Quasi-Circulator Using an Asymmetric Coupler for Tx Leakage Cancellation

electronics

Article Quasi-Circulator Using an Asymmetric Coupler for Tx Leakage Cancellation

Bo-Yoon Yoo, Jae-Hyun Park and Jong-Ryul Yang * ID Department of Electronic Engineering, Yeungnam University, Gyeongbuk 38541, Korea; [email protected] (B.-Y.Y.); [email protected] (J.-H.P.) * Correspondence: [email protected]; Tel.: +82-53-810-2495

Received: 20 July 2018; Accepted: 31 August 2018; Published: 1 September 2018

Abstract: A quasi-circulator is proposed by using an asymmetric coupler with high isolation between the transmitting (Tx) and receiving (Rx) ports. The proposed quasi-circulator consists of quarter-wave transmission lines, which have unbalanced characteristic impedances and the terminated port, which is purposely unmatched with the reference impedance in the coupler. The port compensates for the asymmetric impedances of the coupler using the proposed design parameter. Because of its asymmetric structure and the usage of the unmatched port, the proposed circulator can be accurately designed to have high Tx–Rx isolation without increasing the signal losses in the Tx and Rx paths at the operating frequency. The proposed quasi-circulators show isolation improvements of 9.07 dB at 2.45 GHz and 7.95 dB at 24.125 GHz compared with conventional circulators using the symmetric couplers. The characteristic improvement of the proposed quasi-circulator was demonstrated by the increase of the detectable range of the 2.45 GHz Doppler radar sensor with the quasi-circulator.

Keywords: quasi-circulator; asymmetric coupler; Tx leakage cancellation; planar circulator; hybrid coupler design; passive components

1. Introduction In electromagnetic-wave application systems such as radar sensors and wireless communication systems, the number and size of antennas should be considered in the implementation of the miniaturized module [1–4]. Since the antenna size depends on the operating frequency, it is more effective to reduce the number of antennas. The circulators can be used to reduce two antennas to one but it is not suitable for miniaturization because it is made of expensive directional materials or has disadvantages such as large size and heavyweight [5–7]. The planar quasi-circulator is a useful component in separating transmitting (Tx) and receiving (Rx) signals in miniaturized monostatic radar and communication systems that use a single antenna to transmit and receive wireless signals [4,8,9]. A branch-line coupler with a terminated port is conventionally used for a planar quasi-circulator because of its simple and low-cost design and its realization on a printed circuit board (PCB) [4,8–11]. However, the low isolation of the branch-line coupler between the Tx and Rx paths, which lowers the Rx sensitivity or saturates the receiver, causes a receiver malfunction and limits the maximum detectable range [4,8–13]. In previous studies, quasi-circulators using two branch-line couplers, a coupled-line directional coupler and a directional coupler with a ﬂat coupling were developed as the planar quasi-circulator to improve the isolation [9–12]. The isolation between the Tx and Rx ports increased considerably in previous circulators; however, their size also increased because of the additional components. In addition, insertion losses at all ports increased when the Tx and Rx signals passed through those circulators. Previous studies to reduce the effect of the Tx leakage signals applied to a balanced topology for radar systems [4,14–16]. In this topology, it is necessary for Tx leakage cancellation to generate multiphase Tx signals. The topology also requires a symmetric structure

Electronics 2018, 7, 173; doi:10.3390/electronics7090173 www.mdpi.com/journal/electronics Electronics 2018, 7, 173 2 of 15 consisting of several couplers and a power combiner, which may have a limit to reduce the size of the system. While implementing the conventional quasi-circulator using the branch-line coupler, it is not easy to design the characteristics of the circulator accurately, especially the frequency at which the Tx leakage signal is minimized in the operating frequency band. In particular, since the line width of the transmission line cannot be neglected at the T-junction where the three transmission lines meet at one point, the conventional design parameters of the coupler should be modified. Because of the physical size of the T-junction, it is difficult to accurately design the length of the transmission line to the quarter-wavelength as in the electromagnetic-wave theory [17–19]. The T-junction creates a discontinuous section of the electromagnetic wave propagation in the branch-line coupler, so it can also lead to the performance degradation and frequency shift of the coupler implemented using the simple analytic design method. In addition, the design difficulty increases significantly with shorter wavelengths resulting in increased characteristic errors in the component design with higher operating frequencies. The symmetrical implementation is an important characteristic in the conventional quasi-circulator because the parasitic components and the process variation in the PCB fabrication degrade the operating characteristics at the desired frequency or change the operating frequency which is different from the design [20]. The uniformity of the signal pattern realized by the etching process can also be reduced in the fabrication process [21]. In this paper, a quasi-circulator using an asymmetric branch-line coupler is proposed to improve the isolation between the Tx and Rx ports at the operating frequency based on a novel design methodology. The characteristic impedances of the quarter-wave transmission lines are designed with an asymmetric structure and the impedance at the unused port of the coupler is designed differently from the reference impedance to compensate for the asymmetric characteristic impedances of the transmission lines. Compared with the asymmetric branch-line couplers presented in previous studies [22,23], the asymmetric coupler proposed for the circulator in this work has three input/output ports fixed at the reference impedance of 50 Ω so that the conventional circulator component can be replaced as it is. In contrast to the previous studies, the proposed quasi-circulator adjusts the line widths of the transmission lines asymmetrically and compensates for the asymmetric characteristics using resistor(s) connected to port 4 of the coupler. The asymmetric coupler can be easily implemented because the desired characteristic impedance of the transmission line can be obtained by designing the line width in the microstrip structure. The characteristics due to the asymmetric transmission lines of the coupler are compensated by adjusting the impedance of the single port, which is simply implemented by high-frequency resistors. The proposed quasi-circulator has high Tx–Rx isolation at the operating frequency, while the frequency with high Tx–Rx isolation in the conventional circulator was shifted from the designed frequency. The insertion and return losses of the proposed quasi-circulator are similar to those of the conventional quasi-circulator. The remainder of the paper is organized as follows. Section2 describes the design methodology of the circulator using the asymmetric coupler. The implementation and design parameters of the circulators, operating at 2.45 GHz and 24.125 GHz, are detailed in Section3. Section4 provides the measured performances and comparison with conventional quasi-circulators using symmetric branch-line couplers. A conclusion is presented in Section5.

2. Design of the Quasi-Circulator Using the Asymmetric Coupler The asymmetric branch-line coupler for the proposed circulator consists of four unbalanced quarter-wave transmission lines that have different characteristic impedances. The characteristic impedances are obtained by regulating the line width, deﬁned as Zi (i = 1–4), as shown in Figure1. In the conventional branch-line coupler, the characteristic impedances between the opposing transmission lines are the same, that is, Z1 and Z2 are equal to Z3 and Z4, respectively. Three terminals from ports 1 to 3 are connected to a transmitter, antenna and receiver, respectively, using the transmission lines with a reference characteristic impedance Z50, which is 50 Ω. Port 4 is terminated Electronics 2018, 7, 173 3 of 15

using the certain impedance ZISO, which is purposely unmatched with the reference impedance. The unmatched ratio at port 4 of the proposed asymmetric coupler is defined as the parameter m, which is the ratio of the reflection coefficient between the unmatched and matched impedance conditions and is given as S m = 44 , (1) S44|matched where S44|matched is the reflection coefficient at the matched condition and S44 is the reflection coefficient at port 4 of the proposed branch-line coupler. Because the reflection coefficient at the matched port, S44|matched, should ideally be 0, m can be defined in the range from 1 at the matched condition to infinite at the mismatched condition. When the three ports of the proposed circulator are perfectly matched to the reference impedance 50 Ω, the S-parameter matrix of the proposed asymmetric branch-line coupler can be expressed (2) by the reciprocal property [24] as follows:   0 S21 S31 S41    S21 0 S32 S42  [S] =  . (2)  S31 S32 0 S43  S41 S42 S43 S44

Assuming that each port in the coupler is mutually independent [24], the relationship between the transmitted signals to ports 3 and 4 can be obtained from (2) as follows:

∗ ∗ S31S32 + S41S42 = 0, (3)

∗ ∗ ∗ S31S41 + S32S42 + S43S44 = 0, (4) ∗ ∗ S21S32 + S41S43 = 0, (5) Using the complex conjugate of the S-parameters, (3) and (4) can be modiﬁed as

∗ ∗ ∗ 2 S31 S32S42 + S41|S42| = 0, (6)

2 ∗ ∗ ∗ ∗ ∗ |S31| S41 + S31 S32S42 + S31 S43S44 = 0. (7) The ﬁrst term of (6) and the second term of (7) are cancelled after subtracting (6) from (7) and (8) can be obtained as ∗ 2 2 ∗ ∗ S41(|S31| − |S42| ) + S31 S43S44 = 0. (8) Assuming that the coupler is lossless, the second and the third rows of (2) can be expressed as

2 2 2 |S31| + |S32| + |S43| = 1, (9)

2 2 2 |S21| + |S32| + |S42| = 1, (10) and (8) can be modiﬁed to (11) using the relationship between (9) and (10) as

∗ 2 2 ∗ ∗ S41 |S21| − |S43| + S43S44S31 = 0. (11)

* Substituting (5) into (11) for S31 after modifying (5) for S41 , the magnitude of the Tx leakage isolation |S31| can be expressed as

1 − K2 |S | · |S | | | = | ∗ | = · 21 32 S31 S31 2 . (12) K |S44| where the parameter K is the transmission ratio between the paths from ports 1 to 2 and from ports 3 to 4, which represents the effect of the different characteristic impedances. Considering that the reﬂection Electronics 2018, 7, x FOR PEER REVIEW 4 of 15

Electronics 2018, 7, x FOR PEER REVIEW 4 of 15 reflection signal at port 4 cannot be neglected, K can be expressed using the parameter k, which Electronics 2018, 7, 173 4 of 15 reflectionrepresents signal the effect at port on the 4 cannotdifferent be impeda neglected,nces ofK thecan matched be expressed ports andusing the the parameter parameter m as k, follows: which represents the effect on the different impedances of the matched ports and the parameter m as follows: S43 Kkm==⋅, (13) signal at port 4 cannot be neglected, K can be expressedS using the parameter k, which represents the S4321 effect on the different impedances of the matchedKkm==⋅ ports and, the parameter m as follows: (13) S21 S S 43 matched K =k =43 = k · m,. (13)(14) S21S43 S21 k = matched . (14) S From (1) and (13), (12) can be expressed asS | 21 k = 43 matched . (14) S From (1) and (13), (12) can be expressed as− 2221 SS⋅ =⋅1 km 21 32 S31 23 . (15) From (1) and (13), (12) can be expressed as−km22 SSS⋅ =⋅1 km 4421matched 32 S31 23 . (15) km2 2 1 − k m |S44 | · |S | Figure 2 shows the variation in the| magnitude| = of the · Tx21 leakagematched32 signals in the branch-line coupler S31 2 3 . (15) k m | S44| | Figuredepending 2 shows on k andthe variationm. The design in the equation magnitude D to of minimize the Tx leakage matchedthe Tx leakage signals signalsin the branch-lineis derived from coupler (15) dependingas follows:Figure 2on shows k and the m. variationThe design in equation the magnitude D to minimize of the Tx the leakage Tx leakage signals signals in the branch-lineis derived from coupler (15) dependingas follows: on k and m. The design equation DDkmto=⋅ minimize =1. the Tx leakage signals is derived from (15)(16) as follows: Dkm=⋅ =1. (16) High Tx–Rx isolation can be obtainedD =from|k · (14),m| = which1. shows that k can be compensated (16) by adjusting m to improve the isolation between the Tx and Rx ports. High Tx–Rx isolation can be obtainedobtained from (14), which shows that k cancan bebe compensatedcompensated by adjusting mm toto improveimprove thethe isolationisolation betweenbetween thethe TxTx andand RxRx ports.ports.

Figure 1. Block diagram of the proposed quasi-circulator using an asymmetric branch-line coupler. Figure 1. Block diagram of the proposed quasi-circulatorquasi-circulator using an asymmetric branch-linebranch-line coupler.coupler.

FigureFigure 2.2. CalculatedCalculated TxTx leakageleakage levelslevels dependingdepending onon designdesign parametersparameters kk andand mm.. Figure 2. Calculated Tx leakage levels depending on design parameters k and m. In the conventional circulator using the symmetric coupler, the parameters k and m of (16) are each designed to be 1. Although the conventional circulator theoretically satisﬁes (16), the leakage

Electronics 2018, 7, x FOR PEER REVIEW 5 of 15

ElectronicsIn the2018 conventional, 7, 173 circulator using the symmetric coupler, the parameters k and m of (16)5 ofare 15 each designed to be 1. Although the conventional circulator theoretically satisfies (16), the leakage signals that cannot be ignored in the monostatic radar and the wireless communication systems remainsignals in that the cannot actual beimplemented ignored in circulator. the monostatic The transmission radar and thelines wireless in the coupler communication must be precisely systems implementedremain in the actualwith a implementedquarter-wave circulator. length to Theobtain transmission k as 1. However, lines in since the coupler the deviation must be of precisely the line widthimplemented of the transmission with a quarter-wave lines connected length toat obtainone nodek as cannot 1. However, be neglected, since the realizing deviation the of quarter- the line widthwave transmission of the transmission lines in lines the connected coupler implementa at one nodetion cannot is difficult. be neglected, Figure realizing 3 shows the the quarter-wave difference betweentransmission the ideal lines and in the actual coupler interconnections implementation of th ise difficult. transmission Figure lines3 shows in the the conventional difference between branch- linethe idealcoupler. and actualTheoretically, interconnections the lengths of theand transmission widths of all lines the in transmission the conventional lines branch-line in the branch-line coupler. couplerTheoretically, are designed the lengths to theand quarter-wavelength widths of all the L1 transmission of the operating lines frequency in the branch-line and the widths coupler W1 and are Wdesigned2 by the tocharacteristic the quarter-wavelength impedances ofL 1theof lines. the operating However, frequency the lines andcannot the meet widths at theW1 singleand W point2 by inthe the characteristic implementation impedances of the coupler of the and lines. the However, T-junction the should lines cannotbe used meet to connect at the singlethe lines point as shown in the inimplementation Figure 3b. 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The of line the length physical of the size coupler of the cannot T-junction. be the same The as design the theoretical inaccuracy quarter-wavelength increases in theL1 implementationbecause of the physical of the coupler size of thesince T-junction. the design The that design approximates inaccuracy the increases length to in L1 the’ or implementationL1’’ in Figure 3b 0 00 consideringof the coupler the since dimension the design of the that junction approximates cannot be the regarded length to asL the1 or accurateL1 in Figure design3 bmethodology. considering Thethe dimensionsize of the ofjunction the junction is almost cannot frequency-indepe be regarded asndent the accuratesince it is design mainly methodology. designed by Thethe widths size of Wthe1 junctionand W2 depending is almost frequency-independent on the characteristics sinceof the it issubstrate. mainly designed The design by theerror widths by theW 1junctionand W2 increasesdepending in onthethe implementation characteristics as of the the operating substrate. frequency The design increases error because by the junction the quarter-wavelength increases in the decreasesimplementation but the as size the of operating the junction frequency is almost increases constant because depending the quarter-wavelength on the increase in frequency. decreases This but producesthe size of the the non-ideal junction ischaracteristics almost constant in the depending symmetric on branch-line the increase coupler in frequency. and the This value produces of k in the implementednon-ideal characteristics coupler becomes in the symmetric a different branch-line paramete couplerr with the and design the value value of k inof the1. The implemented multiple reflectionscoupler becomes and transmissions, a different parameterthe substrate with and the propagation design value losses of and 1. Theother multiple parasitic reflections effects should and betransmissions, considered theto substrateaccurately and design propagation the value losses of andk when other implementing parasitic effects the should branch-line be considered coupler. to However,accurately the design complexity the value of of thek when design implementing can increase. the The branch-line characteristics coupler. of the However, coupler the are complexity especially sensitiveof the design to the can change increase. in the The physical characteristics size of the of T- thejunction coupler due are to especiallyfabrication sensitive error. This to theis the change main reasonin the physicalwhy it is sizedifficult of the to T-junctionaccurately design due to the fabrication branch-line error. coupler This on is thethe mainPCB in reason the millimeter- why it is wavedifficult frequency to accurately bands. design The characteristic the branch-line changes coupler caused on the by PCB the in T-junction the millimeter-wave can be modified frequency by optimizingbands. The the characteristic theoretical design changes parameters caused byusing the the T-junction electromagnetic can be modifiedwave simulation by optimizing but there the is atheoretical problem that design the parametersoptimization using process the is electromagnetic required for each wave design simulation every time. but thereFor improving is a problem the isolationthat the optimization by reducing the process effect is of required the T-junction, for each the design proposed every quasi-circulat time. For improvingor using the asymmetric isolation by branch-linereducing the coupler effect of increases the T-junction, the degree the proposedof design freedom quasi-circulator by using using the specific the asymmetric k parameter branch-line and the mcoupler parameter increases to compensate the degree of for design the k freedom parameter. by using When the the specific k parameterk parameter is set and to thea valuem parameter that is excessivelyto compensate smaller for the thank parameter. 1, the width When of the the transmk parameterission line is set becomes to a value thinne thatr is in excessively the realization smaller on thethan PCB 1, the because width of of the the increase transmission in the characteristic line becomes impedance thinner in theof the realization line. The onexcessively the PCB becausethin width of ofthe the increase line can in cause the characteristic performance impedancedegradation of because the line. of Thethe process excessively tolerance thin widthfor the of fabrication the line can of thecause PCB. performance In the proposed degradation quasi-circulator, because of the the process parameters tolerance k and for them fabricationof (16) are of0.947 the PCB.and In1.053, the respectively.proposed quasi-circulator, the parameters k and m of (16) are 0.947 and 1.053, respectively.

(a) (b)

Figure 3. T-junctionT-junction interconnection of the transmission lines lines in in the the conventional conventional branch-line branch-line coupler: coupler: (a) theoretical description usin usingg a single point junction; ( b) physical dimension in layout design for realization on a printed circuit board (PCB).(PCB).

Electronics 2018, 7, 173 6 of 15

3. Implementation of the Proposed and Conventional Quasi-Circulators The conventional quasi-circulator using the branch-line coupler consists of four quarter-wave transmission lines with characteristic impedances Z1 and Z3 of 35.4 Ω and Z2 and Z4 of 50 Ω, as shown in Figure1, respectively [ 24]. Port 4 of the conventional circulator is terminated with Electronics 2018, 7, x FOR PEER REVIEW 6 of 15 the reference impedance Z50. Based on the design parameters k and m in Section2, the characteristic impedances3. ImplementationZ1, Z2, Z3 and of theZ 4Proposedof the and transmission Conventional line Quasi-Circulators and the termination impedance ZISO are obtained in theThe proposed conventional circulator quasi-circulator using theusing asymmetric the branch-line coupler. coupler Toconsists reduce of four the quarter-wave size of the T-junction, the characteristictransmission impedances lines with characteristicZ1 and Z impedances4 in the proposed Z1 and Z3 of circulator 35.4 Ω and Z are2 and designed Z4 of 50 Ω, toas shown 40 Ω and 60 Ω, in Figure 1, respectively [24]. Port 4 of the conventional circulator is terminated with the reference respectively. The value of k in the circulator is obtained by using Z2 of 45 Ω and Z3 of 35 Ω. Unbalanced impedance Z50. Based on the design parameters k and m in Section 2, the characteristic impedances impedances of the transmission lines are compensated by using ZISO of 45 Ω to achieve the desired Z1, Z2, Z3 and Z4 of the transmission line and the termination impedance ZISO are obtained in the value of mproposedat port circulator 4. All circulatorsusing the asymmetric were simulated coupler. To using reduce a Keysight the size of Advanced the T-junction, Design the Systems Momentumcharacteristic electromagnetic-wave impedances Z1 and (EM) Z4 simulatorin the proposed considering circulator theare designed dielectric to constants,40 Ω and 60 loss Ω, tangents and thicknessesrespectively. of PCBs. The value of k in the circulator is obtained by using Z2 of 45 Ω and Z3 of 35 Ω. Unbalanced impedances of the transmission lines are compensated by using ZISO of 45 Ω to achieve the desired 3.1. Quasi-Circulatorsvalue of m at forport 2.45 4. All GHz circulators were simulated using a Keysight Advanced Design Systems Momentum electromagnetic-wave (EM) simulator considering the dielectric constants, loss tangents The asymmetricand thicknesses and of PCBs. symmetric couplers, operating at a 2.45 GHz frequency band, were fabricated × × on an FR43.1. PCB Quasi-Circulators with a dielectric for 2.45 constant GHz of 4.3 of 39 mm 30 mm and 35 mm 30 mm and a thickness of 1 mm, as shown in Figure4. In the conventional circulator using the symmetric coupler, the widths The asymmetric and symmetric couplers, operating at a 2.45 GHz frequency band, were W1 and W2fabricatedof the transmission on an FR4 PCB lineswith awith dielectric the constant characteristic of 4.3 of impedances39 mm × 30 mm 35.4 and 35Ω mmand × 5030 mmΩ were and designed to be 3.3 mma thickness and 1.9 of 1 mm, mm, as respectively. shown in Figure Although 4. In the conventional the quarter-wavelength circulator using the at symmetric 2.45 GHz coupler, is 14.8 mm on the FR4 PCB,the widths the lengths W1 and ofW2 the of the transmission transmission lines lines with in Figurethe characteristic4a were designedimpedances by35.4 including Ω and 50 Ω the size of the junctionwere as designed 16 mm, to 17.9 be 3.3 mm, mm 16and mm 1.9 mm, and respectively. 17.3 mm, Although respectively. the quarter-wavelength Using the designed at 2.45 GHz characteristic is 14.8 mm on the FR4 PCB, the lengths of the transmission lines in Figure 4a were designed by impedancesincludingZ1, Z 2the, Z size3 and of theZ4 junction, the widths as 16 mm, of the17.9 transmissionmm, 16 mm and lines 17.3 mm, in the respectively. proposed Using circulator the were designed todesigned be 2.6 mm,characteristic 2.2 mm, impedances 3.3 mm and Z1, Z 1.32, Z mm,3 and respectively.Z4, the widths Becauseof the transmission the size of lines the in T-junction the was changed byproposed the designed circulator impedances, were designed theto be lengths 2.6 mm, of2.2 themm, transmission 3.3 mm and 1.3 linesmm, respectively. in Figure4 bBecause were modiﬁed to be 16 mm,the size 17.8 of mm, the T-junction 16 mm andwas changed 16.9 mm, by the respectively. designed impedances, The design the lengths accuracy of the can transmission be increased in the lines in Figure 4b were modified to be 16 mm, 17.8 mm, 16 mm and 16.9 mm, respectively. The design proposed circulatoraccuracy can because be increased the in lengths the proposed of the circulat proposedor because circulator the lengths were of reduced the proposed as compared circulator with the lengths ofwere the conventionalreduced as compared circulator. with the The lengths termination of the conventional resistance circulator. at port The 4 termination was realized resistance using a single 45 Ω resistorat port with 4 was the realized size of using 10 mm a single× 5 45 mm Ω resistor made with by Murata the size of Manufacturing 10 mm × 5 mm made Co., by Ltd. Murata (Nagaokakyo, Kyoto, Japan).Manufacturing Co., Ltd (Nagaokakyo, Kyoto, Japan).

17.9 mm 17.8 mm 1.9 mm 3.3 mm 1.9 mm 2.6 mm

16 mm 16 mm 1.9 mm 2.2 mm 17.3 mm 16 mm 16.9 mm 16 mm

1.3 mm

3.3 mm

(a) (b)

Figure 4. 2.45 GHz quasi-circulators fabricated on an FR4 PCB: (a) the conventional circulator using Figure 4. 2.45a symmetrical GHz quasi-circulators branch-line coupler; fabricated (b) the proposed on an FR4circulator PCB: using (a) the an conventionalasymmetrical branch-line circulator using a symmetricalcoupler. branch-line coupler; (b) the proposed circulator using an asymmetrical branch-line coupler.

Figure 5 shows the simulation results for the quasi-circulators. The operating frequency of the Figureconventional5 shows the circulator simulation is lower results than the for design the quasi-circulators.frequency of 2.45 GHz The but operatingthe frequency frequency of the of the conventionalproposed circulator circulator is is lower almost thanthe same the as designthe design frequency frequency. In of addition, 2.45 GHz the proposed but the circulator frequency of the proposed circulator is almost the same as the design frequency. In addition, the proposed circulator shows |S21| of −3.31 dB and |S32| of −3.39 dB at 2.45 GHz, which are similar to each other, whereas the conventional circulator obtains |S21| of −3.05 dB and |S32| of −3.74 dB at this frequency. Electronics 2018, 7, x FOR PEER REVIEW 7 of 15

Electronicsshows |2018S21|, of7, 173–3.31 dB and |S32| of –3.39 dB at 2.45 GHz, which are similar to each other, whereas7 of 15 the conventional circulator obtains |S21| of –3.05 dB and |S32| of –3.74 dB at this frequency.

(a) (b)

FigureFigure 5.5. Simulated SS-parameters-parameters ofof thethe proposedproposed andand conventionalconventional quasi-circulatorsquasi-circulators usingusing 2.452.45 GHzGHz asymmetric and symmetric branch-line couplers, respectively: (a) |S11| denotes the return loss at the asymmetric and symmetric branch-line couplers, respectively: (a) |S11| denotes the return loss at the Tx port; (b) |S21| denotes propagation gains in the Tx path; (c) |S32| denotes propagation gains in the Tx port; (b) |S21| denotes propagation gains in the Tx path; (c) |S32| denotes propagation gains in the Rx path; (d) |S31| denotes the Tx leakage isolation between the Tx and Rx ports. Rx path; (d) |S31| denotes the Tx leakage isolation between the Tx and Rx ports.

3.2.3.2. Quasi-CirculatorsQuasi-Circulators forfor 24.12524.125 GHzGHz As shown in Figure 6, the directional couplers operating at the 24.125 GHz frequency band were As shown in Figure6, the directional couplers operating at the 24.125 GHz frequency band were realized on an RO3003 PCB with a dielectric constant of 3.0 of 17 mm × 13 mm and a thickness of 10 realized on an RO3003 PCB with a dielectric constant of 3.0 of 17 mm × 13 mm and a thickness mil. The losses caused by the length of the transmission line equally occurred in both of two couplers of 10 mil. The losses caused by the length of the transmission line equally occurred in both of because the total size of the two PCBs is the same as each other. The widths of the conventional quasi- two couplers because the total size of the two PCBs is the same as each other. The widths of the circulator were designed to be 1.0 mm and 0.6 mm for the transmission lines with the characteristic conventional quasi-circulator were designed to be 1.0 mm and 0.6 mm for the transmission lines with impedance of 35 Ω and 50 Ω, respectively. The quarter-wavelength at 24.125 GHz is 1.8 mm on the the characteristic impedance of 35 Ω and 50 Ω, respectively. The quarter-wavelength at 24.125 GHz is RO3003 PCB but the lengths of the transmission lines in the conventional circulator were 1.8 mm on the RO3003 PCB but the lengths of the transmission lines in the conventional circulator were implemented as 1.4 mm, 2.0 mm, 1.9 mm and 2.4 mm, respectively. The widths of the lines designed implemented as 1.4 mm, 2.0 mm, 1.9 mm and 2.4 mm, respectively. The widths of the lines designed in the 24.125 GHz proposed quasi-circulator are 0.8 mm, 0.7 mm, 1.0 mm and 0.5 mm, respectively. in the 24.125 GHz proposed quasi-circulator are 0.8 mm, 0.7 mm, 1.0 mm and 0.5 mm, respectively. The line lengths of the proposed circulator were individually designed to be 1.4 mm, 2.0 mm, 1.9 mm The line lengths of the proposed circulator were individually designed to be 1.4 mm, 2.0 mm, 1.9 mm and 2.2 mm, respectively. As in the design of the 2.45 GHz circulators, the length on the left side was and 2.2 mm, respectively. As in the design of the 2.45 GHz circulators, the length on the left side was also reduced by 0.2 mm compared to the length of the conventional one. The resistance of 45 Ω at also reduced by 0.2 mm compared to the length of the conventional one. The resistance of 45 Ω at port port 4 was implemented using high-frequency resistors of 50 Ω and 500 Ω in parallel, which were 4 was implemented using high-frequency resistors of 50 Ω and 500 Ω in parallel, which were made by made by Vishay Intertechnology, Inc. (Malvern, PA, USA). Vishay Intertechnology, Inc. (Malvern, PA, USA). The simulation results in Figure 7 show that the proposed quasi-circulator designed at 24.125 The simulation results in Figure7 show that the proposed quasi-circulator designed at 24.125 GHz GHz but the conventional quasi-circulator designed at a frequency lower than the desired frequency. but the conventional quasi-circulator designed at a frequency lower than the desired frequency. These are similar characteristics to the simulation results of the 2.45 GHz quasi-circulators. The These are similar characteristics to the simulation results of the 2.45 GHz quasi-circulators. The insertion insertion losses in the Tx and Rx paths are well balanced in the simulation results of the proposed losses in the Tx and Rx paths are well balanced in the simulation results of the proposed circulator. Note

Electronics 2018, 7, 173 8 of 15 Electronics 2018,, 7,, xx FORFOR PEERPEER REVIEWREVIEW 8 of 15 circulator.that a conventional Note that circulator a conventional could circulator be well optimized could be to well have optimized accurate to characteristics have accurate at characteristics the operating atfrequency the operating but it frequency takes a long but time it takes to designa long becausetime toto designdesign the physical becausebecause lengths thethe physicalphysical and line lengthslengths widths andand of lineline the widthstransmission of the lines transmission must be determined lines must by be parametric determinedined optimization. byby parametricparametric The optimization. simulationoptimization. results TheThe considering simulationsimulation resultsthe physical considering size of the the physical T-junction size show of the that T-junction the proposed showshow thatthat circulator thethe proposedproposed can be circulatorcirculator exactly designed cancan bebe exactlyexactly at the designeddesired operating at the desired frequency operating without frequency the complicated withoutthout thethe optimization complicatedcomplicated process. optimizationoptimization process.process.

2.0 mm 1.4 mm 2.0 mm 1.4 mm 2.0 mm 1.4 mm 1.0 mm 0.8 mm 2.4 mm 0.6 mm 0.7 mm 2.4 mm 0.6 mm 2.2 mm 0.5 mm 0.7 mm 1.9 mm 1.9 mm 1.0 mm 1.9 mm

(a) (b)

Figure 6. 24.125-GHz24.125-GHz quasi-circulatorsquasi-circulators fabricatedfabricated onon anan RO3003RO3003 PCB:PCB: ((a)) conventionalconventional circulator;circulator; ((b)) Figure 6. 24.125-GHz quasi-circulators fabricated on an RO3003 PCB: (a) conventional circulator; proposed circulator. (proposedb) proposed circulator. circulator.

(a) (b)

Figure 7. SimulatedSimulated S-parameters-parameters ofof the the proposed proposed and and conventional conventional quasi-circulatorsquasi-circulators atat 24.12524.125 GHz:GHz: (a) |S11|; (b) |S21|; (c) |S32|; (d) |S31|. (a) |S1111|;|; (b (b) )| |SS2121|;|; (c) ( c|)S |32S|;32 (|;d) (|dS)31 ||.S31 |.

Electronics 2018, 7, 173 9 of 15

4. Measurement Results of the Proposed Quasi-Circulators.

4.1. Quasi-Circulators for 2.45 GHz Figure8 shows the measured S-parameters of the proposed and conventional quasi-circulators at the frequency band. The isolation between the Tx and Rx ports increased by 9.07 dB at 2.45 GHz and the frequency bandwidth in which the isolation is less than −20 dB extended from 140 MHz to 250 MHz. The frequency with the high Tx–Rx isolation of the proposed circulator is set to be 2.45 GHz, which is the same as the designed operating frequency. On the other hand, the frequency of the conventional circulator shifted from 2.45 GHz to 2.34 GHz despite its precise design. The magnitude of S21 and S32 of the proposed circulator are measured as −3.64 and −3.52 dB, respectively, which are well-balanced compared to those of the conventional circulator. The losses in the Tx and Rx paths of the proposed circulator are similar to those of the conventional circulator. The detailed measurement results of the proposed circulator, compared with the conventional circulator, show an increase of 0.45 dB in the Tx-path loss and a decrease of 0.32 dB in the Rx path loss. As the insertion loss represents the signal attenuation from the Tx and the antenna ports, the Tx path loss can show the efficiency of the circulator for the transmitted signals. Measurement results indicate that the efficiency of the proposed circulator at 2.45 GHz can be less in the transmitted signals than in the conventional circulator, based on the additional Tx path loss of 0.45 dB. On the other hands, the efficiency of the circulator in the received signals at 2.45 GHz can be higher than the conventional circulator because of the difference of 9.39 dB between the Rx path loss and the Tx leakage isolation, which is shown in Figure8e, has a positive effect on the Rx sensitivity. Figure8f shows the difference between the simulation and measurement results of the Tx leakage isolation of the 2.45 GHz quasi-circulators. The difference shows that the design accuracy of the proposed quasi-circulator is higher in the measured frequency band than that of the conventional one. The peak in Figure8f is based on the narrow band characteristics from the simulation result and it can be neglected from determining the accuracy from the comparison between the simulation and measurement because the isolation of less than −30 dB is difficult to realize in the implementation due to the leakage through the PCB substrate itself.

4.2. Quasi-Circulators for 24.125 GHz The measured S-parameters of the proposed and conventional circulators are shown in Figure9. The Tx leakage isolation of the proposed circulator at 24.125 GHz is better than the conventional circulator by 7.95 dB. Similar to the 2.45 GHz circulators, the magnitudes of S21 and S32 of the proposed circulator at 24.125 GHz are also almost the same and well-balanced. The minimum isolation in the proposed and conventional circulators are measured as −19.85 dB at 24.225 GHz and −21.67 dB at 27.115 GHz, respectively. The frequency with high isolation in the proposed circulator is shown in the operating frequency band, while the frequency of the conventional circulator is shown out of the frequency band. Figure9d shows that the design error of the conventional circulator in the 24 GHz frequency band using the symmetric coupler is critically increased compared with the error in the circulator operating at 2.45 GHz in Figure8d. It can be understood that the physical size of the T-junction has a greater effect on the circulator characteristics in the 24 GHz frequency band because the quarter-wavelength of the transmission lines is reduced to be comparable to the width of the lines. Since the effects on the parasitic components and the process errors increase more at the high frequency of 24.125 GHz than 2.45 GHz, the measurement results of the conventional quasi-circulator changed more in the simulation results. Although the parasitic components and the process errors were simultaneously generated between the two circulators in the same fabrication, the characteristics change of the proposed circulator were not shown in the results. The symmetrical coupler used in the conventional quasi-circulator may degrade greatly if the symmetricity of the coupler is broken but the asymmetrical coupler used in the proposed quasi-circulator does not show the degradation in the measurement. Therefore, it can be considered that the performance change in the proposed quasi-circulator is small due to the asymmetric coupler. The efﬁciency of the circulators Electronics 2018, 7, x FOR PEER REVIEW 9 of 15

4. Measurement Results of the Proposed Quasi-Circulators.

4.1. Quasi-Circulators for 2.45 GHz Figure 8 shows the measured S-parameters of the proposed and conventional quasi-circulators at the frequency band. The isolation between the Tx and Rx ports increased by 9.07 dB at 2.45 GHz and the frequency bandwidth in which the isolation is less than −20 dB extended from 140 MHz to 250 MHz. The frequency with the high Tx–Rx isolation of the proposed circulator is set to be 2.45 GHz, which is the same as the designed operating frequency. On the other hand, the frequency of the conventional circulator shifted from 2.45 GHz to 2.34 GHz despite its precise design. The magnitude of S21 and S32 of the proposed circulator are measured as −3.64 and −3.52 dB, respectively, which are well-balanced compared to those of the conventional circulator. The losses in the Tx and Rx paths of the proposed circulator are similar to those of the conventional circulator. The detailed measurement results of the proposed circulator, compared with the conventional circulator, show an increase of 0.45 dB in the Tx-path loss and a decrease of 0.32 dB in the Rx path loss. As the insertion loss represents the signal attenuation from the Tx and the antenna ports, the Tx path loss can show the Electronics 2018 7 efficiency ,of, the 173 circulator for the transmitted signals. Measurement results indicate that the efficiency10 of 15 of the proposed circulator at 2.45 GHz can be less in the transmitted signals than in the conventional in thecirculator, transmitted based signals on the canadditional be almost Tx thepath same loss becauseof 0.45 dB. of theOn similarthe other Tx hands, path losses the efficiency in the circulators of the butcirculator the efﬁciency in the of received the proposed signals circulatorat 2.45 GHz for can the be received higher than signals the atconventional 24.125 GHz circulator can be higher because than of the difference of 9.39 dB between the Rx path loss and the Tx leakage isolation, which is shown in the conventional circulator owing to the difference of 8.04 dB shown in Figure9e. The efﬁciency Figure 8e, has a positive effect on the Rx sensitivity. Figure 8f shows the difference between the degradation in the Rx path is caused by the shift of the operating frequency in the conventional simulation and measurement results of the Tx leakage isolation of the 2.45 GHz quasi-circulators. The circulator. Figure9f shows that the proposed quasi-circulator also has higher design accuracy than a difference shows that the design accuracy of the proposed quasi-circulator is higher in the measured conventional quasi-circulator because the isolation characteristics of less than −30 dB in the simulation frequency band than that of the conventional one. The peak in Figure 8f is based on the narrow band resultscharacteristics can be neglected. from the simulation The results result of the and proposed it can be neglected quasi-circulators, from determining as shown the in accuracy Table1, from reveal thethe improved comparison Tx leakage between isolation the simulation and similar and meas insertionurement losses because at the the designed isolation operating of less than frequency −30 dB is as compareddifficult withto realize those in of the the implementation conventional circulators.due to the leakage through the PCB substrate itself.

(a) (b)

Electronics 2018, 7, x FOR PEER REVIEW 10 of 15 (c) (d)

(e) (f)

Figure 8. Measured S-parameters of the proposed and conventional quasi-circulators at 2.45 GHz: (a) Figure 8. Measured S-parameters of the proposed and conventional quasi-circulators at 2.45 GHz: |S11|; (b) |S21|; (c) |S32|; (d) |S31|; (e) |S32|−|S31|; (f) the difference of |S31| between the simulation (a) |S11|; (b) |S21|; (c) |S32|; (d) |S31|; (e) |S32|−|S31|; (f) the difference of |S31| between the and the measurement. simulation and the measurement. 4.2. Quasi-Circulators for 24.125 GHz The measured S-parameters of the proposed and conventional circulators are shown in Figure 9. The Tx leakage isolation of the proposed circulator at 24.125 GHz is better than the conventional circulator by 7.95 dB. Similar to the 2.45 GHz circulators, the magnitudes of S21 and S32 of the proposed circulator at 24.125 GHz are also almost the same and well-balanced. The minimum isolation in the proposed and conventional circulators are measured as −19.85 dB at 24.225 GHz and −21. 67 dB at 27.115 GHz, respectively. The frequency with high isolation in the proposed circulator is shown in the operating frequency band, while the frequency of the conventional circulator is shown out of the frequency band. Figure 9d shows that the design error of the conventional circulator in the 24 GHz frequency band using the symmetric coupler is critically increased compared with the error in the circulator operating at 2.45 GHz in Figure 8d. It can be understood that the physical size of the T- junction has a greater effect on the circulator characteristics in the 24 GHz frequency band because the quarter-wavelength of the transmission lines is reduced to be comparable to the width of the lines. Since the effects on the parasitic components and the process errors increase more at the high frequency of 24.125 GHz than 2.45 GHz, the measurement results of the conventional quasi-circulator changed more in the simulation results. Although the parasitic components and the process errors were simultaneously generated between the two circulators in the same fabrication, the characteristics change of the proposed circulator were not shown in the results. The symmetrical coupler used in the conventional quasi-circulator may degrade greatly if the symmetricity of the coupler is broken but the asymmetrical coupler used in the proposed quasi-circulator does not show the degradation in the measurement. Therefore, it can be considered that the performance change in the proposed quasi-circulator is small due to the asymmetric coupler. The efficiency of the circulators in the transmitted signals can be almost the same because of the similar Tx path losses in the circulators but the efficiency of the proposed circulator for the received signals at 24.125 GHz can be higher than the conventional circulator owing to the difference of 8.04 dB shown in Figure 9e. The efficiency degradation in the Rx path is caused by the shift of the operating frequency in the conventional circulator. Figure 9f shows that the proposed quasi-circulator also has higher design accuracy than a conventional quasi-circulator because the isolation characteristics of less than –30 dB in the simulation results can be neglected. The results of the proposed quasi-circulators, as shown in Table 1, reveal the improved Tx leakage isolation and similar insertion losses at the designed operating frequency as compared with those of the conventional circulators.

Electronics 2018, 7, 173 11 of 15 Electronics 2018, 7, x FOR PEER REVIEW 11 of 15

(a) (b)

(e) (f)

Figure 9. Measured S-parameters of the proposed and conventional quasi-circulators at 24.125 GHz: Figure 9. Measured S-parameters of the proposed and conventional quasi-circulators at 24.125 GHz: (a) |S11|; (b) |S21|; (c) |S32|; (d) |S31|; (e) |S32|−|S31|; (f) the difference of |S31| between the simulation (a) |S11|; (b) |S21|; (c) |S32|; (d) |S31|; (e) |S32|−|S31|; (f) the difference of |S31| between the simulationand the measurement. and the measurement.

Table 1. Comparisons of the proposed quasi-circulators and the conventional quasi-circulators.

S-parameters |S21| |S32| |S31| (dB) (Tx Path) (Rx Path) (Tx Leakage) Conventional –3.19 –3.84 –17.71 2.45 GHz Proposed –3.64 –3.52 –26.78 Improvement –0.45 0.32 9.07 Conventional –4.80 –4.29 –11.73 24.125 GHz Proposed –4.75 –4.18 –19.68 Improvement 0.05 0.09 7.95

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Table 1. Comparisons of the proposed quasi-circulators and the conventional quasi-circulators.

S-Parameters |S21| |S32| |S31| (dB) (Tx Path) (Rx Path) (Tx Leakage) Conventional −3.19 −3.84 −17.71 2.45 GHz Proposed −3.64 −3.52 −26.78 Improvement −0.45 0.32 9.07 Conventional −4.80 −4.29 −11.73 24.125 GHz Proposed −4.75 −4.18 −19.68 Electronics 2018, 7, x FOR PEER REVIEWImprovement 0.05 0.09 7.95 12 of 15

4.3.4.3. Performance Performance Improvement Improvement by by the the Proposed Proposed Quasi-Circulator Quasi-Circulator at at the the 2.45 2.45 GHz GHz Radar Radar Sensor Sensor TheThe monostatic monostatic Doppler Doppler radarradar sensor shown shown in in Figure Figure 10 10 was was demonstrated demonstrated at 2.45 at 2.45 GHz GHz to verify to verifythe performance the performance improvement improvement by reducing by reducing the Tx the leakage Tx leakage isolation isolation of the of theproposed proposed circulator. circulator. The The2.45 2.45 GHz GHz signal signal with with the the output output power power of − of10− dBm10 dBm is generated is generated at the at Keysight the Keysight N5183B N5183B analog analog signal signalgenerator generator and andthe thesignal signal is divided is divided into into the the Tx Tx and and Rx Rx paths paths by thethe powerpower splitter,splitter, which which is is manufacturedmanufactured by by Mini-Circuits. Mini-Circuits. A Tx A signal Tx signal is transmitted is transmitted to the antenna to the throughantenna thethrough quasi-circulator. the quasi- Thecirculator. 2.45 GHz The signal 2.45 GHz radiated signal from radiated the patchfrom the antenna patch withantenna the with gain the of gain 0 dBi of is 0 reflecteddBi is reflected by the by metronomethe metronome with with 1 Hz 1 vibration Hz vibration and and transmitted transmitted to the to the Rx Rx port port of theof the quasi-circulator quasi-circulator through through the the samesame antenna antenna and and the quasi-circulator.the quasi-circulator. The receivedThe received and the and Tx leakagethe Tx signalsleakage by signals the quasi-circulator by the quasi- arecirculator simultaneously are simultaneously transmitted transmitted to the Rx port to the and Rx amplified port and amplified with the voltagewith the gain voltage of 14 gain dB of in 14 the dB lowin noisethe low amplifier. noise amplifier. The received The signalreceived is mixed signal with is mixed the 2.45 with GHz the reference 2.45 GHz signal reference in the signal frequency in the mixerfrequency to produce mixer a to 1 produce Hz signal a of1 Hz the signal metronome of the metronome motion in the motion baseband. in the Thebaseband. Tx leakage The Tx signal leakage is shownsignal near is shown DC level near after DC passinglevel after through passing the through frequency the mixerfrequency because mixer of thebecause fixed of leakage the fixed path leakage and ispath generally and is larger generally magnitude larger thanmagnitude the received than the signal. received If the signal. sensitivity If the is sensitivity degraded byis degraded the increase by ofthe theincrease Tx leakage of the signal, Tx leakage the amplitude signal, the of amplitude the 1 Hz signal of the of1 Hz the signal metronome of the metronome motion gradually motion decreases gradually becausedecreases the because received the signal received is masked signal by is themasked leakage by th signal.e leakage If the signal. received If the signal received is smaller signal thanis smaller the levelthan determined the level determined by the minimum by the sensitivity minimum of thesensit system,ivity of the the 1 Hz system, signal the cannot 1 Hz be signal detected cannot in the be baseband.detected Therefore,in the baseband. the improvement Therefore, of the the improvement Tx leakage isolation of the Tx by theleakage proposed isolation quasi-circulator by the proposed can bequasi-circulator verified by the increasecan be verified of the receivedby the increase amplitude of the of thereceived 1 Hz signalamplitude and theof the maximum 1 Hz signal detectable and the rangemaximum from the detectable radar sensor range to from the metronome.the radar sensor to the metronome.

FigureFigure 10. 10.Measurement Measurement setup setup for for 2.45 2.45 GHz GHz monostatic monostatic Doppler Doppler radar radar sensor sensor using using the the quasi-circulator. quasi-circulator.

TheThe results results shown shown in Figurein Figure 11 are11 are the the received received signals signals of the of radarthe radar sensors sensors in the in baseband the baseband for the for metronomethe metronome with1 with Hz motion 1 Hz motion measured measured at the distance at the distance from 0.5 from m to 0.5 1 m. m Figureto 1 m. 11 Figuresa,b show 11a,b that show the radarthat sensorthe radar using sensor the proposed using the quasi-circulator proposed quasi-circulator generates a smallergenerates signal a smaller around signal the DC around level due the toDC thelevel improvement due to the improvement of the Tx leakage of the isolation Tx leakage than isolation the sensor than using the sensor the conventional using the conventional quasi-circulator. quasi- Thecirculator. metronome The motionmetronome of the motion around of 1the Hz around signal was1 Hz detected signal was at 0.5detected m by bothat 0.5 sensors m by both using sensors the twousing quasi-circulators the two quasi-circulators as shown in as Figure shown 11 inc butFigure the motion11c but wasthe motion only detected was only at 0.9detected m by theat 0.9 radar m by the radar sensor using the proposed quasi-circulator as shown in Figure 11d. The sensor using the conventional quasi-circulator detected the motion signal at 0.8 m but it did not show the signal at a longer distance than 0.8 m. The magnitude of the motion signal measured at 0.5 m distance was larger in the radar sensor using the proposed quasi-circulator than the sensor using the conventional quasi- circulator. The measurement results of the 2.45 GHz radar sensor show that the improvement of the Tx leakage isolation by the proposed quasi-circulator can improve the system performances of the radar sensor, especially the maximum detectable range.

Electronics 2018, 7, 173 13 of 15

sensor using the proposed quasi-circulator as shown in Figure 11d. The sensor using the conventional quasi-circulator detected the motion signal at 0.8 m but it did not show the signal at a longer distance than 0.8 m. The magnitude of the motion signal measured at 0.5 m distance was larger in the radar sensor using the proposed quasi-circulator than the sensor using the conventional quasi-circulator. The measurement results of the 2.45 GHz radar sensor show that the improvement of the Tx leakage isolation by the proposed quasi-circulator can improve the system performances of the radar sensor, Electronicsespecially 2018 the, 7, maximumx FOR PEER REVIEW detectable range. 13 of 15

(a) (b)

0.5 m 0.9 m

FigureFigure 11. 11. ReceivedReceived signal signal amplitude amplitude in in the the 2.45 2.45 GHz GHz monostatic monostatic Doppler Doppler radar radar sensor: sensor: (a (a) )using using the the proposedproposed quasi-circulator atat the the distances distances from from 0.5 0.5 m to m 1 m;to (1b )m; using (b) theusing conventional the conventional quasi-circulator quasi- circulatorat the distances at the distances from 0.5 mfrom to 1 0.5 m; m (c )to at 1 the m; distance(c) at the of distance 0.5 m; (ofd) 0.5 at them; distance(d) at the of distance 0.9 m. of 0.9 m.

5. Conclusions 5. Conclusions A quasi-circulator that uses an asymmetric branch-line coupler is proposed for obtaining the A quasi-circulator that uses an asymmetric branch-line coupler is proposed for obtaining optimal design characteristics at the operating frequency, including high Tx–Rx isolation. The the optimal design characteristics at the operating frequency, including high Tx–Rx isolation. asymmetric coupler for a highly isolated circulator is designed using the proposed design equation The asymmetric coupler for a highly isolated circulator is designed using the proposed design in (16), wherein the product of the asymmetric ratio of the characteristic impedances of the equation in (16), wherein the product of the asymmetric ratio of the characteristic impedances of the transmission lines and the unmatched ratio in the unused port of the coupler should be 1. The transmission lines and the unmatched ratio in the unused port of the coupler should be 1. The isolation isolation improvements (compared with the conventional circulators) of 9.07 dB at 2.45 GHz and 7.95 improvements (compared with the conventional circulators) of 9.07 dB at 2.45 GHz and 7.95 dB dB at 24.125 GHz are achieved through the implementation of the proposed planar quasi-circulators. at 24.125 GHz are achieved through the implementation of the proposed planar quasi-circulators. The frequencies with high Tx–Rx isolation in the conventional quasi-circulators are shifted but those The frequencies with high Tx–Rx isolation in the conventional quasi-circulators are shifted but those of the proposed quasi-circulators are realized in the operating frequency bands. The insertion losses of the proposed quasi-circulators are realized in the operating frequency bands. The insertion losses of the proposed quasi-circulators are balanced between the Tx and Rx paths and are similar to those of the proposed quasi-circulators are balanced between the Tx and Rx paths and are similar to those of the conventional quasi-circulators. The maximum detectable range of the 2.45 GHz Doppler radar of the conventional quasi-circulators. The maximum detectable range of the 2.45 GHz Doppler radar sensor was increased when the proposed quasi-circulator was used in the monostatic radar sensor. sensor was increased when the proposed quasi-circulator was used in the monostatic radar sensor.

Author Contributions: J.-R.Y. identified the framework of this study and defined the research theme. B.-Y.Y. and J.-R.Y. designed the circulators. B.-Y.Y. and J.-H.P. performed the measurements. All authors analyzed the data, wrote the paper and edited the manuscript.

Funding: This research was funded by the 2017 Yeungnam University Research Grant No. 217A580039. And this work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (grant number 2017R1C1B2002285).

Conflicts of Interest: The authors declare no conflict of interest.

Electronics 2018, 7, 173 14 of 15

Author Contributions: J.-R.Y. identified the framework of this study and defined the research theme. B.-Y.Y. and J.-R.Y. designed the circulators. B.-Y.Y. and J.-H.P. performed the measurements. All authors analyzed the data, wrote the paper and edited the manuscript. Funding: This research was funded by the 2017 Yeungnam University Research Grant No. 217A580039. And this work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (grant number 2017R1C1B2002285). Conflicts of Interest: The authors declare no conflict of interest.

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