Bandwidth improvement of rat‐race couplers having left‐handed transmission‐line sections

A novel broadband rat‐race coupler has been investigated. The coupler utilizes an artificial left‐handed transmission line section for broadband phase response realization. Moreover, a narrowband model of left‐handed section has been shown to prove the couplers equivalent circuit at the center frequency. To broaden the operational bandwidth multisection quarter‐wave transformers have been proposed. The exemplary rat‐race coupler with two‐section impedance transformers has been designed and manufactured. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:341–347, 2014.     

Modeling and performance improvement of folded coupled lines in miniaturized Quasi‐lumped directional couplers

An efficient modeling method for folded coupled inductors for application in quasi‐lumped directional couplers designed in both symmetric and asymmetric structures has been proposed. The presented model takes into account the deteriorative effects which occur when coupled inductors realized as electrically short coupled‐lines matched to the high impedance standard are folded into, for example, meander or spiral pattern. The deteriorative influence of coupled‐lines' folding on the performance of the resulting directional coupler has been modeled as additional lumped inductors and capacitors on the schematic diagram of a quasi‐lumped directional coupler's subsection. Moreover, it has been shown that this deteriorative influence can be substantially minimized when values of lumped elements constituting the directional coupler are appropriately changed. The proposed design procedure has been experimentally verified by measurements of two 3‐dB single‐section directional couplers designed in symmetric and asymmetric structures as quasi‐lumped couplers with folded coupled inductors. The measurement results show a good agreement with both circuit and electromagnetic analysis which proves the correctness and usefulness of the presented methods. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:1–9, 2015.     

Miniaturized wide‐band rat‐race coupler

New designs of wide‐band rat‐race couplers are proposed. The wide‐band operation is achieved with the use of the microstrip nonuniform transmission line sections for the branches of the conventional rat‐race coupler. The design formulas are developed using ABCD matrix and the even‐ and odd‐mode analysis. The theoretical analysis has been verified by measurements of the two manufactured wideband rat‐race couplers, one operate within 0.85–1.92 GHz and other within 1.55–3.55 GHz frequency range with the equal normalized characteristic impedance functions. For both fabricated couplers, the isolation parameter is better than 15 dB over a 77% relative bandwidth. Also, it is shown that the designed wide‐band rat‐race coupler can be realized in higher frequency bands with the fixed fractional bandwidth. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 23: 675–681, 2013.     

Design method for quasi‐optimal multiband branch‐line couplers

In this article, the design approach, the implementation, and experimental results of multiband branch‐line couplers operating at arbitrary frequencies are presented. The conventional branch‐line coupler structure is adapted to multiband operation by shunting its four ports with multiband reactive networks. The performance of the proposed multiband couplers is theoretically analyzed and optimized through the even‐odd mode circuit analysis. Dual‐band (2.4–3.5 GHz), triple‐band (1.5–2.4–4.2 GHz), and quad‐band (1.5–2.4–3.5 GHz) microstrip branch‐line couplers have been realized and tested to verify the design method. The good experimental results (input return loss greater than 15 dB and amplitude imbalance lower than 0.7 dB) show excellent agreement with theoretical and simulated ones, thus validating the proposed approach. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:117–129, 2014.     

Broadband magic‐Ts with the use of coupled‐line directional couplers and left‐handed transmission line sections

The problem of broadband magic‐T realization with the use of coupled‐line directional couplers and left‐handed (LH) transmission‐line sections has been comprehensively investigated. Broadband amplitude characteristics of the proposed networks are ensured with the use of coupled‐line couplers, whereas the required phase characteristics have been achieved by the appropriate selection of right‐ and left‐handed transmission line sections. To analyze the properties of the proposed networks, a model of an ideal left‐handed transmission line has been utilized. The presented concept has been verified by two designs of broadband magic‐Ts operating in 2.5–3.5 GHz and 0.8–2.3 GHz, respectively. The obtained results proved that the proposed magic‐T networks allows one to obtain broadband amplitude and phase responses together with its compact structure. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:513–521, 2014.     

Wideband 3‐dB broadside‐coupled CPW coupler with ACPW and DGS for directivity enhancement

Phase velocity compensation technologies using asymmetric coplanar waveguides (ACPW) with or without defected ground structures (DGS) are proposed to enhance the directivity of wideband 3‐dB broadside‐coupled coplanar waveguide (BC CPW) couplers. First, even‐ and odd‐mode analyses of BC CPW couplers with four shunt capacitors are performed to obtain closed‐form design formulas. Then, ACPW instead of the shunt capacitors are inserted to the input and output ports of the coupler to improve the directivity. Lastly, DGS combining with ACPW are applied to further improve the directivity and broaden the bandwidth. The measured results show that only using ACPW, the bandwidth for the directivity of better than 20 dB is 44.2% with a maximum directivity of 42.6 dB. When both ACPW and DGS are used, the bandwidth for the directivity of better than 20 dB is 63.3%, and that for the directivity of better than 30 dB is enhanced to 21.3% with a maximum directivity of 55.9 dB, which is more than 10 dB over the existing BC CPW couplers. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:59–67, 2014.     

Design of compact rat‐race couplers with enhanced bandwidth

This article presents a method to design compact rat race couplers with improved bandwidth values. The coupler consists of three coupled‐line sections of different electrical lengths and characteristic impedances. First, design equations are obtained by imposing the coupler conditions using a lossless transmission line model. Input impedance matching, isolation, phase, and amplitude imbalances, all four conditions for both the sum and the difference port excitations are considered for bandwidth calculations. Then, an algorithm is developed to solve for the coupled‐line parameters. Considering the limitations of fabrication, guidelines are provided for selecting the right physical parameters according to bandwidth requirement. As an example, a rat race coupler is fabricated that occupies 10% area of a conventional coupler without compromising the bandwidth values. Measurement results shows that the coupler provides 50% of 15 dB return loss bandwidth, 41.7% of 20 dB isolation bandwidth, 15% of ±5° phase imbalance bandwidth, and 62.5% of ±0.5 dB amplitude imbalance bandwidth which are more than those of a conventional 3λ/2 rat race coupler.     

Via‐hole coupled oversized microstrip line and its band‐pass filter application

A simple via‐hole coupled oversized microstrip line filter is proposed and demonstrated in this article. The via‐hole in this case works as an inductor coupling structure whereas the oversized microstrip line resonator has a higher Q‐factor than its conventional counterpart. Full‐wave‐based circuit models of a series of via‐holes embedded in the oversized microstrip line are extracted by using our proposed numerical calibration technique combined with a commercial method‐of‐moments simulator. A simple 3‐pole via‐hole coupled oversized microstrip line filter is designed and fabricated on the basis of the extracted circuit models of via‐holes. Measured results show that the demonstrated filter has a center frequency of 1.853 GHz, a bandwidth of 6.98% and an insertion loss of 1.36 dB. Measured results of the fabricated filter sample are in agreement with its simulated results, showing a good performance of the proposed scheme. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008.     

Design of X‐band Moreno cross‐guide coupler based on superformula curves

This article deals with the design of Moreno cross‐guide couplers based on supershapes for X‐band applications. Crossed‐waveguide couplers are mainly used due to their compact structures. In these couplers, cross‐aperture structures are usually employed to offer flat coupling and high isolation. In the present article, the possible shapes for apertures and metal inserts that can be derived by the superfomula curves are explored and the effects of variations of superformula parameters are investigated on the performance of Moreno coupler. Finally, the proposed Moreno coupler is validated through fabrication and measurement. The experimental validation shows an excellent agreement with the simulated results. In the frequency range from 8 to 12.5 GHz, the measured coupling value changes from 18.8 to 20.8 dB and the directivity is better than 38 dB and 29 dB from 8 to 11 GHz and 11 to 12.5 GHz, respectively. The results are valuable for the design and evaluation of broadband high directive waveguide couplers.     

Surrogate‐assisted EM‐driven miniaturization of wideband microwave couplers by means of co‐simulation low‐fidelity models

This article proposes a methodology for rapid design optimization of miniaturized wideband couplers. More specifically, a class of circuits is considered, in which conventional transmission lines are replaced by their abbreviated counterparts referred to as slow‐wave compact cells. Our focus is on explicit reduction of the structure size as well as on reducing the CPU cost of the design process. For the sake of computational feasibility, a surrogate‐based optimization paradigm involving a co‐simulation low‐fidelity model is used. The latter is a fundamental component of the proposed technique. The low‐fidelity model represents cascaded slow‐wave cells replacing the low‐impedance lines of the original coupler circuit. It is implemented in a circuit simulator (here, ADS) and consists of duplicated compact cell EM simulation data as well as circuit theory‐based feeding line models. Our primary optimization routine is a trust‐region‐embedded gradient search algorithm. To further reduce the design cost, the system response Jacobian is estimated at the level of the low‐fidelity model, which is sufficient due to good correlation between the low‐ and high‐fidelity models. The coupler is explicitly optimized for size reduction, whereas electrical performance parameters are controlled using a penalty function approach. The presented methodology is demonstrated through the design of a 1‐GHz wideband microstrip branch‐line coupler. Numerical results are supported by experimental validation of the fabricated coupler prototype.     

Isolation enhanced circularly polarized patch antenna array using modified electric‐field‐coupled resonator

A modified electric‐field‐coupled (MELC) resonator featuring negative permittivity is proposed to enhance the inter‐element isolation of a circularly polarized (CP) patch antenna array operated at Chinese compass navigation satellite system (CNSS) downlink band. The resonator comprises two capacitive gaps and a common inductive strip connected to the ground plane by two metal vias. A suspended microstrip line excitation is employed to efficiently design and investigate the MELC resonator whose constitutive parameters are subsequently extracted. A dual‐element CNSS antenna array has been prototyped and measured. The experimental results demonstrate that under the assistance of the proposed MELC resonator, a mutual coupling reduction of 15 dB has been achieved while maintaining good impedance matching and CP radiation performance. Details of the design considerations along with simulation and measurement results are presented and discussed.     

In‐line stepped ridge coaxial‐to‐rectangular waveguide transition with capacitive coupling

An original in‐line coaxial‐to‐rectangular waveguide transition including stepped ridged sections is presented. This device differs from the state of the art because of the coaxial inner conductor not being in electrical contact with the ridge, leading to considerable mechanical advantages. In addition to this, capacitive coupling can extend the bandwidth even with a few matching sections when compared with the traditional counterpart presenting DC contact with the first ridge. An ultra‐compact transition is designed, manufactured, and tested in back‐to‐back configuration. From measured data, a return loss better than 20 dB on a bandwidth of 1.27 GHz (12.2%) is observed in this configuration. An average insertion loss of 0.11 dB can be derived for the standalone transition.     

Dual‐wideband filtering power divider based on center‐fed three‐line coupled structure

In this article, a dual‐wideband filtering power divider is proposed by using a center‐fed three‐line coupled structure with three open stubs and two isolation resistors. The center‐fed three‐line coupled structure can generate two wide passbands separated by a transmission zero (TZ). The three open stubs can achieve four TZs around the two passbands, which is conducive to the frequency selectivity. Compared with the reported designs, the bandwidth is extended and the performance of isolation, insertion loss and circuit size can reach balance. The proposed design is implemented with size of 0.22 λ_g × 0.39 λ_g (λ_g is the guided wavelength at the center frequency of the lower passband) which exhibits the 3‐dB fractional bandwidths of 56.5%/24.27% and the insertion loss of 0.51/0.68 dB at the center frequency of two passband (f₁/ f₂) of 1.94/4.2 GHz, while the isolation at f₁/f₂ are higher than 22.5/20.1 dB.     

A Q‐band compact high‐performance double‐ridged orthomode transducer

In this article, a Q‐band compact waveguide orthomode transducer (OMT) based on a stepped double‐ridged waveguide is presented. By using a symmetrical structure, good output return losses of above 20 dB are obtained from 35 to 50 GHz. In particular, the measured insertion losses are as low as 0.15 to 0.4 dB over the entire band for both polarization channels with an improved assembly configuration. The measured cross‐polarization levels are well below ?37 dB, while the circular‐polarized output port isolation is less than ?20 dB combined with a feed horn and a differential phase shifter. The developed OMTs have been used in a Q‐band two‐beam cryogenic receiver on the Tianma Radio Telescope, contributing only 1 to 2 K noise at an operating temperature around 20 K.     

Wideband bandpass filtering branch‐line balun with high‐isolation

The wideband bandpass filtering branch‐line balun with high isolation is presented in this paper. The proposed balun can be designed for wideband performances by choosing a proper characteristics impedance of input vertical transmission line and odd‐mode impedance of parallel‐coupled lines. The proposed balun was designed at a center frequency (f₀) of 3.5 GHz for validation. The measured results are in good agreement with the simulations. The measured power divisions are ?3.31 dB and ?3.24 dB at f₀ and ?3 ± 0.17 dB within the bandwidth of 0.95 GHz (3 GHz to 3.95 GHz). The input return loss of 24.09 is measured at f₀ and higher than 20 dB over the same bandwidth. Moreover, the measured output losses are better than 11 dB within a wide bandwidth. The isolation between output ports is 20.32 dB at f₀ and higher than 13.2 dB for a broad bandwidth from 1 GHz to 10 GHz. The phase difference and magnitude imbalance between two output ports are 180° ± 4.5° and ± 0.95 dB, respectively, for the bandwidth of 0.95 GHz.     

Performance of stripline‐type ferrite coupled line circulators

The normal‐mode approach is used to design stripline‐type ferrite coupled line circulators with alumina as a substrate and ferrite as a superstrate. The measurements that are taken clearly demonstrate the nonreciprocal behavior of these devices, with an average insertion loss of 5 dB and an isolation of ?12 dB. It is found that the three‐port circulator has a broader bandwidth (9–14 GHz) than that of the four‐port circulator (9–12 GHz), and the hybrid coupler has been identified as the bottleneck in the latter. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 13: 173–179, 2003.     

Cost‐efficient design methodology for compact rat‐race couplers

In this article, a reliable and low‐cost design methodology for simulation‐driven optimization of miniaturized rat‐race couplers (RRCs) is presented. We exploit a two‐stage design approach, where a composite structure (a basic building block of the RRC structure) is first optimized using a pattern search algorithm, and, subsequently, the entire coupler is tuned by means of surrogate‐based optimization (SBO) procedure. SBO is executed with the underlying low‐fidelity model implemented as cascaded response surface approximations (RSAs) of the composite structure. Full‐wave analysis of the entire coupler is required at the tuning stage only. By combining SBO with coupler decomposition and RSA surrogates, the overall cost of the design process corresponds (in terms of CPU time) to less than three electromagnetic simulations of the compact RRC, and results in highly miniaturized structure (82% footprint reduction compared to conventional coupler) that exhibits perfect return loss and isolation (almost ?60 dB at the operating frequency), as well as a strong harmonic and spurious suppression (below ?20 dB) in, approximately, 3–9.5 GHz frequency band. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:236–242, 2015.     

Rapid surrogate‐assisted design optimization of minimum‐size broadband branch‐line couplers with variable topology

This work discusses simulation‐driven design of miniaturized wideband branch‐line couplers with a variable topology. Size reduction is enabled here by replacing uniform transmission lines of the original coupler with slow‐wave structures in the form of cascaded compact cells and meander lines. The primary goal is to determine a number of cells in the cascade and particular cell dimensions for which the minimum size of the coupler as well as its required operating conditions are ensured. To this end, we employ a surrogate‐assisted technique involving a trust‐region gradient search framework. Computational efficiency of the design process stems from estimating the Jacobian of circuit responses at the level of a low‐fidelity model of the cascade. The latter is composed in a circuit simulator from duplicated EM‐evaluated data blocks of a single cell and is well correlated with the corresponding high‐fidelity model. The key advantage of this work is the utilization of a reconfigurable, cheap, and well‐aligned low‐fidelity model. The proposed approach is demonstrated through design of a minimum‐size two‐section branch‐line coupler with quasi‐periodic dumbbell‐shaped cells and meander lines. Excellent circuit performance as well as its small size showcase the reliability and usefulness of the presented method. Experimental verification is also provided.     

Directional‐coupler‐based microwave sensors for differential angular‐displacement measurement

The novel application of microwave directional couplers to develop angular‐displacement microwave sensors is reported. The proposed sensor approach employs as stator a branch‐line‐type coupler arranged in transversal mode by loading its direct and coupled ports with two distinct‐length open‐ended stubs. Thus, by taking the isolated port of the coupler as the stator output node, a bandpass filtering transfer function with transmission zeros (TZs) is created. Then, a rotor made up of an angularly‐moveable open‐ended stub is attached to a curved section of the longest loading stub of the stator through physical contact, so that their interconnection point varies with the angular‐displacement of the rotor. In this manner, the sensor transfer function is altered with the stub rotation through TZ reallocation, angular‐displacement sensing capabilities are achieved. The theoretical operational foundations of the conceived branch‐line‐coupler‐based microwave angular‐displacement sensor, which features single/multi‐band sensing properties in terms of inter‐TZ spacing and stop band attenuation levels, along with design examples and curves are provided. The extrapolation of this sensor principle to other classes of power‐distribution circuits, such as the rat‐race‐type directional coupler, is also demonstrated. Finally, for experimental‐validation purposes, two 920 MHz microstrip prototypes of the conceived branch‐line‐coupler‐based angular‐displacement microwave‐sensor approach are built and measured.     

Accurate and efficient design technique for wideband substrate integrated waveguide directional couplers

In this article, we present an efficient technique for the accurate design of wideband substrate integrate waveguide directional couplers. By tapering the coupling section, the bandwidth of substrate integrated waveguide (SIW) directional couplers can be enlarged. Two design aspects are involved in this approach. First, the even‐mode propagation constant in the tapered coupling section is accurately extracted with the help of a numerical thru‐reflect‐line calibration technique. Then, it is fitted into the model of a uniform dielectric‐filled rectangular waveguide and thereafter extrapolated to the operation range of the odd mode. Second, equivalent circuit models of the waveguide bifurcation effects are also presented together with parametric values. Based on the results of extraction, a 90° 3‐dB directional coupler is developed to validate the proposed design approach. To achieve the reverse phasing at two output ports, the prototyped 90° 3‐dB directional coupler is subsequently integrated with a novel broadband fixed phase shifter developed with the SIW technology, of which a systematic synthesis procedure has been proposed in this article. Measured performance of both 90° and 180° 3‐dB couplers confirms the accuracy of our proposed design approach. This kind of wide‐band directional coupler can find applications in wideband power dividing/combining circuits within a single‐layer platform. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.