Abstract: A multiple-input–multiple-output dielectric resonator antenna with enhanced isolation is proposed in this letter for the future 5G millimeter (mm)-wave applications. Two rectangular dielectric resonators (DRs) are mounted on a substrate excited by rectangular microstrip-fed slots underneath DRs. Each DR has a metal strip printed on its upper surface moving the strongest part of the coupling field away from the exciting slot to improve the isolation between two antenna elements. The proposed antenna obtains a simulated impedance bandwidth ( S 11 ≤ –10 dB) from 27.25 to 28.59 GHz, which covers the 28 GHz band (27.5–28.35 GHz) allocated by the Federal Communications Commission for the 5G applications. A maximum improvement of 12 dB on the isolation over 27.5–28.35 GHz is achieved. The mechanism of the isolation improvement and the design procedure are given in this letter. A prototype is manufactured and measured as a validation of the proposed decoupling method.

Abstract: The operating frequency of future communication systems will cover unlicensed millimeter-wave bands as well as existing microwave bands. Large frequency ratio antennas that can be applied to both frequency bands simultaneously and maintain the high isolation between the two channels are difficult to design. This paper presents a new design of dual-band shared-aperture antenna based on the concept of structure reuse. The antenna consists of a patch antenna working at 3.5 GHz and a $12 \times 12$ substrate integrated waveguide (SIW) slot array antenna working at 60 GHz. The frequency ratio of this shared-aperture antenna is 17. In this design, the overall structure of the SIW slot array antenna is employed as the radiator of the patch antenna. With this new scheme, the high aperture reuse efficiency can be achieved. Meanwhile, the millimeter-wave antenna based on the SIW technology has the high-pass nature to reject the lower frequency signal. A compact microstrip resonant cell that acts as a low-pass filter is connected in series on the feedline of the microwave antenna to suppress the upper frequency signal. Thus, the channel isolation between the patch and the SIW slot array antennas can be more than 130 dB at 3.5 GHz and 65 dB at 60 GHz.

Abstract: An approach is proposed to reduce mutual coupling between two closely spaced radiating elements. This is achieved by inserting a fractal isolator between the radiating elements. The fractal isolator is an electromagnetic bandgap structure based on metamaterial. With this technique, the gap between radiators is reduced to $\sim 0.65\lambda $ for the reduction in the mutual coupling of up to 37, 21, 20, and 31 dB in the ${X}$ -, Ku -, ${K}$ -, and Ka -bands, respectively. With the proposed technique, the two-element antenna is shown to operate over a wide frequency range, i.e., 8.7–11.7, 11.9–14.6, 15.6–17.1, 22–26, and 29–34.2 GHz. Maximum gain improvement is 71% with no deterioration in the radiation patterns. The antenna’s characteristics were validated through measurement. The proposed technique can be applied retrospectively and is applicable in closely placed patch antennas in arrays found in multiple-input multiple-output and radar systems.

Abstract: This article presents a broad review on optical, radio-frequency (RF), microwave (MW), millimeter wave (mmW) and terahertz (THz) biosensors. Biomatter-wave interaction modalities are considered over a wide range of frequencies and applications such as detection of cancer biomarkers, biotin, neurotransmitters and heart rate are presented in detail. By treating biological tissue as a dielectric substance, having a unique dielectric signature, it can be characterized by frequency dependent parameters such as permittivity and conductivity. By observing the unique permittivity spectrum, cancerous cells can be distinguished from healthy ones or by measuring the changes in permittivity, concentration of medically relevant biomolecules such as glucose, neurotransmitters, vitamins and proteins, ailments and abnormalities can be detected. In case of optical biosensors, any change in permittivity is transduced to a change in optical properties such as photoluminescence, interference pattern, reflection intensity and reflection angle through techniques like quantum dots, interferometry, surface enhanced raman scattering or surface plasmon resonance. Conversely, in case of RF, MW, mmW and THz biosensors, capacitive sensing is most commonly employed where changes in permittivity are reflected as changes in capacitance, through components like interdigitated electrodes, resonators and microstrip structures. In this paper, interactions of EM waves with biomatter are considered, with an emphasis on a clear demarcation of various modalities, their underlying principles and applications.

Abstract: This paper proposes a polarization-orthogonal co-frequency dual antenna pair suitable for fifth-generation (5G) multiple-input multiple-output (MIMO) smartphone with metallic bezels. The proposed spatial-reuse dual antenna pair consists of a split on the metallic bezel and a slot on the mainboard ground, where the slot is centered on the split. Two orthogonal degenerate characteristic modes operating at half wavelength including in-phase current and slot modes can be excited on the same antenna structure. An end-shorted microstrip line that crosses the split is used to excite the in-phase current mode, producing $y$ -polarized radiation, while a symmetrical slot-centered Y-shaped feeding network with its shorted ends crossing the slot is adopted to excite the slot mode, producing $x$ -polarized radiation. To the best of authors’ knowledge, this is the first time that a co-frequency dual antenna pair suitable for smartphone with metallic bezels is proposed. A sub-6 GHz dual antenna pair operating at 3.5 GHz that occupies a footprint of $25 \times 7 \times 1.5$ mm3 shows a simulated isolation better than −24.1 dB without any external decoupling measures and an envelope correlation coefficient (ECC) lower than 0.008. A four-antenna MIMO array is fabricated and measured to verify the feasibility. Across the operation band, the measured isolation of the antenna pair is better than −20.1 dB and other isolation levels are all better than −12.7 dB, the ECCs of any two antennas are lower than 0.13, and the total efficiency range of the four antennas is 35.2%~64.7%. The proposed dual antenna pair exhibits a great potential to be applied in future 5G MIMO smartphone.

Abstract: A novel ultra-wideband (UWB) multiple-input multiple-output (MIMO) Vivaldi antenna with dual band-notched characteristics is presented and fabricated. The antenna is comprised of an improved ground plane and two microstrip feeding lines with a compact size of $26\times 26$ mm 2 . The evolutionary process of the antenna is given. By etching the T-shaped slot on the ground plane, the port isolation between individual antenna elements can be greatly increased. Meanwhile, by adding two split ring resonator (SRR) of different sizes next to the microstrip feed lines, dual notches can be achieved to filter the interfere of WLAN and X-band communication satellites. The notched mechanism is analyzed from the surface current distributions. The experimental results show that the impedance bandwidth of the designed MIMO antenna is from 2.9 to 11.6 GHz, along with two notched bands covering 5.3-5.8 GHz and 7.85-8.55 GHz, and the mutual coupling is less than -16 dB in the whole working bandwidth. The MIMO antenna has good radiation characteristics, stable gain, and very low envelop correlation coefficient (ECC <;0.02), which is suitable for UWB MIMO system applications. The idea in this paper also has a certain guiding significance for the study of band-notched MIMO Vivaldi antenna.

Abstract: A simple decoupling method of using metallic vias to improve the isolation of millimeter-wave multiple-input-multiple-output (MIMO) dielectric resonator antenna (DRA) elements is investigated. The vias are vertically added to the DRA elements, at appropriate positions. By means of the interaction with the electromagnetic fields, the vias can potentially affect the filed distributions and further reduce the coupled fields effectively. The isolation between the MIMO DRA elements can, therefore, be enhanced substantially. As the vias are placed inside the DRA elements, no extra footprint is needed, making the entire antenna system very simple and compact. Two typical examples, including an H-plane and an E-plane, coupled $1\times2$ MIMO DRA arrays, have been designed, fabricated, and measured to demonstrate the feasibility and universality of this method. The results show that by using the vias appropriately, the isolation of the H-plane coupled MIMO DRA array can be enhanced from ~15.2 to 34.2 dB, while that of the E-plane array can be improved from ~13.1 to 43 dB at 26 GHz.

Abstract: We present, for the first time, a novel dual-linear polarized tightly coupled dipole array with integrated balun operating across a 9:1 bandwidth, from 2 to 18 GHz. This dual-linear polarized array employs a tightly coupled dipole topology and scans down to 60° from boresight. The overall geometry consists of cross-located tightly coupled dipoles integrated with a folded Marchand balun that serves as an impedance transformer to achieve a wideband feeding network. Notably, the traditional dielectric superstrate is replaced with double layers of frequency selective surfaces to enable scanning down to 60° in both E-, H-, and diagonal (D)-planes across the entire band. The design achieves VSWR $\lambda _{low}$ /10). The design is validated through fabrication and testing of an $11 \times 11$ prototype. The measurements of this prototype are presented and are in good agreement with simulations.

Abstract: A design of an ultra-wideband eight-port multiple-input multiple-output (MIMO) antenna array in a smartphone with an open-slot metal frame for fifth-generation (5G) communications is presented. Each element is fed by a microstrip line with a tuning stub, consisting of a U-slot on the ground plane and an open slot on the metal frame. Each slot element on the ground only occupies an area of 15 x3 mm. The antenna array can operate in 3.3-6 GHz (S 11 <; -6 dB) that is ultra-wide bandwidth for the future 5G communications. The antenna array is manufactured and measured. Measured antenna isolation is higher than 11 dB without any decoupling structures applied. Moreover, measured radiation patterns, antenna efficiencies, and envelop correlation coefficients are also given in this paper. High agreement between the measured and simulated results is obtained, which means that the proposed antenna is promising in engineering application.

Abstract: A novel 1-bit 256-element reconfigurable transmitarray antenna (RTA) at the Ku-band is presented. The element consists of two orthogonal H-shaped slots as receiving and transmitting structures, and the power transmission is realized by a coupling microstrip line in between. Two p-i-n diodes integrated on the coupling line can be electronically controlled to reverse the excitation directions, thus generating two states with 180° phase difference and low transmission loss. A subwavelength element spacing of $\lambda _{0}$ /3 is utilized to stabilize the element performance under oblique incidence. A transmitarray prototype with $16 \times 16$ elements and aperture size of $5.3\lambda _{0}\times 5.3\lambda _{0}$ at 12.5 GHz is fabricated and measured for the experimental verification. The measured results show that the maximum gain is 17.0 dBi with an aperture efficiency of 14.0%, and 2-D scanning beams within ±50° angular range are obtained. The 3-dB gain bandwidth of the broadside beam is 9.6%.

Abstract: A glucose concentration sensor for microliter-volume water–glucose solutions is presented. The proposed sensor is composed of an open-loop microstrip resonator with a dielectric liquid holder (5–25- $\mu \text{L}$ volume) glued onto the gap between the line ends. The resonator is coupled to two microstrip lines forming a two-port network whose S-parameter response provide information about the dielectric properties of the liquid under study. Three versions of the sensor at resonant frequencies between 2 and 7 GHz are presented. The sensors are assessed by measuring the complex permittivity of standard liquids reported in the scientific literature. Models of the sensors are presented, which properly match the experimental results. This paper presents an experimental study of the sensors as glucose concentration retrievers. The main novelty is the use of $Q$ factor and the maximum S21 of the resonance as sensing magnitudes. The dependence of these parameters on the glucose concentration of the solutions obeys almost linear relationships. Good sensitivities have been obtained for the proposed sensors, which are coherent with previous works by other authors.

Abstract: A compact quasi-isotropic dielectric resonator (DR) antenna (DRA) with filtering response is first investigated in this communication. The cylindrical DRA is fed by a microstrip-coupled slot, exciting in its ${\text {HEM}}_{11 \delta }$ mode which radiates like a magnetic dipole. A small ground plane is used for this DRA and it radiates like an electric dipole. The combination of the two orthogonal dipoles leads to a quasi-isotropic radiation pattern, with gain deviation as low as 5.8 dB in the 360° full space. To integrate the filtering function, the microstrip feed-line and the ground plane are turned upside down, and further two stubs with different lengths are used together to excite the DR. Due to the different loading effects of the feeding stubs, two resonances of the DR ${\text {HEM}}_{11 \delta }$ mode are excited in the passband, effectively enhancing the bandwidth of DRA ( $\varepsilon _{r} = 20$ ) to 7%. Furthermore, two controllable radiation nulls are generated by the DR loaded microstrip feed-line, bringing about high frequency selectivity at the edges of the passband and a quasi-elliptic bandpass response. For demonstration, a prototype operating at 2.4 GHz was fabricated and tested; reasonable agreement is obtained between the simulated and measured results.

Abstract: A multiple-input multiple-output (MIMO) antenna system is proposed for fifth generation (5G) and fourth generation (4G) mobile communication. The design meets all of the requirements of both 5G and 4G antennas using only a single structure. Since 5G will work at millimeter-wave (mm-Wave), the proposed design serves triple bands at mm-Wave (28, 37 and 39 GHz) for 5G in addition to 2 GHz band (1.8–2.6) for 4G. Each MIMO element consists of a slot-based antenna, fed by two microstrip feeders for the 5G and 4G bands. The design works as a tapered slot antenna at mm-Wave offering end-fire radiation for 5G and works as an open-ended slot antenna for a 2 GHz band offering omni-direction radiation for 4G. The slot antenna type used in the proposed design produces wide bandwidths for the 5G and 4G. The overall volume of each MIMO antenna element is $0.21\times 0.10 \times 0.003\,\,\lambda ^{3}$ , where $\lambda $ is the wavelength of the lowest operating frequency. As a proof of concept, a prototype is developed and tested. The measured results show a wide impedance bandwidth of $|S_{11}| −10 dB covering the band 27.5–40 GHz for 5G, and impedance bandwidth of $|S_{11}| −6 dB covering the band 1.8–2.6 GHz for 4G.

Abstract: The capability of a magnetodielectric superstrate to suppress the surface wave propagation is investigated. It is shown that the capacitively loaded loop (CLL) metamaterial (MTM) superstrate exhibits a high degree of surface wave attenuation. One of the important applications of such structure is to reduce the mutual coupling between the microstrip patch elements in array antennas. The results of numerical simulations show a significant reduction in the mutual coupling between the antenna elements. To verify and confirm the simulation results, a prototype of the CLL-MTM superstrate incorporating an array of rectangular patch antennas are fabricated and tested. The proposed two-element array antenna has a dimension of approximately $1.6\lambda \times 0.9\lambda \times 0.16\lambda $ while achieving a gain and an efficiency of 8.2 dB and 97%, respectively. Measurement shows the mutual coupling reduction of over 55 dB. The CLL-MTM superstrate provides the gain and efficiency almost the same of unloaded array antennas with 0.1 dB and 2% changes, respectively.

Abstract: In this work, a broadband millimeter-wave (mm-wave) multiple-input–multiple-output (MIMO) antenna system for upcoming fifth generation (5G) networks is presented. The MIMO antenna system is two ports and realized using two antenna arrays, aligned in opposite directions. Each array consist of three elements in each, as each element is a simple recognized printed wide-slot antenna proximity excited by microstrip line with a widened tuning stub; manipulated for operating in the Ka-band, which includes the 28 and 38 GHz bands, as potential candidates for 5G communications. An electromagnetic band-gap (EBG) reflector is placed behind the antenna structure toward the feeding network to decrease the backward radiation and improve the front-to-back (F/B) ratio. Results show that the proposed MIMO antenna system with EBG reflector provides wideband impedance bandwidth >27 GHz (from 22.5 to >50 GHz) and good radiation characteristics with a total realized gain up to 11.5 and 10.9 dBi at the two frequencies of interest, respectively. The envelope correlation coefficient (ECC) and diversity gain (DG) were evaluated and showed good MIMO performance. These remarkable features with the benefits of design simplicity and easily expansion to large-scale antenna system make the proposed design suitable for mm-wave communications.

Abstract: Internet of Things (IoT) based application requires integration with the wireless communication technology to make the application data readily available. In this paper, a modified meander shape microstrip patch antenna has been proposed for IoT applications at 2.4 GHz ISM (Industrial, Scientific and Medical) band. The dimension of the antenna is 40×10×1.6 mm 3 . The antenna design is comprised of an inverse S-shape meander line connected with a slotted rectangular box. A capacitive load (C-load) and parasitic patch with the shaped ground are applied to the design. Investigations show that the antenna designed with an inverse S-shape patch and connecting rectangular box in the microstrip line has a higher efficiency and gain compare to the conventional meander shape antenna. The C-load is applied to the feed line to match the impedance. Moreover, parametric studies are carried out to investigate the flexibility of the antenna. Results show that, the gain and efficiency can be improved through adjusting the rectangular box with applying parasitic element and the shaped ground. The parasitic element has high impact on the bandwidth of the antenna of 12.5%. The finalized antenna has a peak gain of -0.256 dBi (measured) and 1.347 dBi (Simulated) with 79% radiation efficiency at 2.4 GHz. To prove the efficiency and eligibility in IoT applications, the measurement of the power delivered and received by the antenna at 2.4 GHz is performed and compared with the results of a dipole antenna. The antenna is integrated with 2.4 GHz radio frequency module and IoT sensors to validate the performance. The antenna novelty relies on the size compactness with high fractional bandwidth that is validated through the IoT application environment.

Abstract: This letter presents a novel microwave-based rotation sensor having a wide dynamic range to detect and measure the angular displacement in terms of the change in resonant frequency. The proposed sensor is based on the microstrip technology, where a rotor comprised of a complementary split-ring resonator (CSRR) placed on the ground plane of the microstrip line is free to rotate around its axis. The mechanical rotation of CSRR determines a change in the cross coupling between the microstrip line and the CSRR, thus changing the overall inductance. The proposed planar unloaded microwave sensor, working around ISM band of 5.8 GHz, is quite sensitive to detect angular rotation in the wide dynamic range of 0°–90°. The linearity in dynamic range is achieved in the range of 30°–60°. Operating frequency and bandwidth can be adjusted by loading the rotor with dielectric. Depending on the type of dielectric loading of CSRR, it is possible to select the center frequency from a wide range of 4.67–5.94 GHz, with the bandwidth ranging from 116 to 250 MHz. Due to its features, the proposed sensor can be useful for various industrial applications.

Abstract: A novel polarization-reconfigurable cut ring microstrip antenna with high gain is proposed. This simple structure microstrip antenna consists of a ring radiation patch, two switches (p-i-n diodes), and six nonmetallic columns. By controlling the switches, the antenna can be operated on three polarized states: one state for linear polarization (LP), one state for left-hand circular polarization (LHCP), and one state for right-hand circular polarization (RHCP). In addition, the antenna peak gain can reach 10.0 dB for LP, LHCP, and RHCP over the operating bandwidth and stable unidirectional radiation patterns. The proposed antenna is fabricated and verified. By analyzing simulated and measured results, the measured common operating bandwidth of the proposed antenna on three states is from 3.86 to 3.98 GHz with the relative bandwidth of 3.1%. The cross-polarization of the antenna on the whole states is very low. Good polarization-reconfigurable characteristics of the proposed antenna have been obtained from 3.86 to 3.98 GHz. The proposed antenna is also a good candidate for advanced wireless communication systems.

Abstract: A wideband, low-profile, tightly coupled antenna array with a simple feed network is presented. The dipole and feed networks in each unit cell are printed on both sides of a single RT/Duroid 6010 substrate with a relative dielectric constant of 10.2. The feed network, composed of meandered impedance transformer and balun sections, is designed based on Klopfenstein tapered microstrip lines. The wide-angle impedance matching is empowered by a novel wideband metasurface superstrate. For the optimum design, scanning to 70° along the E-plane is obtained together with a very high array figure of merit PA = 2.84. The H-plane scan extends to 55°. The broadside impedance bandwidth is 5.5:1 (0.80–4.38) GHz with an active voltage standing-wave ratio value ≤2. The overall height of the array above the ground plane is $0.088\lambda _{\mathrm {L}}$ , where $\lambda _{\mathrm {L}}$ is the wavelength at the lowest frequency of operation. A prototype was fabricated and tested to confirm the design concepts.

Abstract: This communication presents a wide stopband filtering antenna element and its application in multi-input multi-output (MIMO) system. Microstrip terminated coupled lines and a rectangle patch are utilized to generate the filtering function. By varying the length of the parallel coupled lines and open-circuited stubs, multiple transmission zeros can be introduced in the spurious band. Appropriate positioning the location of these transmission zeros enables a wide harmonic suppression. To demonstrate the proposed method, a wide stopband filtering antenna is designed and further applied in a four-element MIMO array. The measured and simulated results agree well. The proposed MIMO antenna obtains an attenuation of 12.6 dB up to $6.6\times $ the center frequency.

Abstract: This letter presents a low-cost, single-layer, dual circularly polarized (CP) series-fed antenna based on the sequential rotation technique for millimeter-wave (MM-wave) applications. The series-fed working form endows the proposed antenna with more compact structure and lower ohmic, dielectric losses caused by the feeding network compared with parallel-fed antennas. The curved microstrip transmission lines are adopted as radiation elements; therefore, the whole antenna can be etched on a single-layer laminate using standard printed circuit board technology. These characteristics make the antenna a competitive candidate for MM-wave applications. To validate the proposed design, a dual-CP antenna operating at 30 GHz is fabricated. Based on the measurement, the proposed antenna has a 3 dB gain bandwidth of 1.97 GHz, the measured $| {{S_{21}}} |$ is lower than −20 dB, and the axial ratios are lower than 3 dB in the aforementioned frequencies.

Abstract: In this study, we propose a design of a multi-band slot antenna array applicable for fourth-generation (4G) and fifth-generation (5G) smartphones. The design is composed of double-element square-ring slot radiators fed by microstrip-line structures for easy integration with radio frequency (RF)/microwave circuitry. The slot radiators are located on the corners of the smartphone printed circuit board (PCB) with an overall dimension of 75 × 150 mm2. The proposed multiple-input multiple-output (MIMO) antenna is designed to meet the requirements of 4G and 5G mobile terminals with essential bandwidth for higher data rate applications. For −10 dB impedance bandwidth, each single-element of the proposed MIMO design can cover the frequency ranges of 2.5–2.7 GHz (long-term evolution (LTE) 2600), 3.45–3.8 GHz (LTE bands 42/43), and 5.00–5.45 GHz (LTE band 46). However, for −6 dB impedance bandwidth, the radiation elements cover the frequency ranges of 2.45–2.82 GHz, 3.35–4.00 GHz, and 4.93–5.73 GHz. By employing the microstrip feed lines at the four different sides of smartphone PCB, the isolation of the radiators has been enhanced and shows better than 17 dB isolation levels over all operational bands. The MIMO antenna is implemented on an FR-4 dielectric and provides good properties including S-parameters, efficiency, and radiation pattern coverage. The performance of the antenna is validated by measurements of the prototype. The simulation results for user-hand/user-head impacts and specific absorption rate (SAR) levels of the antenna are discussed, and good results are achieved. In addition, the antenna elements have the potential to be used as 8-element/dual-polarized resonators.

Abstract: In this communication, a corporate stacked microstrip and substrate integrated waveguide (SIW) feeding structure is reported to be used to broaden the impedance bandwidth of a $4 \times 4$ patch array antenna. The proposed array antenna is based on a multilayer printed circuit board structure containing two dielectric substrates and four copper cladding layers. The radiating elements, which consist of slim rectangular patches with surrounding U-shaped parasitic patches, are located on the top layer. Every four radiation elements are grouped together as a $2 \times 2$ subarray and fed by a microstrip power divider on the next copper layer through metalized blind vias. Four such subarrays are corporate-fed by an SIW feeding network underneath. The design process and analysis of the array antenna are discussed. A prototype of the proposed array antenna is fabricated and measured, showing a good agreement between the simulation and measurement, thus validating the correctness of the design. The measured results indicate that the proposed array antenna exhibits a wide $\vert \text {S}_{11}\vert dB bandwidth of 17.7%, i.e., 25.3–30.2 GHz, a peak gain of 16.4 dBi, a high radiation efficiency above 80%, and a good orthogonal polarization discrimination of higher than 30 dB. In addition, the use of low-profile substrate in the SIW feeding network makes this array antenna easier to be integrated directly with millimeter-wave front-end integrated circuits. The demonstrated array antenna can be a good candidate for various Ka -band wireless applications, such as 5G, satellite communications and so on.

Abstract: A coupling-matrix approach for the theoretical design of a type of input-reflectionless RF/microwave bandpass filters (BPFs) and bandstop filters (BSFs) is presented. They are based on diplexer architectures with arbitrary-order bandpass and bandstop filtering channels that feature complementary transfer functions. The transmission behavior of these reflectionless filters is defined by the channel that is not loaded at its output, whereas the input-signal energy that is not transmitted by this branch is completely dissipated by the loading resistor of the other channel. Analytical formulas for the coupling coefficients for the first-to-fourth-order filter designs are provided and validated through several synthesis examples. This theoretical design methodology, along with an optimization step, is also exploited to design input-quasi-reflectionless quasi-elliptic-type BPFs with a transmission-zero-(TZ)-generation cell in their bandpass filtering channel. In addition, the application of the proposed input-reflectionless BPF and BSF networks to input-quasi-reflectionless multiplexer design is approached. It is shown that a single resistively terminated multi-band BSF branch can absorb the input-signal energy not transmitted by the multiplexer channels in their common stopband regions to achieve quasi-reflectionless characteristics at its input. Moreover, experimental microstrip prototypes consisting of 2-GHz third-order BPF and BSF circuits, a 2-GHz sharp-rejection third-order BPF with two close-to-passband TZs, and a second-order diplexer device with channels centered at 1.75 and 2.1 GHz are developed and measured.

Abstract: A type of compact single/multi-band coupled-multi-line filtering section and its application to the design of RF filtering devices are presented. This one-port filtering cell is made up of ${N+1}$ intercoupled quarter-wavelength transmission-line segments for the generation of a transfer function with a total of ${N+1}$ transmission zeros (TZs) distributed at both sides of all its ${N}$ first-order passbands when arranged in the transmission mode. Design formulas for the produced poles and TZs of its corresponding normalized coupling-routing diagram and illustrative theoretical responses are provided. Furthermore, for experimental demonstration purposes, microstrip prototypes of the following RF filtering components that exploit the conceived filtering section are manufactured and measured: 1) a quasi-elliptic-type diplexer with a dual-band bandpass filter (BPF) junction; 2) two input-reflectionless triple-band BPF and bandstop filter (BSF) circuits; and 3) an out-of-phase equal-power-division bandpass filtering coupler with input-quasi-reflectionless behavior.

Abstract: A new class of in-phase and out-of-phase power dividers with constant equal-ripple frequency response and wide operating bandwidth is presented in this paper. The proposed design is based on microstrip-to-slotline transitions and slotline resonators. A slotted T-junction is adopted to split the power into two parts and obtain wideband isolation between the two output signals at the same time. The characteristic impedance of the transitions and resonators determines the operating bandwidth and in-band magnitude response. By reversing the placement direction of the slotline-to-microstrip transition, the electrical field is reversed, thus resulting in out-of-phase responses between output ports. A thorough analysis of the relations between the structure and the characteristic functions is provided to guide the selection of parameters of the structure in order to meet the design objectives. In the structure, simulation and measurement are conducted to verify the design method. For both in-phase and out-of-phase cases, more than 110% bandwidth has been achieved with excellent matching at all ports and isolation of output signals. Constant in-band ripple is obtained within the operating band of the power dividers, indicating that the proposed design can realise minimal power deviations, which is extremely desired in wireless systems.

Abstract: A two-port structure-shared planar ultrawideband (UWB) antenna with high isolation is presented in this communication. The proposed antenna consists of a planar monopole and two back-to-back tapered slots etched on the monopole. The monopole and the tapered slots are fed by the microstrip line and the differential feeding network, respectively. The differential feeding network of the tapered slots excites the modes, which are orthogonal to those of the monopole. Therefore, high isolation between the two ports can be achieved without additional decoupling structures. The measured results show that the proposed antenna can operate from 3 to 10 GHz with the reflection coefficient of the two ports lower than −10 dB and the isolation higher than 38 dB. The measured realized peak gains are above 2.6 dB, and the measured envelope correlation coefficients are less than $9 \times 10^{-5}$ . The proposed UWB multiple-input-multiple-output (MIMO) antenna can be applied in the MIMO communication systems, especially those requiring high isolation.

Abstract: The next generation of mobile communication system is the fifth generation (5G) communication systems that used mm-wave bands to achieve high data rates and increase the user capacity. 5G communication system require a low profile, lightweight, high gain and simple structure antennas to ensure reliability, mobility, and high efficiency. Due to the low atmospheric absorption rate of electromagnetic waves at 28GHz, this paper aims to design a directional single element slotted microstrip antenna with compact size to operate at 28GHz for 5G applications. The proposed antenna is simulated by using High Frequency Structure Simulator (HFSS) software on FR4 substrate with 4.4 dielectric constant, 0.8 mm thickness, and 0.02 loss tangent. In order to achieve the requirements of the proposed antenna, the patch is modified by loading specific slots; and the performance of the antenna is studied in terms of antenna parameters that are return loss, VSWR, gain, and radiation pattern, and compared with published results.

Abstract: In this paper, we propose a humidity sensor based on a negative resistance oscillator using conducting polymer (CP) PEDOT:PSS film at room temperature. The proposed humidity sensor basically consists of a microstrip line and a negative resistance circuit, which creates a negative resistance for high-frequency oscillation. The microstrip line has a circuit structure that is connected through a via hole between the ground plane and the signal line with a CP thin film. From our experimental results, we find that the oscillation frequency of the sensor gradually decreases as the relative humidity (RH) increases. Owing to the conductivity variation of the CP film with the RH value, the oscillation frequency of the sensor sensitively changes in real time. Therefore, we suggest that our microwave-oscillator-based sensor scheme is a good candidate for the design of a robust and stable humidity sensor.