations can be made can in bethe made context in theof fabrication context of fabricationaccuracy. accuracy.

（a）（0 b）1 0.9 -10 0.8 0.7 -20 0.6 0.5 PCR R 0.4 -30 yx R 0.3 xx -40 0.2 Reflection Coefficient/dB Reflection 0.1 0 -50 10 15 20 25 30 35 40 45 50 10 15 20 25 30 35 40 45 50 Frequency/GHz Frequency/GHz FigureFigure 2. The 2. The simulated simulated results. results. (a) The (a) Thereflection reflection coefficients coefficients in the in range the range of 10 ofGHz–50 10–50 GHz; GHz; ( (bb)) the thepolarization polarization conversion conversion ratio. ratio.

TheThe PCR PCR can can be becalculated calculated using using [16]: [16 ]:

2 2 RR = yxyx PCRPCR= 2 . .(1) (1) 2 2+ 2 |RRRxx| + yxRyx

TheThe PCR PCR defines defines the the fractional fractional power power of ofthe the y-polarizedy-polarized component component in inthe the reflected reflected power.power. It is It found is found that that the the majority majority region region of ofthe the passband passband shows shows a PCR a PCR as ashigh high as as0.94. 0.94. TheThe evolution evolution of ofthe the design design is ispresented presented in inFigure Figure 3a.3a. Slant Slant dipole dipole array array has has been been reportedreported to tobe beable able to toproduce produce polarization polarization rota rotationtion [32]. [32 However,]. However, it is it isseen seen that that single single layerlayer slant slant dipole dipole array array does does not not produce produce very very good good bandwidth. bandwidth. The The end end is is then loadedloaded to to minimize the design, design, which which is is manifested manifested by by the the shift shift of of the the first first resonance resonance to tothe the lower lower frequencyfrequency side. side. The The loading, loading, however, however, makes makes the in-band insertioninsertion loss loss worse, worse, which which is is due dueto to that that the the far far distance distance between between the the first firs andt and the secondthe second resonances. resonances. Therefore, Therefore, the middle the middleof the of dipole the dipole is loaded is loaded with with a patch, a patch, which which creates creates another another resonance resonance near near 27 GHz.27 GHz. This design eventually reaches a good balance between bandwidth and in-band insertion loss. This design eventually reaches a good balance between bandwidth and in-band insertion loss.

Electronics 2021, 10, 916 4 of 11 Electronics 2021, 10, x FOR PEER REVIEW 4 of 12

（a） 0

-10 (1) -20

-30 Ryx (1) dB (2) Ryx (2) -40 Ryx (1) (2) Rxx (1) Rxx (2) -50 (3) (1) Rxx (1) -60 10 15 20 25 30 35 40 45 50 (3) Frequency/GHz

y （b） v u x

Rv Ru v-direction u-direction

Z 0 Z 0 L C jθ jθ e Z s e Z s

FigureFigure 3. 3. TheThe evolution evolution of the of thedesign design and the and equivalent the equivalent circuit. circuit.(a) Evolution (a) Evolution process: design process: (1) is design (1) a skewed strip array; design (2) is an end-loaded array; and design (3) is further loaded in the middle iswith a skewed a patch; ( stripb) Equivalent array; design circuit. (2)The is v-direction an end-loaded is an inductive array; and effect; design the u-direction (3) is further is a capac- loaded in the middleitive effect. with a patch; (b) Equivalent circuit. The v-direction is an inductive effect; the u-direction is a capacitive effect. An equivalent circuit for slant array can be described using Figure 3b, where the unit cell actsAn as equivalent an inductor circuit to the v for-direction, slantarray and a capacitor can be described to the u-direction using Figure[33,34].3 There-b, where the unitfore, cellthe reflection acts as an coefficients inductor can to thebe writtenv-direction, as: and a capacitor to the u-direction [33,34]. Therefore, the reflection coefficients can be written as:  R =−Δ()1cosϕ /2  yx  p , (2)  RRyx=+Δ=()1cos(1 − ϕcos /2∆ϕ)/2  xx p , (2) Rxx = (1 + cos ∆ϕ)/2

where ∆ϕ is the phase difference between Rv and Ru [33]. In this connection, for a polariza- tion rotator, the phase difference ∆ϕ shall be −180◦.

To shed some light on the principle, the current distribution at the three resonate frequencies is plotted in Figure4. The first resonance takes place at 18.2 GHz, where the Electronics 2021, 10, x FOR PEER REVIEW 5 of 12

Electronics 2021, 10, 916 5 of 11 Δϕ where is the phase difference between Rv and Ru [33]. In this connection, for a polar- ization rotator, the phase difference Δϕ shall be −180°. To shed some light on the principle, the current distribution at the three resonate frequencieslength of the is plotted dipole in (including Figure 4. The the loadedfirst resonance stubs) takes corresponding place at 18.2 to GHz, half wavelengthwhere the in the lengthsubstrate, of the which dipole can (including be estimated the loaded using: stubs) corresponding to half wavelength in the substrate, which can be estimated using: λ c =λ √ 0 =c √ λλ==g 0 , , (3) g εεεeff f0 εeff (3) efff 0 eff ε where εeffeffis isthe the effective effective dielectric dielectric [35]. The [35 peak]. The region peak on the region bottom on layer the can bottom be clearly layer can be recognized.clearly recognized. For the second For the resonance second at resonance 27 GHz, it at is 27clearly GHz, seen it is from clearly the current seen from distri- the current butiondistribution on the onbottom the bottomlayer that layer standing that standingwave takes wave place. takes One place.peak and One two peak valleys and can two valleys becan seen be seenon the on bottom the bottom layer. layer.For the For 39.4 the GHz 39.4 resonance, GHz resonance, the current the is current almost is restricted almost restricted toto the the patch patch region. region. The current The current distribution distribution does verify does that verify the patch that creates the patch more reso- creates more nancesresonances in comparison in comparison to the loaded to the dipole. loaded dipole.

Figure 4. 4. TheThe current current distribution distribution at 18.2 atGHz 18.2 (top GHz row (),top 27 GHz row ),(middle 27 GHz row (),middle and 39.4 row GHz), and(bot- 39.4 GHz tom(bottom row). rowThe ).first The column first column is for the is front for the layer front and layerthe second and the column second is for column the bottom is for layer. the bottom layer.

ToTo investigate investigate the the sensitivity sensitivity of each of each parameter, parameter, systematic systematic parameter parameter study has study been has been conducted. The The analysis analysis is presented is presented in Figure in Figure 5. Since5. Sincethe designed the designed value for value w was for onlyw was only 120 µm, the step was set to 10 µm (the fabrication accuracy), roughly 8% relative error to the designed value. A much larger step (100 µm) was used for other parameters. This step would produce 10% error to the parameter g (designed value 1 mm), very close to the relative error of w. It is seen that the most sensitive parameter is the length of the unit cell p. And it is found that the second resonance is more liable to change of p. The length of the patch a and the width b also produce variation to the second resonance, however, is less influential in comparison to p. Other parameters are less sensitivity. These simulation results imply that 10 µm fabrication accuracy is sufficiently good for this design. Electronics 2021, 10, 916 6 of 11 Electronics 2021, 10, x FOR PEER REVIEW 7 of 12

（a）（Parametric study-aa b） Parametric study-bb 0 0 Ryx Rxx Rxx -10 -10 Ryx -20 a=1.8 -20

-30 Ryx (1.8) dB -30 dB Ryx (1.0) a=1.6 Ryx (1.7) b=1 Ryx (1.1) -40 a=1.7 Ryx (1.6) -40 b=1.2 Ryx (1.2) Rxx (1.8) b=1.1 Rxx (1.0) -50 Rxx (1.7) -50 Rxx (1.1) Rxx (1.6) Rxx (1.2) -60 -60 10 20 30 40 50 10 20 30 40 50 Frequency/GHz Frequency/GHz （c）（Parametric study-g d） Parametric study-hh 0 0 Rxx Rxx -10 Ryx -10 Ryx -20 -20

-30 Ryx (1.1) -30 Ryx (1.6) dB dB g=1.1 Ryx (1.0) Ryx (1.5) -40 -40 h=1.6 g=0.9 Ryx (0.9) Ryx (1.4) g=1 Rxx (1.1) h=1.5 h=1.4 Rxx (1.6) -50 Rxx (1.0) -50 Rxx (1.5) Rxx (0.9) Rxx (1.4) -60 -60 10 20 30 40 50 10 20 30 40 50 Frequency/GHz Frequency/GHz p （e）（Parametric study-ll f） Parametric study-p 0 0 Rxx Ryx Rxx -10 Ryx -10

-20 -20

Ryx (3.9) -30 B -30 Ryx (3.8) d dB Ryx (3.8) Ryx (3.9) l=3.9 Ryx (3.7) p=4 -40 -40 Ryx (4.0) l=3.7 Rxx (3.9) p=3.8 l=3.8 Rxx (3.8) -50 Rxx (3.8) -50 p=3.9 Rxx (3.7) Rxx (3.9) Rxx (4.0) -60 -60 10 20 30 40 50 10 20 30 40 50 Frequency/GHz Frequency/GHz Parametric study-ww Parametric study-θ （g）（0 h）0 Rxx Rxx -10 Ryx -10 Ryx -20 -20 B -30 Ryx (0.13) Ryx (10 ) d -30 ° dB Ryx (0.12) Ryx (0°) w=0.11 w=0.13 -40 Ryx (0.11) -40 Ryx (20°) Rxx (0.13) θ=10° w=0.12 Rxx (10°) θ=20° -50 Rxx (0.12) -50 Rxx (0°) ° Rxx (0.11) Rxx (20°) θ=0 -60 -60 10 20 30 40 50 10 20 30 40 50 Frequency/GHz Frequency/GHz Figure 5. Parametric study for parameters (a) a, (b) b, (c) g, (d) h, (e) l, (f) p, (g) w, (h) θ. The fabrication accuracy is 10 μm, Figure 5. a a b b c g d h e l f p g w h θ µ the stepParametric is 10 μm studyfor w and for 100 parameters μm for other ( ) parameters.,( ) ,( ) ,( ) ,( ) ,( ) ,( ) ,( ) . The fabrication accuracy is 10 m, the step is 10 µm for w and 100 µm for other parameters.

3. Fabrication and Measurement

The fabricated polarization rotator is shown in Figure6. The fabricated dimensions are 200 mm long and 200 mm wide. The substrate was first cut using a high-precision six-axis Electronics 2021, 10, x FOR PEER REVIEW 8 of 12

3. Fabrication and Measurement Electronics 2021, 10, 916 7 of 11 The fabricated polarization rotator is shown in Figure 6. The fabricated dimensions are 200 mm long and 200 mm wide. The substrate was first cut using a high-precision six- axis drilling/milling machine (HANS-F6M, Mason PCB, (Shenzhen, China)). Then, the patterndrilling/milling was etched machine out using (HANS-F6M, the etching Mason proces PCB,s. Surface (Shenzhen, quality China)). control Then,process the was pattern em- ployedwas etched during out usingfabrication. the etching By using process. an in Surfacedustry qualitymicroscope control (S processuperEyes, was (Shenzhen, employed China)),during fabrication. the fabrication By usingaccuracy an industrycan be examined microscope and (SuperEyes,measured after (Shenzhen, calibration. China)), The key the parametersfabrication accuracyare marked can using be examined the rule andtool measuredof the microscope. after calibration. It can be Theseen key that parameters the fabri- cationare marked accuracy using is thebetter rule than tool ofthe the expected microscope. value It of can 10 be μm, seen showing that the satisfactory fabrication accuracyfabrica- tionis better accuracy. than the expected value of 10 µm, showing satisfactory fabrication accuracy.

Figure 6. The fabricated prototype. (a) Photography in comparison with a ruler; (b) image under an industrial microscope Figure 6. The fabricated prototype. (a) Photography in comparison with a ruler; (b) image under an industrial microscope with marked size. with marked size.

MeasurementMeasurement was was conducted conducted using using a a free free sp spaceace method, method, as as shown shown in in Figure 77.. TheThe systemsystem was was connected to a pairpair ofof broadbandbroadband hornshorns workingworking fromfrom 12–4012 GHz–40 GHz. AGHz. vector A vectornetwork network analyzer analyzer (VNA, (VNA, Cyear AV3276D,Cyear AV3276D, (Qingdao, (Qingdao, China)) generatedChina)) generated the input the signal input to signalthe transmitting to the transmitting horn and horn collected and thecollected signal the from signal the receiving from the horn. receiving Radar horn. absorption Radar absorptionmaterial (RAM) material was (RAM) mounted was surrounding mounted thesurrounding sample to reducethe sample scattering to reduce waves. scattering In order waves.to eliminate In order residual to eliminate noise, time-gating residual noise, technique time-gating was applied. technique The was elevators applied. were The fixed ele- vatorson the were wooden fixed ground, on the wooden and the ground, horns were and the fixed horns on thewere top fixed panel on the of thetop elevator.panel of the By elevator.averaging By several averaging rounds several of measurement rounds of me inasurement combination in withcombination time-gating with technique, time-gating the technique,measured the results measured demonstrated results demonstrated good repeatability. good repeatability. Due to the physical Due to sizethe physical of the horns, size ofwe the only horns, managed we only to managed measure theto measure case of θthe= 20case◦. of However, θ = 20°. However, it is sufficient it is forsufficient the proof for theofconcept. proof of concept. The measurement procedures are as follows: 1. Background measurement on open air. The measurement was conducted by placing the bottom layer of the sample to the sample location so as to produce total reflection. This is due to that the bottom layer is the metal layer. 2. Recording the transmission coefficient of the VNA (S21_air). This was due to using the bi-static method to collect the reflected signal from the sample, as can be seen from Figure7. 3. Measurement on sample for the reflection from x-polarization to x-polarization Rxx, and recording the transmission coefficient of the VNA (S21_xx). 4. Measurement on sample for the reflection from x-polarization to y-polarization Ryx, and recording the transmission coefficient of the VNA (S21_yx). 5. Calculating the reflection coefficients by using Rxx = S21_xx − S21_air, and Ryx = S21_yx − S21_air. This is done to remove the background effects.

Electronics 2021, 10, 916 8 of 11 Electronics 2021, 10, x FOR PEER REVIEW 9 of 12

Figure 7. The measurement setup. (a) Schematic diagram using bistatic method; (b) instrumenta- Figure 7. The measurement setup. (a) Schematic diagram using bistatic method; (b) instrumentation tion based on VNA technology. based on VNA technology. The measurement procedures are as follows: The measured results are plotted in Figure8. It is seen that the measured results are in 1. goodBackground agreement measurement with the simulation. on open For air. the The reflection measurement coefficients, was conducted the measurement by placing was conductedthe bottom from layer 12 of GHz the tosample 40 GHz. to the The sample resonate location frequencies so as toagree produce with total the reflection. simulation, onlyThis show is due slight to that frequency the bottom shift. layer The depthis the metal of resonance layer. for the measure results are less than that of simulation. In addition, the measured PCR is also satisfactory good, larger 2. Recording the transmission coefficient of the VNA ( S 21_ air ). This was due to using thanthe 0.92 bi-static in in mostmethod part to of collect the passband. the reflected The discrepancy signal from between the sample, the measurement as can be seen and simulation may be attributed to that the PCB produces more loss than simulation. Larger from Figure 7. loss in turn causes lower resonance and more insertion loss. R 3. MeasurementA comparison on ofsample this work for the with reflection that in thefrom literature x-polarization is presented to x-polarization in Table2. Thesexx ,

selectedand recording designs arethe alltransmission of reflection coefficient type fabricated of the VNA with ( noS 21_ morexx ). than two layers. The 4. comparisonMeasurement is made on sample in regard for the to bandwidth,reflection from layers x-polarization and realized to y PCR.-polarization It is seen R that, the bandwidth of structures made of two layers is wider than the design in this work.yx and recording the transmission coefficient of the VNA ( S ). However, compared to single substrate, this work provides21_ broadbandyx operation, up to 100%. Furthermore, the PCR is satisfactorily high, up to 0.95 by simulation,=− and better than 5. Calculating the reflection coefficients by using RSxx21_ xx S 21_ air , and 0.92 for measurement. Such results demonstrate that this design can be used for broadband RS=− S . This is done to remove the background effects. polarizationyx21_ yx rotation 21_ air with sufficiently good PCR. The measured results are plotted in Figure 8. It is seen that the measured results are in good agreement with the simulation. For the reflection coefficients, the measurement

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was conducted from 12 GHz to 40 GHz. The resonate frequencies agree with the simula- tion, only show slight frequency shift. The depth of resonance for the measure results are less than that of simulation. In addition, the measured PCR is also satisfactory good, larger Electronics 2021, 10, 916 than 0.92 in in most part of the passband. The discrepancy between the measurement and 9 of 11 simulation may be attributed to that the PCB produces more loss than simulation. Larger loss in turn causes lower resonance and more insertion loss.

(a)0 (b) 1 0.8 -20 0.6 dB PCR 0.4 -40 Ryx (1) 0.2 Rxx (1) -60 0 10 20 30 40 50 10 20 30 40 50 Frequency/GHz Frequency/GHz Figure 8. Comparison between measurement and simulation. (a) Rxx and Ryx; (b) PCR. Measure- Figure 8. Comparison between measurement and simulation. (a) Rxx and Ryx;(b) PCR. Measurement ment was conducted at 20°. was conducted at 20◦. A comparison of this work with that in the literature is presented in Table 2. These Tableselected 2. A designs performance are allcomparison of reflection of type reflective fabricated polarization with no rotators more than presented two layers. in literature. The comparison is made in regard to bandwidth, layers and realized PCR. It is seen that the bandwidthRef. of structures Freq. made (GHz) of two layers 3 dBis wider Bandwidth than the (%) design in Layers this work. How- PCR ever, [compared16] to single 5.5–10.5 substrate, this work provides 62.5% broadband operation, 1 up to 100%. 0.88 Furthermore,[17] the PCR 5–9.7, is satisfactorily 11.2–15 high, up to 63.5%, 0.95 by 29% simulation, and better 1 than 0.92 0.6 for measurement.[18] Such 5–10.8results demonstrate that this 73% design can be used 1for broadband 0.6 polarization[19] rotation with 5.6–11.4 sufficiently good PCR. 66% 1 0.75 [20] 6–34 140% 2 0.8 Table [2.21 A] performance comparison 15–55 of reflective polarization 116% rotators presented in 2 literature. 0.89 [22] 2.42–3.52 37% 2 - Ref.[23] Freq. 8–18 (GHz) 3 dB Bandwidth 68.6% (%) Layers 1 PCR 0.65 [[16]24] 5.5–10.5 8–23 62.5% 100% 1 10.88 0.9 [[17]25] 5–9.7, 4–6 11.2–15 63.5%, 40% 29% 1 20.6 0.9 [[18]26] X-band,5–10.8 Ku-band 16.1%, 73% 20% 1 20.6 0.86 This[19] work 5.6–11.4 15–45 66% 100% 1 10.75 0.95 [20] 6–34 140% 2 0.8 Actually,[21] to achieve15–55 wideband property, 116% more resonances2 have to be0.89 created. The [22] 2.42–3.52 37% 2 - method used in [17] is a dual split-ring structure. Unfortunately, the PCR or bandwidth was [23] 8–18 68.6% 1 0.65 not improved, though two resonances were created. The reason might be that the coupling [24] 8–23 100% 1 0.9 between the two resonances was not balanced. The same issue is also found with [20], [25] 4–6 40% 2 0.9 where the PCR is 0.8 over the passband. The perforated array is itself a multi-resonance [26] X-band, Ku-band 16.1%, 20% 2 0.86 structure, although the fabrication is a bit more complicated. In [24] the fractal structure is This work 15–45 100% 1 0.95 also a multi-resonance structure producing wideband operation with 0.9 PCR. Actually, the slantActually, I shape to achieve is also capablewideband of property, creating mo polarizationre resonances rotation. have to By be loading created. theThe patch at themethod middle, used more in [17] resonances is a dual split-ring are induced. structure. This Unfortunately, is instructive tothe design PCR or wideband bandwidth reflective polarizationwas not improved, rotator though by using two multi-resonance resonances were structures.created. The The reason only might challenge be that is the to increase thecoupling angular between stability. the two One resonances possible approachwas not balanced. is to minimize The same the issue unit is also cell. found with [20], where the PCR is 0.8 over the passband. The perforated array is itself a multi-reso- 4.nance Conclusions structure, although the fabrication is a bit more complicated. In [24] the fractal struc- A broadband polarization rotator has been realized based on single substrate. The prototype was measured using the free-space method. The unit cell consists of an I-shaped dipole loaded at the center with a rectangular patch so as to create more resonance. The

fractional −3 dB bandwidth reads 100% and the measured PCR is better than 0.92. Such a structure can be used as wideband polarization rotator with very high PCR. The design based on single substrate enable easy fabrication using common PCB materials. In regard to the angular stability, only 20◦ is realized, and there still space for further improvement by using miniaturization methods. The bandwidth enhancement is also a good direction for further improvement. Electronics 2021, 10, 916 10 of 11

Author Contributions: Data curation: X.Y. and T.Q.; investigation: Y.Z. and X.L.; methodology: G.L. and Q.C.; resources: Y.Z.; writing—original draft: X.Y.; writing—review and editing: X.L. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (61871003), the State Key Laboratory of Complex Electromagnetic Environment Effects on Electronics and Information System (CEMEE2021Z0201B). Conflicts of Interest: The authors declare no conflict of interest.

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