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Background: The urban lifestyle and environment pose a constant immune challenge to city dwellers. A major such challenge is influenza, which creates substantial public health and socio-economic burdens. The global healthcare paradigm has begun emphasizing the importance and cost-effectiveness of self-care in partnership with healthcare professionals such as community pharmacists for the management of mild ailments. For the general public, micronutrient supplementation is an affordable and potentially feasible self-care strategy for immunity enhancement and disease management. At the same time, micronutrient deficiencies are a serious public health concern in both developing and developed areas. Objective: This review focuses on the clinical evidence for the efficacy and safety of three key micronutrients — vitamins C, D and zinc — on respiratory infections. Key findings: These micronutrients are important for optimal immune function through their complementary roles in supporting both innate and adaptive immunity, as well as epithelial barriers. The need to improve public awareness of self-care in prevention and health management is highlighted by recent public health issues and the global fight against antimicrobial resistance. Community pharmacists could play a crucial role in empowering patient autonomy. Conclusion: With this review we aim to offer insights into the supplementation of these micronutrients as a self-care approach to the management of immune health.

Load-variation insensitivity, for impedance matching between power amplifiers (PAs) and transmitting antennas, contributes to challenging the design of millimeter-wave wireless systems. In this paper, a $W$ -band two-way balanced PA based on a compact quadrature coupler with a broadside coupled strip-line (BCSL) as the core is presented to enhance the load-variation insensitivity and stability. The proposed coupler is truly broadband with low amplitude and phase imbalance. The proposed $W$ -band balanced PA achieves higher power-added efficiency (PAE) and unsaturated output power ${P} _{\mathrm{ sat}}$ over wide frequency bandwidth. The $W$ -band balanced PA is implemented in a 0.13- $\mu \text{m}$ SiGe BiCMOS process and achieves a measured ${P} _{\mathrm{ sat}}$ of 16.3 dBm and a peak PAE of 14.1% at 100 GHz (with 1.6-V power supply). The measured ${P} _{\mathrm{ sat}}$ with 1-dB bandwidth is from 91 to 102 GHz. The measured results present the feasibility of the compact quadrature coupler. The total chip surface area (with pads) is 0.64 mm2, where the size of the proposed quadrature coupler area is only 0.04 mm2.

This letter proposes a compact and broadband quadrature coupler with a center frequency of 55 GHz, which consists of a 90° broadside coupled-line to support the differential signal propagation and two T-type L-C networks to support the common signal propagation. To analyze the proposed coupler, an equivalent circuit model is provided for estimation of the distributed and lumped component values. The measured results of the proposed on-chip quadrature coupler show that the return loss and isolation are greater than 20 dB with a bandwidth of 105%, while the insertion loss is about −0.85 dB. The magnitude imbalances are less than 1 dB within the bandwidth of 56% and the phase differences are with ±1° errors within the bandwidth of 96.9%. The chip size, excluding the test pads, is only $0.31\times0.22$ mm2.

We investigated a dielectric transmission line consisting of three dielectric substrate layers for millimeter-wave (mmW) applications. This line is referred to as a narrowed dielectric microstrip line (N-DML). The effective dielectric constant method is used to analyze the propagation characteristics of the N-DML. The result of the analysis of the normalized phase constant is consistent with the simulation. For demonstration, two J -band N-DML prototypes of different lengths with transitions to rectangular waveguides were designed, fabricated, and measured. Good agreement between the simulated and measured results is observed. In particular, due to its nonmetal structure, the N-DML shows a low average attenuation constant of only 0.52 dB/cm over 220-280 GHz in the experiment. Due to its good performance, ease of fabrication, low loss, and low cost, the N-DML is a suitable candidate for various mmW applications.

A study of lumped transformer-based directional couplers is proposed by converting a conventional two-port transformer into a four-port directional coupler. The directional coupler of this kind is a backward coupler that consists of a ferrite-core transformer and four or five associated lumped reactive elements. Four different topologies of the coupler are proposed with four different reactive element arrangements, namely, ring topology, cross topology, H-shaped topology, and twisted topology. Analysis and design equations are presented. Four different circuits with different frequency responses are derived. Four 13.56-MHz couplers with these four different topologies are designed, fabricated, and measured. Wideband, compact, and backward wave coupling features are theoretically and experimentally confirmed.

A multilayer technology is applied to design a dual-band branch-line coupler for the dual-band balanced amplifier. The proposed coupler is designed based on a three-layer printed circuit board. The dual-band operation is achieved by four additional shunt stubs at the middle layer, while the upper layer is the core and active circuits layer and the bottom is ground layer. The proposed coupler with equal power division is designed at two frequencies of 1.1 and 1.9 GHz. Besides, a balanced amplifier is also designed which achieves gain of 11.6 and 10.1 dB at 1.1 and 1.9 GHz, respectively. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1680–1683, 2014

A wideband 90° hybrid coupler has been presented and implemented in planar microstrip circuit. With similar structure of conversional 2-section branch-line coupler, the proposed coupler consists of a lumped high-pass network but not the quarter wavelength transmission at the center. The values of all lumped elements were optimized to replace a quarter-wavelength transmission line with a phase inverter. To demonstrate the proposed concept, a 1-GHz prototype was fabricated and tested. It achieves 90% impedance bandwidth with magnitude of S11 less than −10 dB. Within this bandwidth, more than 13 dB port-to-port isolation, less than 5.0 degree phase imbalance, and less than 4.5 dB magnitude imbalance are achieved, simultaneously. The proposed coupler not only achieves much wider bandwidth but also occupies less circuit area than that of the conversional 2-section branch-line coupler.

A meandered substrate integrated waveguide (SIW) has been proposed for the frequency scanning array that has potential application of the low cost 24-GHz Doppler radar system. All radiating slots are connected in series using a short circuit ended SIW; therefore, its radiation beam can be steered with frequency. The modified antenna with meandering structure is proposed to enhance the scanning range. The physical slot spacing is kept at about half free space waveguide, whereas the electrical spacing is about 540° at the center frequency of 24 GHz. A broadside radiation is achieved at 24-GHz frequency and a ±30° continuous scanning range is experimentally confirmed with the frequency band from 23 to 25 GHz. Another antenna with center frequency of 77 GHz is designed and tested. It also achieves a ±30° continuous scanning range with the frequency band from 75 to 79 GHz. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1478–1483, 2014

This paper presents a class of wideband 180° hybrid (rat race) couplers implemented by parallel-strip line. By replacing the 270° arm of a conventional 180° hybrid coupler by a 90° arm with phase inverter, the bandwidth of the coupler is greatly enhanced and the total circuit size is reduced by almost half. Simple design formulas relating the characteristic impedance of the arms and power division ration are derived. To demonstrate the concept, four couplers with different power division ratios of 1, 2, 4, and 8 were designed, fabricated, and tested. -parameters of the coupler are simulated and measured with good agreement. All working prototypes operate more than 112% impedance bandwidth with more than 25 dB port-to-port isolation and less than 5° absolute phase imbalance. The proposed 180° hybrid couplers can be employed as a wideband in-phase/differential power divider/combiner, which are essential for many RF and microwave subsystem designs.

A phase conjugated array capable of sending two individually modulated signals towards the direction of an interrogator is proposed. Each of the array elements is a dual-fed patch with one input port connected to a 90° phase shifting element and a resistive field effect transistor (FET) mixer and the other port just the mixer alone, making the array a balanced structure. A 4-element working prototype was designed at the center frequency of 5.8 GHz. A linearly polarized retrodirectivity has been experimentally confirmed by measuring bi-static radiation patterns with the interrogating signal coming at 0°, +30°, +60°, -15°, and -45°. A 8.6-dB peak power variation of re-radiation within ±60° normal to plane of the array has been shown in the measured mono-static radiation patterns. To test the ability of direct conversion, an active integrated direct conversion receiver employing the same array has been designed to receive and demodulate the response signals from the phase conjugated array. A 5 MHz square and 8 MHz sinusoidal waves as two base-band signals were carried through the phase conjugated array and successfully recovered by the direct conversion receiver.

A printed log-periodic dipole antenna (PLPDA) with multiple notched bands is proposed for ultrawideband (UWB) applications. The impedance bandwidth of 3.1 GHz-10.6 GHz with VSWR less than 2 is achieved based on the wideband property of the PLPDA as well as the half mode substrate integrated waveguide (HMSIW) Balun. Different from omnidirectional UWB antennas, the end-fire radiation pattern of the PLPDA is more stable within the UWB band. Multiple notched bands are generated by integrating U-shaped slots into the PLPDA for blocking the interference from other narrow band wireless communication systems. Several antennas with the notched frequencies of 3.5 GHz, 5.5 GHz, 6.8 GHz, and 8.5 GHz are designed, fabricated, and measured. The measured results are in agreement with the predicted results.

Nowadays, demands for compactness, light weight, and broadband have a great impact on design issues at both component and system levels in wireless communications. Multilayered structures fabricated with low-temperature co-fired ceramic (LTCC) technologies have been developed recently in response to these demands. With this trend, double-sided printed circuits using conventional fabrication techniques with lower cost have been receiving plenty of attention. The parallel-strip line, belongs to the family of balanced transmission lines. The conventional printed circuit board technology is able to realize it easily. It is a simple structure comprised of a piece of dielectric substrate sandwiched by two conducting traces. Signals flowing on the upper and lower conductors are always equal in magnitude and 180° out of phase; therefore, parallel-strip line has an inherent advantage of easy realization of a balanced configuration. Balanced or double balanced microwave circuits, such as amplifier, mixer, and antenna, are frequently employed in high-performance wireless communications systems, and they can be directly fed by parallel-strip lines. In contrast, microstrip line is an unbalanced transmission line; therefore, extra effort of designing balanced to unbalanced structures (balun) is required for using microstrip transmission lines.

This paper presents a new class of wideband 90° hybrid coupler with an arbitrary coupling level. The physical size of the proposed coupler is close to that of a conventional two-section branch-line coupler, but it has an additional phase inverter. The impedance bandwidth of the proposed coupler is close to that of a four-section branch-line coupler. The proposed coupler is a backward-wave coupler with a port assignment different from that of a conventional branch-line coupler. The design formulas of the proposed coupler are proved based on its even- and odd-mode half structures. We demonstrated three couplers at the center frequency of 2 GHz with different design parameters.

In this study, a wideband compact parallel-strip 180° hybrid coupler with the center frequency of 2 GHz is presented. By replacing the 270° section in the 180° hybrid coupler with a parallel-strip phase reversal swap, the total circuit size is reduced by half. In this structure, the phase gradients of the four arms of the coupler are equalized; therefore, the operation bandwidth is dramatically enhanced. S-parameters of the coupler are simulated and measured with good agreement. As predicted from the measured data, the bandwidth of the 90% combining efficiency is increased from 45 to 124%. The proposed design achieves more than −25 dB signal suppression within the whole measured frequency range (0–4 GHz) as opposed to 1.8 to 2.2 GHz of the conventional design. The proposed 180° hybrid coupler can serve as a wideband in-phase/differential power divider/combiner, which are essential for many RF and microwave subsystem designs. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3271–3274, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23926

A 1-D four-element and 2-D eight-element phase-conjugated retrodirective arrays employing low conversion-loss resistive field-effect transistor mixers and stub-loaded patch elements are proposed. The mixer, systematically designed using a multi- harmonic loading technique, offers a low 5.2-dB conversion loss at a 4.5-dBm local oscillator power level. The stub-loaded patch element is a square patch with a capacitive open stub loaded at each of its four corners, leading to patch-size reduction, lower mutual coupling between adjacent elements, and broader beam width. With the same element spacing of half a free-space wavelength at 5.8 GHz, the mutual coupling is suppressed by 3.1 and 3.3 dB in the E- and if-planes, respectively, when compared to the conventional square patches. Within an angle of plusmn60deg normal to the arrays, distinct linearly polarized retrodirectivities in terms of pattern deformations are experimentally confirmed for both the H-plane of the 1-D array and both E- and H-planes for the 2-D array. Additionally, circularly polarized retrodirectivity are also experimentally confirmed for 2-D arrays.

In this paper, a new power divider concept, which provides high flexibility of transmission line characteristic impedance and port impedance, is proposed. This power divider is implemented on a parallel-strip line, which is a balanced transmission line. By implementing the advantages and uniqueness of the parallel-strip line, the divider outperforms the conventional divider in terms of isolation bandwidths. A swap structure of the two lines of the parallel-strip line is employed in this design, which is critical for the isolation enhancements. A lumped-circuit model of the parallel-strip swap including all parasitic effects has been analyzed. An equal power divider with center frequency of 2 GHz was designed to demonstrate the idea. The experimental results show that the equal power divider has 96.5% -10-dB impedance bandwidth with more than 25-dB isolation and less than 0.7-dB insertion loss. In order to generalize the concept with an arbitrary power ratio, we also realize unequal power dividers with the same isolation characteristics. The impedance bandwidth of the proposed power divider will increase with the dividing ratio, which is opposite to the conventional Wilkinson power divider. Unequal dividers with dividing ratios of 1 : 2 and 1 : 12 are designed and measured. Additionally, a frequency independent 180 power divider has been realized with less than 2 phase errors.

The realization of a highly efficient linearized amplifier has emerged as a paramount issue in the design of advanced mobile handsets. In this paper, a new RF amplifier linearization scheme using a compensating transistor combination is proposed. The devised approach ably utilizes all terminals of an additional transistor that offers a unified pre-post-distortion and cubic distortion characteristic for performance improvement. Meticulous modeling along with a power-dependent Volterra series is performed to identify contributions on each mechanism under various power levels. An experimental four-tone test reveals a maximum 28-dB reduction for the intermodulation distortion at 1.95 GHz, which outperforms typical pre-distortions of 5-10 dB. A single-ended two-stage amplifier module demonstrates a state-of-the-art power efficiency of 55% with 27-dB transducer gain at 24-dBm output power. Meanwhile, the adjacent channel power ratio (ACPR 1) is maintained with good margins of -35dBc for a four-channel wideband code-division multiple-access signal under all output dynamics. Graceful degradations on modulation bandwidths, tone spacing, bias, and gain variations are also discussed, showing superb performance with virtually no dedicated retuning circuit parameters for multicarrier applications. By combining a bias control along with the proposed linearization technique, the average efficiency (12%) is 3times higher than that of the fixed bias (3.94%), demonstrating the potential utility on further prolonging battery lifetime in practical scenarios

In this paper, three folded couplers and their application have been presented. The Wilkinson power divider, and 90° and 180° hybrids are folded for size reduction in the multilayered printed circuit media. The folded hybrid ring is employed as the transition between the microstrip line (MSL) and the parallel-strip line (PSL) for balanced circuit applications. The PSL has an inherent advantage of easy realization of balanced configuration, which is suitable for push–pull designs, especially for high power applications. The PSL makes the design of circuits more flexible than MSL because the lump components and solid state devices can be easily mounted in both series and shunt manners without any difficulties in fabrication. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2285–2288, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21913

A high-performance X-band subharmonic self-oscillating mixer (SOM) is proposed. By using a reconfigurable transistor pair in conjunction with open- and short-circuit stubs, the proposed SOM is capable of down-converting four RF frequency bands with center frequencies at 7.35, 8.65, 11.35, and 12.65 GHz with a conversion gain ranging from 7.83 to 18.7 dB. The measured phase noise of the free-running oscillation is −85.4 dBc/Hz at an offset frequency of 100 kHz. With injection locking, the phase noise is reduced to less than −100 dBc/Hz at 10 kHz, without affecting the conversion gains. This is the first demonstration of a multiband subharmonic SOM. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 415–419, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20321

A downconverting second harmonic self-oscillating mixer (SOM) is developed for low-cost wireless communications applications. Incorporating resonant cell in the SOM, we can provide suitable oscillation for generating LO and terminations to all major unwanted mixing products, leading to high conversion gain design. The proposed SOM was measured with 8.5 dB downconversion gain at the RF frequency of 8.2 GHz RF, LO frequency of 4.0 GHz, and IF frequency of 0.2 GHz. The proposed design achieves higher conversion gain than that of the SOM without resonant cell.

A novel design of a third-harmonic self-oscillating mixer (SOM) based on a nonlinear optimization technique, namely multi-harmonic load, has been designed. Power consumption is reduced by terminating harmonic properly, and conversion gain is optimized in the form of harmonic termination networks at dc, RF, and LO frequency, and its harmonics. In this way, a novel SOM circuit at an RF frequency of 5.79 GHz has been designed and fabricated yielding an excellent conversion gain of +11.5 dB. Gate and drain voltage supplies are -1.18V and +1.5 V, respectively and dc power consumption is about 9 mW.

The GLH proteins belong to a family of four germline RNA helicases in Caenorhabditis elegans. These putative ATP-dependent enzymes localize to the P granules, which are nonmembranous complexes of protein and RNA exclusively found in the cytoplasm of all C. elegans germ cells and germ cell precursors. To determine what proteins the GLHs bind, C. elegans cDNA libraries were screened by the yeast two-hybrid method, using GLHs as bait. Three interacting proteins, CSN-5, KGB-1, and ZYX-1, were identified and further characterized. GST pull-down assays independently established that these proteins bind GLHs. CSN-5 is closely related to the subunit 5 protein of COP9 signalosomes, conserved multiprotein complexes of plants and animals. RNA interference (RNAi) with csn-5 results in sterile worms with small gonads and no oocytes, a defect essentially identical to that produced by RNAi with a combination of glh-1 and glh-4. KGB-1 is a putative JNK MAP kinase that GLHs bind. A kgb-1 deletion strain has a temperature-sensitive, sterile phenotype characterized by the absence of mature oocytes and the presence of trapped, immature oocytes that have undergone endoreplication. ZYX-1 is a LIM domain protein most like vertebrate Zyxin, a cytoskeletal adaptor protein. In C. elegans, while zyx-1 appears to be a single copy gene, neither RNAi depletion nor a zyx-1 deletion strain results in an obvious phenotype. These three conserved proteins are the first members in each of their families reported to associate with germline helicases. Similar to the loss of GLH-1 and GLH-4, loss of either CSN-5 or KGB-1 causes oogenesis to cease, but does not affect the initial assembly of P granules.

This paper presents a multilayer dielectric transmission line — the dielectric microstrip line (DML), for the terahertz (THz) applications. This DML is a metal free transmission line eliminating metal loss, which can be a very serious problem at THz. The E⁁ mode is the main mode in the DML for the electromagnetic wave propagation. A DML with standard waveguide operating within 0.220THz–0.325THz is designed, fabricated and measured for demonstration. With TRL calibration method, phase constant and attenuation constant of the DML are obtained.

We report that four putative germline RNA helicases, GLHs, are components of the germline-specific P granules in Caenorhabditis elegans. GLH-3 and GLH-4, newly discovered, belong to a multi-gene glh family. Although GLHs are homologous to Drosophila VASA, a polar granule component necessary for oogenesis and embryonic pattern formation, the GLHs are distinguished by containing multiple CCHC zinc fingers. RNA-mediated interference (RNAi) reveals the GLHs are critical for oogenesis. By RNAi at 20°C, when either loss of GLH-1 or GLH-4 alone has no effect, loss of both GLH-1 and GLH-4 results in 97% sterility in the glh-1/4(RNAi) offspring of injected hermaphrodites. glh-1/4(RNAi) germlines are under-proliferated and are without oocytes. glh-1/4(RNAi) animals produce sperm; however, spermatogenesis is delayed and the sperm are defective. P granules are still present in glh-1/4(RNAi) sterile worms as revealed with antibodies against the remaining GLH-2 and GLH-3 proteins, indicating the GLHs function independently in P granule assembly. These studies reveal that C.elegans can use GLH-1 or GLH-4 to promote germline development.

A compact nonresonating, dual-band bandstop filter is proposed. By connecting two transmission lines with proper characteristic impedances and phase delay (one is 90° and the other one is 450° in electrical length at center frequency), a dual stopband with two transmission zeros at each stopband is obtained. A prototype is demonstrated for 2.4/5.8-GHz applications. The measured results show the dual fractional stopband bandwidths with 20-dB signal attenuation are 25.2% and 12.4%. Also, the theoretical simulation, electromagnetic simulation, and the measured results show good agreement with this simple design. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2952–2954, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24753

An oversize microstrip patch antenna using multifeed has been proposed. By employing 1-to-8 in phase power divider, the antenna has been excited using edge feeds and resonates at fundamental mode. The proposed patch antenna is designed for high antenna gain of 10.9 dBi at 5.8 GHz. The occupancy area of proposed antenna is 34% smaller than that of conventional 8-element microstrip patch antenna array. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2749–2751, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23792

A new wideband parallel-strip bandpass filter using phase inverter with stubs is proposed. With the wideband phase inverter in the parallel-strip line structure, the measured fractional passband bandwidth is 123% with a flat group delay response. Also, over 90% impedance bandwidth is obtained for the return loss higher than 20 dB. The theoretical simulation, electromagnetic simulation, and measured results show good agreement with this proposed design.

A new passive 2-D Van Atta array is introduced with star-shaped antenna elements. The star-shaped antenna is proposed for low mutual coupling in the array. A single antenna element designed at 5.8 GHz has two orthogonal feeding ports with 23 dB port return loss and 28 dB isolation between ports. The mutual coupling among the antenna elements is less than -15.5 dB, which is 5 dB better than the conventional square patch antenna. Experimental results show that this array keeps a constant retrodirective characteristic at arbitrary injection polarization and direction. A stable bistatic radar cross section (BRCS) within 120deg angle can be achieved at 5.8 GHz

A Wilkinson power divider with a differential output implemented in parallel-strip-line (PSL) is proposed. Taking full advantages of the PSL technology and a three-stage cascaded design, more than 170% impedance and isolation bandwidths are obtained. Inherent to the PSL structure, the 180deg differential output is frequency-independent. A class-B push-pull power amplifier employing the devised concept is designed, showing a peak efficiency of 44% over a 4-GHz bandwidth. Without exploiting any extra and external low-pass filters, the proposed design can produce startling second-harmonic suppressions (more than 50dB) over the whole working dynamics and operated bandwidth

A lumped quadrature coupler based on a ferromagnetic core is proposed for high-frequency (HF) application. The four terminals of the transformer are connected to the four ports, whereas two inductors and two capacitors are connected between two adjacent ports. The bandwidth is only limited by magnitude imbalance. A 13.56 MHz prototype is designed and tested. A 69% bandwidth is achieved with ±3 dB magnitude imbalance. Within the frequency band, the measured return loss is greater than 31.57 dB, port-to-port isolation is greater than 33.21 dB and the phase imbalance is

A novel and compact four-element retrodirective array (RDA) prototype is realized. It demonstrates excellent performance in terms of retrodirectivity and sensitivity. Each element employs a phase conjugator (a biased FET mixer) with a conversion gain of 5.46 dB. The LO is set at 19.61 GHz and is pumped into the element while the RF operates at 9.8 GHz. The mixer generates an IF signal with a frequency offset of 10 MHz from the RF to separate the transponded signal from the interrogating sig- nal. Retrodirectivity is observed over a wide range of incidence angles from 50° to 50°. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 409 - 411, 2006; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.21365

A compact microstrip line to dielectric microstrip line (DML) transition is investigated, where DML is a dielectric waveguide that has quasi-planar structure line like conventional microstrip. DML is much more convenient to be used for various circuit constructions, comparing to other dielectric waveguides such as non-radiative dielectric guide and image guide. However, DML is not a standard transmission line. Transition is required for the measurement. In our experiment, the measured insertion loss of a section of DML with two back-to-back transitions ranges from 3.1 dB to 5.3 dB during 42 GHz to 54 GHz.

A parallel-strip phase inverter with a pair of simple impedance matching networks is designed. The phase inverter introduces the almost frequency independent 180° phase shift and was employed in the wideband parallel-strip balun. The balun was designed and measured with the maximum magnitude imbalance of 0.5 dB and the maximum phase imbalance of 6.0°. The proposed balun is used as input network for the wideband balanced frequency doubler. The proposed frequency doubler achieves significant conversion gain from 0.1 GHz to 1.7 GHz. The frequency doubler achieves 7.4 dB conversion gain and 23 dB fundamental signal suppression at 1 GHz.

This paper presents an alterative implementation of a shunt open-circuited stub for multilayer microwave circuit. With making use of the proposed implementation, a simple and compact bandstop filter with a district bandstop characteristics and a first spurious at the third harmonic is proposed. The proposed filter exhibits about 12.5% fractional bandwidth of −10 dB signal rejection at the center frequency of 2.04 GHz and flat group delay at the pass bands. Besides, by cascading a number of the proposed bandstop filters designed at the different frequencies, multiband bandstop filters can be easily realized. A dual- (tri-) band design at the center frequencies of 2 GHz and 3 GHz (and 4 GHz) is designed, realized, and measured. The proposed bandstop filter well suits the nowadays multilayer and compact radio frequency integrated circuit design.