Rat-race Coupler Research Articles

Impedance transforming rat-race couplers with modified Lange section

The relation between load impedances and the characteristic impedance of the rat-race ring has been derived. To minimize the circuit size, the 270°-long section has been replaced by a shorted coupled-line section. Moreover, to achieve appropriate coupling coefficient in the required coupled-line section, a modified Lange structure consisting of five-coupled lines has been applied. Furthermore, the second structure has been proposed having improved operational bandwidth, in which coupled-line matching sections are added. The theoretical analysis has been verified by measurements of the fabricated rat-race couplers operating at the center frequency 1 GHz. The designed couplers transforms 50 Ω input impedance into 10 Ω, which proves their practical applications.

Performance Analysis of 180° HRR Coupler Used for Direction Finding with an Antenna Array

This paper presents the performance analysis of hybrid rat race coupler, widely used in radio frequency (RF)/wireless communication systems to couple the power in the desired way. The hybrid ring coupler consists of 4 ports, two for the input signals and two for the output signals, where sum and difference patterns of the applied two signals can be obtained at two output ports and usually called sum and difference ports. In this work, the couplers have been designed and simulated at central frequencies (fo) of 2.4 and 10 GHz using different types of substrates such as RT Duroid 5880 and FR4. Furthermore, the coupler has been used for direction finding (angle-of-arrival estimation) application, where we combine the designed hybrid rat race coupler with a simple two antenna elements array (at fo=10GHz and RT Duroid =2.2) and fabricate the circuit in order to validate the performance of the coupler by measuring the direction of arrival (DoA) from and ports. The obtained results show that good performance can be achieved with the designs considered in this paper.

Rapid design closure of microwave components by means of feature-based optimization and adjoint sensitivities

In this article, fast design closure of microwave components using feature-based optimization (FBO) and adjoint sensitivities is discussed. FBO is one of the most recent optimization techniques that exploits a particular structure of the system response to “flatten” the functional landscape handled during the optimization process, which leads to reducing its computational complexity. When combined with gradient-based search involving adjoint sensitivities, the design cost becomes even lower, allowing us to find the optimum design using just a few electromagnetic (EM) simulations of the structure at hand. Here, operation and performance of the algorithm is demonstrated using a waveguide filter and a miniaturized microstrip rat-race coupler (RRC). Comparative studies indicate considerable savings that can be achieved even compared with adjoint-based gradient search. In case of RRC, numerical results are supported by experimental validation.

A broadband circularly polarized cavity-backed Archimedean spiral array antenna for C-band applications

In this letter, a new configuration of sequentially rotated circularly polarized (CP) array antenna with a modified gain is investigated. In order to realize the presented antenna, four Archimedean spiral radiators are applied on a substrate, backed by a circularly slotted ground plane. Also a combination of 180° rat-race coupler and 90° branch-line couplers prepare the employed feeding network in this study. Furthermore, cavity-backed structure is utilized to enhance the antenna gain and achieve a directional radiation pattern over the operating frequency band. This configuration also extends the return loss (RL) and the axial ratio (AR) bandwidths, simultaneously. According to the experimental results, the 2×2 CP array antenna has RL bandwidth of 64.28%, AR bandwidth of 54.28%, and peak gain of 10.84dBic. The close correspondence between the measured and the simulated results validates the proposed design.

Compact substrate integrated waveguide mono-pulse antenna array

A compact monopulse antenna array based on substrate integrated waveguide technology is presented through this article. The design is fabricated on Printed Circuit Board (PCB) technology consisting of a double-layered 8-cell array antenna with a slot in the middle-ground metal used for aperture-coupling excitation and reducing unwanted spurious emissions from feed network. The Impedance bandwidth and AR bandwidth are enhanced due to optimal feed network, including Rat-Race coupler to generate sum and difference patterns for mono-pulse applications operating at 10 GHz. The prototype of the proposed antenna with the size of 124*25 mm2 is fabricated and tested. Measured results compared very well to simulation results obtained by CST microwave studio and show −10-dB impedance bandwidth of 4% and −22 dB null-depth in difference mode.

Compact reconfigurable rat-race coupler with tunable frequency and tunable power dividing ratio

A compact reconfigurable rat-race coupler with tunable frequency and tunable power dividing ratio is proposed for the first time. Varactors and two single control voltages are used to obtain both the tunable frequency and the tunable power dividing ratio in this article. The structure of the rat-race coupler involves 50 Ω parallel-strip lines only and a phase inverter is used for size reduction. Theoretical equations for the relationship among S-parameters and the capacitance of varactors are derived. The graphic method is used to choose capacitance for the desired operation frequency and the desired power dividing ratio. For demonstration, a prototype is designed and fabricated. The measured results show that the rat-race coupler's frequency and the power dividing ratio can be effectively tuned in 0.69 GHz ∼ 0.81 GHz and 3 dB ∼ 14 dB, respectively with isolation better than 20 dB, phase difference less than 7°and return loss better than 20 dB. The theoretical simulation, electromagnetic simulation, and measured results show good agreement in this design.

High Gain Modified Dual-Band and Dual-Circularly Polarized Array Antenna

A novel design of double-layer dual-band circularly polarized array antennas (DDCPAAs) is presented in this paper. First, a DDCP single antenna is introduced as the array element. This element is composed of four truncated patches on the top layer and one other patch on the bottom layer. Then, with the aim of performance enhancement and extension of the antenna functionalities, 2 × 2 and 4 × 4 DDCPAAs are outlined. In the case of 2 × 2 structure, impedance bandwidth lies within 5.21–5.88 and 9.32–9.48 GHz. As well, it demonstrates CP characteristic within 5.24–5.86 and 9.32–9.46 GHz with gain values between 9 and 10.2 dBic. In the case of 4 × 4 DDCPAA, four 2 × 2 sub-arrays are implanted on the structure. In this configuration, the adjacent patches in neighboring sub-arrays are shared to decrease the elements spacing. Technically speaking, this feature suppresses the grating lobes, effectively. The proposed 4 × 4 DDCPAA exhibits an increased impedance bandwidth in two applicable frequency bands accommodating 5.05–6.01 and 9.16–9.98 GHz. Similarly, it improves the CP characteristics to attain a broader range namely 5.18–5.90 and 9.26–9.52 GHz. Moreover, gain values vary between 12 and 13.5 dBic. In both of DDCPAAs, feed network composed of two 3 dB/90-degree branch line coupler and a 3 dB/180-degree rat-race coupler is utilized. Detailed simulation studies extracted from Ansoft high frequency structure simulator are compared with that of measurement results to explore the efficiency of the proposed design. Results are discussed in depth.

Dual-Mode Branch-Line/Rat-Race Coupler Using Composite Right-/Left-Handed Lines

A novel dual-operational mode directional coupler, functioning as a branch-line coupler at one frequency but identically to a rat-race coupler in the other one, is proposed and investigated. To fulfill the unusual design, the characteristic impedance and phase response of a conventional composite right-/left-handed line are controlled simultaneously at both operating frequencies. Analytical design equations are derived and verified by experimental results. The dual-mode coupler shows typical responses of a 90° and 180° coupler, respectively, in the first and second (1.5/2.5 GHz) operating bands. The fractional bandwidths are 8% and 16%.

A Slow-Wave Rat-Race Coupler Using Substrate Integrated Suspended Line Technology

This paper presents a slow-wave rat-race coupler using substrate integrated suspended line (SISL) technology. Double-sided interconnected strip line (DSISL) is utilized to further reduce the loss of the suspended line circuit. Using the DSISL concept, a slow-wave structure based on modified meander line configuration is proposed. Formed by simple circuit topology with only three parameters of one unit cell, the characteristic impedance and slow-wave factor of the proposed slow-wave DSISL can be controlled separately. Utilizing the proposed slow-wave DSISL, a rat-race coupler at 1.5 GHz has been designed and fabricated by standard printed circuit board process. A 90.5% size reduction is achieved, and the fractional bandwidth is 26.5%. Benefiting from SISL technology, the proposed slow-wave rat-race coupler is self-packaged and has advantages of low cost and light weight.

A 280-GHz 10-dBm Signal Source Based on InP HBT Technology

A 280-GHz high-power signal source has been developed in this work based on a 250-nm InP heterojunction bipolar transistor (HBT) technology. The fabricated signal source is composed of two in-phase locked common-base cross-coupled oscillators, the output of which is on-chip combined by a pair of rat-race couplers and a Wilkinson power combiner for enhanced output power. The developed signal source exhibits an oscillation frequency of 276.4 GHz and a phase noise of–89 dBc/Hz at 1 MHz offset. The output power of the signal source is measured to be 10 dBm (10 mW), while consuming a dc power of 196 mW (dc-to-RF efficiency of 5.1%).

Antipodal Vivaldi Antenna for Sum and Difference Radiation Patterns With Reduced Grating Lobes

A new design of an E -plane double-element Vivaldi antenna with sum and difference radiation patterns is presented in this letter. The antenna operates in the frequency range of 2.3–15 GHz (ratio of 6.5:1) with the highest grating lobe level of $-$ 10 dB at 15 GHz and competitive gain performance from 6.8 dBi at 2.3 GHz up to 14.7 dBi at 15 GHz. The grating lobes are reduced effectively by decreasing the distance between the two input ports to one wavelength at the highest operating frequency. The active S -parameters of the antenna are improved using modifications of the geometry, including a new method consisting of an increased length of the internal flares, as well as combinations of known methods such as elliptical corrugations on the outer edges, and optimal shaping of inner edges of the antenna. The gain is also improved by the addition of a dielectric lens, which also provides extra space for lengthening the internal flares. The proposed double-element antenna satisfactorily provides the sum and difference radiation patterns when fed by a wide band rat-race coupler. To warrant a near-ideal beam symmetry, the antenna is designed so that the directive radiation pattern occurs when the antenna elements are fed by out-of-phase signals, i.e., when the difference ( $\Delta$ ) port of the coupler is excited.

Compact paper‐substrate rat‐race coupler deploying modified stepped impedance stub and interdigitated slot resonator for wide‐band harmonic suppression

This study presents the design of a flexible compact rat-race coupler operating at 0.72 GHz on a 1 mm thick paper substrate. The ultra-wide stop-band effect caused due to the combination of the modified stepped impedance stub and the interdigitated resonator slot, offers suppression up to the 14th harmonic. The slow-wave effect caused due to the resonator slot leads to 59.6% size reduction compared with the conventional coupler at the design frequency. The signal transmission through the coupler for different substrate deformation steps has been investigated. Stable responses are reported for the same, making the prototype suitable for flexible electronics applications.

A 7.5-GHz uniplanar 180° hybrid coupler on flexible polyimide substrate

A uniplanar hybrid rat-race coupler has been fabricated on a flexible polyimide film for wearable and flexible RF applications. To enhance the reliability and repeatability of its operation in flexible environment, the coupler is designed using an asymmetric coplanar strip (ACPS) structure. Compared to the microstrip line or coplanar waveguide (CPW) structures, the ACPS requires no via holes through the substrate and reduces the number of air bridges. The dimension of the ACPS is determined by both analytic design equations and full electromagnetic simulation. The coupler was fabricated through photolithography and e-beam evaporation processes. The measurement shows that the insertion loss and phase error are less than 6 dB and 7°, respectively, from 7.4 to 7.9 GHz. The port matching and isolation performance are better than 13.4 dB over the same frequency band.

Miniaturised dual‐band rat‐race coupler with harmonic suppression using synthetic transmission line

A dual-band 90° unit module with harmonic suppression (HS) is proposed. This unit module consists of a pair of back-to-back stepped impedance sections and an open stub. The unit module is realised by synthetic transmission lines, and it is successfully implemented into a novel miniature dual-band (2.45/5.8-GHz) rat-race coupler with HS (suppression level ≥16 dB, 6.8–13 GHz). Compared with traditional 1.5λ microstrip ring hybrid, the proposed dual-band rat-race with HS only occupies 13.84% of its area.

Surrogate-assisted multi-objective optimization of compact microwave couplers

This work presents a rigorous methodology for expedited simulation-driven multi-objective design of microwave couplers with compact footprints. The proposed approach is a viable alternative for computationally expensive population-based metaheuristics and exploits a surrogate-assisted point-by-point Pareto set determination scheme that utilizes – for the sake of computational efficiency – space-mapping-corrected equivalent circuit models. The technique is showcased using a complex design example of a compact rat-race coupler, for which a set of nine alternative design solutions is efficiently identified. The latter design solutions illustrate the best possible trade-offs between conflicting design objectives for the structure at hand, that is, its operational bandwidth and the layout area. The overall design cost corresponds to approximately 20 high-fidelity electromagnetic simulations of the miniaturized coupler. Several selected trade-off designs have been manufactured and measured for the purpose of method validation.

Rapid Microwave Design Optimization in Frequency Domain Using Adaptive Response Scaling

In this paper, a novel methodology for cost-efficient microwave design optimization in the frequency domain is proposed. Our technique, referred to as adaptive response scaling (ARS), has been developed for constructing a fast replacement model (surrogate) of the high-fidelity electromagnetic-simulated model of the microwave structure under design using its equivalent circuit (low-fidelity model). The basic principle of ARS is a nonlinear frequency and amplitude response scaling aimed at accommodating the discrepancies between the low- and high-fidelity models at the reference design and, subsequently, at tracking the low-fidelity model changes that occur during the optimization run. The surrogate model prediction is obtained by applying appropriately composed scaling functions to the high-fidelity model at the reference design. ARS is a parameterless and simple-to-implement method that can be applied to a wide range of microwave structures. The ARS surrogate features excellent generalization capability that translates into improved reliability and reduced design cost. It is demonstrated using an eighth-order microstrip bandpass filter and a miniaturized rat-race coupler. Comparison with several space mapping algorithms is provided. The numerical results are supplemented by measurements of the fabricated optimum designs of the considered structures.

Highly compact wideband double-section rat-race hybrid with harmonic suppression using series and shunt stepped impedance transmission lines

This paper presents the design of a size miniaturized and harmonic suppressed wideband double-section rat-race coupler (RRC). Series and shunt stepped-impedance transmission line units are proposed to replace the quarter wavelength transmission lines in the conventional design of wideband (50% fractional bandwidth) two-stage rat-race coupler. Design equations are derived using the lossless transmission line model. The proposed double-section RRC occupies only 14% area of the conventional coupler, with good return loss and isolation performances. In addition, the full-wave simulation and measured responses exhibit harmonic suppression up to at least fifth harmonic. The proposed design can be easily implemented using the standard printed circuit board etching process without any via-holes, bonding wires, and lumped elements, which make it very useful for wireless communication systems

Arbitrary Power Division Ratio Rat-Race Coupler With Negative Group Delay Characteristics

In this letter, we present theoretical and experimental investigations of a rat-race coupler with negative group delay (NGD) characteristics and an arbitrary power division ratio. From the theoretical analysis, the NGD characteristics can be obtained through various transmission paths and have small variations with the power division ratio. The power division ratio is controlled by only the characteristic impedance of the transmission lines. Ideal port isolation and return loss characteristics are obtained at a center frequency for any arbitrary power division ratio. For experimental demonstration, a microstrip line rat-race coupler is implemented with center frequency of 2.14 GHz. The measurement results agree well with simulation results and theoretically predicated values.

Quad-Band Rat-Race Coupler With Suppression of Spurious Pass-Bands

In this letter, a quad-band rat-race (RR) coupler with spurious pass-band suppression is presented. A new design method of laterally offset dual ring (LODR) resonator backed with concentric dual split ring (CDSR) slots has been deployed. The transmission line model of the laterally offset dual-ring resonator has been characterized and presented. The prototype offers quad-band response at 1.45, 3.8, 4.3, and 6.3 GHz, corresponding to International Maritime Satellite Organisation (INMARSAT), Satellite C-band downlink, aeronautical radio navigation, and Satellite C-band uplink applications respectively. It offers 180° coupler characteristics at each of the four design frequencies and also provides 30.4% size reduction compared to a conventional RR coupler operating at the lowest band. This RR coupler suppresses spurious pass-bands beyond the fourth band. The proposed model has been fabricated and tested to validate the performance.

Analysis and Design of a Planar Rat-Race Coupler with Arbitrary Power Division Ratio and nth Harmonics Suppression

ABSTRACTIn this article, a planar, unequal rat-race hybrid with harmonic suppression is presented. It is constructed by replacing the quarter-wavelength transmission lines of unequal rat-race coupler with their equivalent T-shaped transmission lines. Compared with the conventional coupler, the proposed structure can achieve arbitrary power division ratios, deep harmonic suppression, and size reduction. For verification, a prototype operating at 1 GHz with third-order harmonic suppression is designed, simulated, and measured. The validity of this design has been proved by the consistency of the simulation and measurement results. Moreover, the performance variations with the different parameters are mainly simulated and analyzed.