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.     

Harmonic suppressed compact wideband branch‐line coupler using unequal length open‐stub units

In this article, the design of a broadband branch‐line coupler (BLC) with reduced size and suppressed harmonic passband response is presented. The proposed approach can be used to replace the low impedance λ/4 lines of the conventional BLC by an equivalent structure almost λ/12 in length. The main advantage of the proposed BLC is that, it has approximately the same bandwidth as that of a conventional BLC. A prototype broadband coupler having fractional bandwidth >50% at 1.1 GHz and of size less than one third of a conventional three‐section wideband BLC topology is realized. In addition, at least 20 dB suppression of up to fourth harmonics is achieved. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

A dual‐band branch line coupler for LTE 0.7 GHz and LTE 2.6 GHz frequencies

In this paper, a dual‐band branch line coupler (BLC) for Long Term Evolution (LTE) 0.7 GHz and LTE 2.6 GHz frequencies is designed and developed. A dual‐band quarter wave transmission line (DBQWTL) capable to perform dual band operation for frequency ratio >3 is also proposed. The dual band BLC is designed by replacing the quarter wave transmission line of the conventional single band BLC with the proposed DBQWTL. By means of even and odd mode analysis, the closed form design equations of the proposed DBQWTL are obtained. Considering the implementation viewpoint of the proposed BLC, the circuit parameter analysis is carried out. The proposed BLC performs dual band operation with maximum amplitude imbalance of 0.26 dB and phase deviation of 3.07°. It is found in the comparative analysis that the proposed BLC has novelty in terms of its operating frequency ratio range.     

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.     

A phase tunable hybrid coupler with enhanced bandwidth

The majority of the previous works on tunable coupler concentrates on the tunability of the frequency and amplitude. The capability to control the phase characteristics draws little attention. However, the tunability in phase becomes more and more important, as the flexible control in phase affects the performance significantly in the modern wireless communication systems. Regarding this, several phase tunable couplers were proposed but with narrow bandwidth. In this article, a novel phase‐tunable coupler with enhanced bandwidth is proposed. It is constructed by introducing two varactor loaded coupled line sections between two branch line sections. By varying the capacitances loaded on and between the coupled lines, the continuously tunable phase difference can be achieved between two output ports without affecting the equal power division characteristics. The analysis of the ideal model and detailed circuit has been conducted to obtain the design formulas and guidelines. For demonstration, a phase tunable coupler operating at 1.8 GHz was designed, fabricated, and measured. It exhibits a tunable phase difference from 60° to 120° by varying the two biasing voltages. Meanwhile, the equal power division, good return loss, and high isolation are still maintained. The desired characteristics have been implemented over a wide bandwidth from 1.6 GHz to 2.0 GHz.     

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.     

A novel dual‐band ring coupler based on dual‐band phase inverter

A novel dual‐band ring coupler based on dual‐band phase inverter is proposed. And two types of dual‐band phase inverters (Type I and Type II) are designed in this article. The design method of dual‐band ring coupler is simpler than the traditional ways like replace the single‐band λ/4 transmission line with dual‐band λ/4 transmission line. Its main idea is replacing the wide‐band phase inverter with dual‐band phase inverter. Two dual‐band ring couplers (0.9/2.88 and 0.9/2.43 GHz) using the two types of dual‐band phase inverter, respectively, are simulated and measured. The measured results validate the proposed method.     

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.     

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.     

Frequency tunable circularly polarized antenna with branch line coupler feed network for wireless applications

In this article, frequency tuning and circularly polarized concentric circular microstrip antenna is investigated. The proposed antenna consist of varactor diode for frequency tuning and branch line coupler (BLC) feed network to achieve the circular polarization (RHCP/LHCP). By changing the varactor diode capacitance between 12.33 pF (0 V) to 1.30 pF (15 V) attain the frequency tuning (2.34‐2.68 GHz). The right hand circular polarization (RHCP) and left hand circular polarizations (LHCP) are realized in the antenna through BLC feed network output ports. The impedance bandwidth (2.05‐3.13 GHz) of BLC feed network is well‐matched with the circular microstrip antenna frequency tunable bandwidth. The proposed antenna is fabricated, and simulated results are verified using the mathematical modeling and experimental verification.     

Synthesis and design of tunable bandpass filters with constant absolute bandwidth using varactor‐loaded microstrip resonator

This article presents two new types of tunable filters with constant absolute bandwidth using varactor‐loaded microstrip resonators. First, the second‐ and third‐order Butterworth tunable filters are designed based on the parallel coupled‐line J inverters. Second, a fourth‐order Chebyshev tunable filter is designed based on the alternative J/K inverters, in this design, two adjacent resonators are coupled with each other through a short‐circuited transmission line as the K inverter. The proposed two topologies can be easily extended to high‐order tunable filter. Three tunable bandpass filters with J and alternative J/K inverters, respectively, are built with a tuning range from ～1.8 to ～2.3 GHz. The measured second‐order filter has a 3‐dB bandwidth of 160 ± 6 MHz and an insertion loss of 2.4–3.8 dB. The third‐order filter shows a 3‐dB bandwidth of 197 ± 5 MHz and an insertion loss of 3.8–4.8 dB. The fourth‐order filter shows a 3‐dB bandwidth of 440 ± 5 MHz and an insertion loss of 2.1–2.6 dB. For all the designed filters, the measured results are found in excellent agreement with the predicted and simulated results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:681–689, 2014.     

Complementary meander‐line‐inspired dielectric resonator multiple‐input‐multiple‐output antenna for dual‐band applications

The communication presents a simple dielectric resonator (DR) multiple‐input‐multiple‐output (MIMO) dual‐band antenna. It utilizes two “I”‐shaped DR elements to construct an “I”‐shaped DR array antenna (IDRAA) for MIMO applications. The ground plane of the antenna is defected by two spiral complementary meander lines and two circular ground slots. In the configuration, two “I”‐shaped DR elements are placed with a separation of 0.098λ. The antenna covers dual‐band frequency spectra from 3.46 to 5.37 GHz (43.26%) and from 5.89 to 6.49 GHz (9.7%). It ensures the C‐band downlink (3.7‐4.2 GHz), uplink (5.925‐6.425 GHz), and WiMAX (5.15‐5.35 GHz) frequency bands. Each DR element is excited with a 50‐Ω microstrip line feed with aperture‐coupling mechanism. The antenna offers very high port isolation of around 18.5 and 20 dB in the lower band and upper band, respectively. The proposed structure is suitable to operate in the MIMO system because of its very nominal envelope correlation coefficient (<0.015) and high diversity gain (>9.8). The MIMO antenna provides very good mean effective gain value (±0.35 dB) and low channel capacity loss (<0.35 bit/s/Hz) throughout the entire operating bands. Simulated and measured results are in good agreement and they approve the suitability of the proposed IDRAA for C‐band uplink and downlink applications and WiMAX band applications.     

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.     

A self‐matched negative group delay circuit with tunable center frequency and group delay

A compact self‐matched negative group delay circuit (NGDC) with tunable center frequency and tunable group delay (GD) is proposed. Two varactors and a variable resistor are utilized to implement the tunability of the center frequency and GD of the proposed NGDC. To verify the design concept, a tunable NGDC is designed and fabricated. The measured center frequency is tuned from 0.8 to 1.3 GHz and the measured NGD time is tuned from ?1 to ?10 ns. The insertion loss varies from 16.0 to 34.5 dB. In the process of tuning the center frequency and NGD time, the return losses keep better than 30 dB.     

A miniature coupled‐line‐based 3‐dB directional coupler using GaAs PHEMT process

A novel complementary‐conducting‐strip (CCS) coupled‐line (CL) design is proposed to achieve compact size by applying two‐dimensional layout and standard gallium‐arsenide (GaAs) thin‐film technology. To obtain high coupling and satisfy the design rules of GaAs process, mixed‐couple mechanism with edge and broadside coupling are also used. A CCS CL‐based Ka‐band 3‐dB directional coupler is fabricated using WIN 0.15‐μm GaAs pseudomorphic high electron mobility transistor technology. Experimental results show that the proposed directional coupler can cover the entire Ka‐band (26–40 GHz) with through and coupling of approximately 3.7 ± 0.25 dB, and isolation of better than 13 dB. In addition, the phase difference between the two output ports is approximately 90° ± 5°. The occupied area of the prototype (without I/O networks) is only 220 × 220 μm². © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:21–26, 2016.     

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.     

Millimeter‐wave wideband circularly polarized monopulse antenna using the sequential rotation feeding technique

This article presents the design and implementation of a single‐layer wideband millimeter‐wave circularly polarized (CP) monopulse cavity‐backed antenna based on substrate integrated waveguide (SIW) technology. The antenna consists of a 2× 8 array of CP cavity‐backed antenna elements, a 90° 3‐dB coupler, power dividers, and phase shifters. In order to enhance the operating bandwidths, the sequential rotation feeding technology is adopted in the design of the monopulse antenna. To validate the proposed concept, a prototype operating at 42 GHz was fabricated and measured. The measured 3‐dB axial ratio (AR) bandwidth for the sum beam can cover a frequency range from 37 to 46 GHz. The measured gain for the sum beam at the center frequency of 42 GHz is 17.5 dBiC, while the null‐depth of the difference beam is measured to be ?36.8 dB. The proposed monopulse antenna has advantages of low‐cost, easy‐fabrication, and easy integration with planar circuits.     

Frequency reconfigurable RF circuits using photoconducting switches

Designs for a frequency switchable dual‐band branch‐line coupler and a reconfigurable S‐band power amplifier input matching network with photoconducting switches are presented. Frequency switching is achieved by increasing the power of the laser applied to the highly resistive silicon wafer and changing the properties of silicon under optical illumination. The advantages of this approach are high‐speed switching, electromagnetic transparency (no interference), and thermal and electrical isolation between the device and the control circuit. A branch‐line coupler frequency shift of 35% and 10% has been achieved from all switches off to all switches on in lower (900 MHz) and upper (1800 MHz) frequency bands, respectively. Frequency switchable class AB power amplifier with silicon switch in the input matching circuit has obtained the frequency tuning range of 2.5–3.5 GHz with no significant loss in efficiency and linearity. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

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.     

A low‐profile quad‐port UWB MIMO antenna using defected ground structure with dual notch‐band behavior

A compact (45 × 45 × 1.6 mm³) ultrawide‐band (UWB), multiple‐input multiple‐output (MIMO) design using microstrip line feeding is presented. The proposed design comprises four elliptical monopoles placed orthogonally on a cost‐effective FR‐4 substrate. In order to improve the impedance bandwidth and lessen the return loss of the MIMO antenna, defects in ground plane are created by etching symmetrical square slots and half‐rings. Moreover, a different method (of unsymmetrical H‐shaped slot with C‐shaped slot) was proposed into the patch to introduce dual‐band rejection performance from UWB at center frequency 5.5 GHz (covering lower WLAN as well as upper WLAN) and 7.5 GHz (X band). In addition, a stub is introduced at the edge of each defected ground structure to obtain isolation >–22 dB covering entire performing band from 2 to 16.8 GHz (where, S₁₁ < –10 dB). The proposed design has miniaturized size, very low envelop correlation coefficient less than 0.1, stable gain (2‐4 dBi except for notch bands). Furthermore, various MIMO performance parameters are within their specifications, such as diversity gain (= 10 dB), total active reflection coefficient (<–5 dB, and channel capacity loss (<0.35 bits/s/Hz). The presented design is optimized using the HFSS software, and fabricated design is tested using vector network analyzer. The experimental results are in good agreement with the simulation results.