Microstrip Directional Coupler Research Articles

A directional coupler with suppressed Tx leakage for UHF RFID reader applications

A directional coupler with a high isolation characteristic for radio frequency identification reader systems is presented. Compensation capacitors are used to improve the inherently poor isolation of a microstrip directional coupler due to unequal even- and odd-mode phase velocities. The transmitter leakage signal, which is reflected by the mismatch of the antenna impedance, is substantially reduced by optimization of the compensation capacitor values and the value of an additional resistor attached to the coupling port. The proposed directional coupler exhibits an isolation of 76.8 dB, a coupling factor of 16.6 dB, and an insertion loss of 0.28 dB for a matched antenna impedance at 910 MHz. It also has an isolation of more than 60 dB for mismatched antenna load impedances corresponding to a reflection coefficient of 0.33 at 910 MHz.

High directivity DGS-based coupler

This paper reports a new design of microstrip directional coupler with high directivity. This directional coupler uses corrugated coupled lines and floating conductor in the ground plane of microstrip to enhance coupling. Based on this structure, directional coupler having 4.4% bandwidth has been designed at 4500 MHz with 10 dB coupling and 35 dB directivity. The designed directional coupler has been fabricated and tested for the validity of the design. Measured results are presented in this paper.

High‐performance microstrip directional coupler for radio‐astronomical receivers at cryogenic temperature

A microstrip directional coupler with high directivity is proposed for microwave applications requiring a weak coupling factor. This component has been used in the L-band radio-astronomical receiver of the Sardinia radio telescope. To improve the performance of the receiver, the operating temperature is lowered to 20 K. An in-house cryostat has been manufactured to perform measurements on the directional coupler, and very good agreement with the design specification is achieved.

Optically Controlled Coupled Microstripline Microwave Power Attenuator

This paper presents an optically controlled coupled microstripline microwave powerattenuator on a high resistivity silicon substrate in the 3GHz-7GHz frequency band. The structure is based on the10dB microstrip directional coupler. Attenuation control is performed by optical control of the open end resistances. The open end act as a variable resistance under sufficient optical illumination conditions. The attenuator has been designed for 10dB and 25dB attenuation control for one-port and two port optical illuminations respectively. The analysis has been carried out for one port illumination using a 30mW, 850nm laser diode and two ports illumination using two 20mW, 650nm laser diode. The attenuator has been modeled by the Computer Simulation Technique's Micro Wave Studio (CST MWS) 3D EM simulator. At 5GHz, the structure provides a continuous variation of S41 between 0 and 25dB with both open end illumination.

Using Phase Relations in Microstrip Directional Couplers to Achieve High Directivity

We analyze the microstrip directional couplers with a special focus on the phase difference between the coupled and isolated ports. The analysis uses the even-odd mode decomposition technique and network theory. The results show that the phase difference between the signals at the coupled port and the isolated port is close to π in a very wide band. This property can be used to achieve a broadband directivity by inducing a voltage cancellation at the isolated port of the couplers. Two different cancellation methods are investigated. Analytical formulas for the required component values of both methods are given. Higher directivity can be obtained at the expense of reduced bandwidth.

CALCULATION OF MICROSTRIP DIRECTIONAL COUPLERS WITH DISPERSION AND LOSSES IN TRANSMISSION LINES

Simple analytical relations for calculating parameters of microstrip directional couplers are obtained. It is shown that depending on the power splitting ratio k in the output ports of the circuit the technological restrictions of couplers are changed. At k ≥1 for double-stab four-port directional coupler, unlike three- and four-stab couplers, the range of characteristic impedance satisfies the technological restrictions. However, at k <1 three- and four-stab directional couplers become more technological. Analytical formulas for high precision calculation of the dispersion of effective permittivity and losses in the substrate valid for the main microstrip T-wave in a wide frequency range are also presented.

Dual-Polarization Frequency Scanning Microstrip Array Antenna With Low Cross-Polarization for Weather Measurements

The design of a dual-polarization frequency scanning array of microstrip patch antennas is presented. The coupling between the patch elements and the transmission lines is realized using microstrip directional couplers. Cross polarization suppression is achieved by applying a differential feed configuration to each element for the vertical polarization and an imaged feed arrangement with respect to the columns for the horizontal polarization. The antenna has been fabricated and test results are provided verifying the accuracy of the simulated results. In addition, the imaged feed configuration is used for two neighboring columns of the array to compensate for the spurious radiations and achieve a lower cross polarization level.

EDGE coupled variable microfluidic directional coupler

A microstrip directional coupler with a microfluidic superstrate in the coupling gap is investigated.In the microfluidic channel, relative dielectric constant values ranging from εr = 81 to εr = 1, deionized water and air are studied, respectively. The tuning range of the coupler is evaluated experimentally and compared to its full-wave simulation. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:756–758, 2013; View this article online at wileyonlinelibrary.com. DOI: 10.1002/mop.27436

Compact UWB directional coupler with one notch-band

An ultra-wideband (UWB) parallel coupled-line directional coupler (DC) with one notch-band based on folded stepped impedance resonator (SIR) is investigated in this paper. The proposed UWB DC is comprised of one conventional microstrip directional coupler, two folded shunt quarter-wavelength short-circuited stubs and one SIR with metalized via hole to ground. The SIR is studied and employed to generate one desired notched band. The predicted results are compared with measured data and good agreement is reported.

Design of novel microstrip directional coupler for differential signal decoupling

This study describes a concept of a novel microstrip directional coupler for differential signal decoupling, which can be used to digital signal overhearing on printed-circuit-boards. The complete design method is proposed with rules given analytically. Considered methodology is suitable for synthesis of couplers with low coupling factors, which have negligible influence on the transmission in main line. Theoretical considerations are confirmed by numerical tests performed using full-wave simulator for the practical case of the coupler project and measurement results. The authors demonstrate that the accuracy of the design method is sufficient for practical system design both in terms of required coupling factor and influence on the main transmission line.

High-Directivity Enhancement With Passive and Active Bypass Circuit Techniques for GaAs MMIC Microstrip Directional Couplers

This paper describes monolithic microwave integrated circuit microstrip directional couplers based on a new directivity enhancement technique. This technique utilizes a bypass circuit-a phase shifter and an attenuator-placed between the coupling and isolation ports to cancel out backward wave leakage to the coupling port, thereby enhancing directivity of the couplers. The bypass circuit can be implemented with a simple passive LCR circuit or an active heterojunction bipolar transistor (HBT) phase-inversion attenuation circuit. Edge-coupled-type microstrip spiral couplers using the passive and active bypass circuits were fabricated in a GaAs HBT process. Measurements are as follows. The coupler with the passive LCR bypass circuit delivers a -21-dB coupling factor and a 0.14-dB insertion loss (IL) at 2.6 GHz while keeping enhanced directivity of more than 30 dB with a 23.1% relative bandwidth. This corresponds to more than 21-dB improvement of directivity at the same frequency, compared to the coupler without the bypass circuit. For the coupler with the active attenuator, a peak directivity of 46 dB and more than 30-dB directivity with a 120% relative bandwidth are achieved while a -21-dB coupling factor and a 0.13-dB IL are delivered at 2 GHz. Additional measurements of HBT power detectors integrated with the couplers show that the couplers with passive and active bypass circuits can suppress detection errors of less than ±0.20 and ±0.10 dB, respectively, under 4:1 voltage standing-wave ratio load mismatching conditions, thus proving the effectiveness of the bypass techniques.

A new approach to analytical calculation of microstrip directional couplers with full structure symmetry

A new approach to analytical calculation of directional couplers that have two symmetry planes, which is based on using input impedances of co-phase (symmetrical) and opposite-phase (asymmetrical) partial two-poles is suggested. Examples of using such approach for calculations of directional couplers on the basis of a coupled lines segment with three directivity types are discussed.

Size‐reduced defected ground microstrip directional coupler

AbstractIn this article, size reduction and harmonic suppression in coupled‐lines microstrip directional couplers is attained using dumbbell, and reshaped dumbbell slots, as defected patterns, specifically placed under the metering position in the ground plane. Quantitative investigation of the performance of the directional coupler as controlled by the variations of the DGS rectangular slots height, DGS gap length, and the DGS reshaped rectangular slot height is presented. This concept is further illustrated experimentally for a 12 dB coupler realized on RT/Duroid 5880 substrate. The proposed DGS design confers size reduction because of the shift of the operating frequency to lower values. The measurements are in a good agreement with the simulated results and emphasize the size reduction and the harmonics suppression of the DGS coupled‐lines microstrip directional coupler. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1933–1937, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25410

A Design Method for Microstrip Directional Couplers Loaded With Shunt Inductors for Directivity Enhancement

An accurate design method is proposed for directivity enhancement of microstrip directional couplers loaded with shunt inductors. The parasitic effects of junction discontinuities in various parts of such microstrip directional couplers have critical effects especially on the directivity, and therefore they must be taken into account. Without proper modeling of these parasitic effects, directivity enhancement becomes extremely difficult especially for weak coupling levels. The demonstrated method of analysis can be applied to obtain exact designs of all previous microstrip directional couplers that are loaded symmetrically with series and/or shunt reactance for directivity enhancement, regardless of the coupling levels. Based on the proposed method, a 20-dB microstrip directional coupler is designed at 2.4 GHz. A maximum directivity of 56 dB has been measured, which is an improvement of 48 dB over a conventional microstrip directional coupler. A 16.3% bandwidth at 2.4 GHz has been measured in which the directivity remains above 20 dB, while the maximum variation in the coupling level is 0.5 dB. This is the first work to demonstrate directivity of more than 50 dB for a 20-dB microstrip directional coupler.

An Inductor-Loaded Microstrip Directional Coupler for Directivity Enhancement

A directivity-enhancement technique is demonstrated for a microstrip directional coupler. By loading the coupler with shunt inductors, the phase velocities of the even and odd modes are altered in such a way that an isolation null is achieved. Compared with other compensation techniques for directivity enhancements, the proposed method provides the advantages of being simple yet easily application to high frequencies. For a 10 dB directional coupler at 0.9 GHz, a maximum directivity improvement of nearly 38 dB is measured over the conventional counterpart.

Symmetrical Coupled Microstrip Lines With Epsilon Negative Metamaterial Loading

In this paper, we show how metallic plate inclusions can be used to enhance the coupling values of microstrip directional couplers. The analysis and the design of symmetrical coupled microstrip lines with high values of forward-wave coupling with a coupling length very small are presented. A quasi-static circuit model of the structure is developed and physical insights on the operation of the proposed component and on the role of Epsilon NeGative (ENG) material slab loading are also given. Full wave numerical simulations are shown for different geometries.

The Complete Design of Microstrip Directional Couplers Using the Synthesis Technique

A symmetrical microstrip directional coupler design using the synthesis technique without prior knowledge of the physical geometry of the directional coupler is analytically given. The introduced design method requires only the information of the port impedances, the coupling level, and the operational frequency. The analytical results are first validated by using a planar electromagnetic simulation tool and then experimentally verified. The error between the experimental and analytical results is found to be within 3% for the worst case. The design charts that give all the physical dimensions, including the length of the directional coupler versus frequency and different coupling levels, are given for alumina, Teflon, RO4003, FR4, and RF-60, which are widely used in microwave applications. The complete design of symmetrical two-line microstrip directional couplers can be obtained for the first time using our results in this paper.

A new directional coupler for UHF RFID reader

AbstractA conventional microstrip directional coupler has poor isolation due to its unequal phase velocity between the even and odd modes. In this article, a compact dual‐stub microstrip directional coupler for the UHF RFID reader is proposed. The operating frequency band is 902–928 MHz. It is found that the coupler can obtain a good isolation of −65 dB at 920 MHz. Modeling and simulation results are also presented which are in good agreement with measurement data. © Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1973–1975, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23495

Aperture‐coupled cavity‐fed back‐to‐back microstrip directional coupler

AbstractAn analysis and design theory of an aperture‐coupled cavity‐fed back‐to‐back microstrip directional coupler is presented for efficient and optimized design. For this purpose, an equivalent network is developed, and a simple yet accurate circuit element value calculation is described. Design equations of the coupler are derived based on the equivalent circuit. To determine various structural design parameters, the evolutionary hybrid optimization method based on a genetic algorithm and the Nelder–Mead method is used. As a validation check of the proposed theory and optimized design method, a 10 dB directional coupler was designed and fabricated. The measured coupling was 10.3 dB at 3 GHz, and the return loss and isolation were 31.8 dB and 30.5 dB, respectively. The directional coupler also showed very good quadrature phase characteristics. Good agreements between the measured and the design specifications fully validate the proposed network analysis and design procedure. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 551–555, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23145

Microstrip directional coupler with flat coupling and high isolation

Based on the open-circuited slotline to realise capacitive compensation, a microstrip directional coupler with high isolation is investigated. Meanwhile, by adding external matching networks at the input/output ports, bandwidth is enhanced and a flat coupling is obtained. Centred at 2.4 GHz, a 3 dB directional coupler is designed, fabricated and examined. Both simulated results and measured data are presented, and good agreement between them is observed.