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A simple, high gain, and dual-polarized patch antenna array for Wi-Fi 6/6E application is proposed in this communication. Different from the conventional feeding method, the patches are driven by coupling dipoles that are printed on the same board. The dipoles are specially designed to be T-shaped to adjust the impedance matching. Broadband covering both 5 and 6 GHz bands of Wi-Fi 6/6E is achieved. The feeding circuit is easily integrated on the board with only parallel dual line utilized. A prototype is fabricated and tested to validate the concept. The measured and simulated results are in good agreement. The measured bandwidth with $S_{11} < -10$ dB for two ports are 5.12–7.50 and 5.15–7.50 GHz with the isolation higher than 30 dB. Moreover, the measured gains are higher than 9.76 and 9.61 dBi for two ports over the operating band with a peak value of 12.05 dBi. The proposed antenna array shows great potential in the application of Wi-Fi mesh. [ABSTRACT FROM AUTHOR]

In this communication, we present a longitudinally slotted substrate integrated slot (SIS) leaky-wave antenna (LWA) for point-to-point communication. To mitigate the dispersion of traditional substrate integrated waveguide (SIW) LWAs, an SIS is proposed by etching a long straight slot along the SIW. The loaded slot mitigates the dispersion of SIW due to the special electric field distribution. Anti-TE10 mode is excited inside the SIS. Then, longitudinal slots are symmetrically loaded on both sides of the long slot. The prototype of SIS LWA is validated by the simulated and experimental results. A frequency bandwidth of 10.3% is realized with the beam squinting within 7° and realized gain of 13.9 ± 0.7 dB at a fixed angle of −7°. The proposed antenna is intended for point-to-point communication. [ABSTRACT FROM AUTHOR]

In this article, a two-port microstrip antenna (MSA) with high isolation is presented for simultaneously supporting wireless fidelity 6 (Wi-Fi 6) and wireless fidelity 6E (Wi-Fi 6E) applications. Due to the narrow guard band between Wi-Fi 6 (5.15–5.83 GHz) and Wi-Fi 6E (5.92–7.125 GHz), the mutual coupling near common edge frequencies cannot be sufficiently suppressed by existing filters. To mitigate the mutual coupling over the guard band of a symmetrical two-port MSA, the common-mode (CM)/differential-mode (DM) cancellation theory was utilized. Then, the operating frequencies of two ports were tuned to Wi-Fi 6 and Wi-Fi 6E bands, without deteriorating the original decoupling performance. Finally, two filters were loaded to realize high isolation over the whole working bandwidth. For the demonstration, a prototype was fabricated and measured. The measured results show that the proposed two-port MSA achieves the impedance bandwidth of 5.49–5.9 and 5.89–7.14 GHz and the isolation over 20 dB across the whole band. The results indicate that the proposed antenna is a promising candidate for Wi-Fi 6 and Wi-Fi 6E applications. [ABSTRACT FROM AUTHOR]

A broadband and omnidirectional coverage dual-antenna pair with horizontally polarized (HP) radiation for Wi-Fi 6/6E application is presented in this article. The dual-antenna pair is composed of two open cavity antennas (OCA) with the apertures partially sealed. By sealing the aperture partially, the dimension of the proposed half-OCA (HOCA) is reduced dramatically without changing the matching bandwidth and the radiation performance. Compared with other OCAs with the omnidirectional radiation, the proposed dual-antenna pair realizes omnidirectional coverage in azimuth plane by the back-to-back combination of two unidirectional OCAs, which also makes the antenna have multiple-input multiple-output (MIMO) performance. Besides, a broadband covering both 5 and 6 GHz bands of Wi-Fi 6/6E is also achieved. A prototype was fabricated and tested to validate the concept. In well agreement with the simulated results, the measured bandwidth with $S_{11} < -10$ dB for two ports are 5.03–7.35 GHz and 5.09–7.53 GHz, respectively, with the isolation higher than 22.67 dB. Besides, the measured gain > 5 dBi and ECC < 0.05 are achieved over the operating band. The proposed antenna shows great potential in the Wi-Fi 6/6E application. [ABSTRACT FROM AUTHOR]

This article demonstrates an electronically scanned reflectarray antenna based on liquid crystal (LC). The LC-based reflectarray element was designed as a periodically loading of parallel H-shaped polygons on two metal layers. To realize a tunable reflection phase and reduce the inhomogeneity effect of LC, an $8~\mu \text{m}$ -thick LC layer was designed as a varactor rather than a substrate. In simulations, the adjustable reflection-phase range of the proposed element was 180° in the frequency band 21–21.5 GHz. The design concept was verified on a reflectarray prototype containing 26 rows of the basic elements. In addition, a 32-channel control circuit was designed and fabricated for biasing. The measured results confirmed a tunable phase range of 150° at 23.8 GHz. The disagreements between the simulated and measured results are discussed, and a test process is proposed. As a proof of concept, the main beam was steered to 0°, −40°, and −60° in one plane. The result confirmed the high beam-steering capability of the proposed reflectarray with measured gains above 18 dB at 23.8 GHz. This work provides useful references for the design and measurement of LC-based reflectarray antennas. [ABSTRACT FROM AUTHOR]

This article describes a compact dual-polarized endfire antenna array for fifth-generation (5G) millimeter-Wave (mm-Wave) mobile application. The dual-polarized endfire antenna element is constructed by a compact open-ended cavity with cavity mode for vertical polarization (VP) radiation and open-slot mode for horizontal polarization (HP) radiation. With the integrated design of VP and HP elements, the proposed antenna shows a much more compact size compared to existing schemes. To enhance the bandwidth of low-profile VP radiation, a slot is etched on the cavity to create a new mode for dual-resonance response, realizing a wide bandwidth of 24.5–29.5 GHz for both polarizations. In addition, radiation pattern modification techniques are employed to achieve good endfire beams with a high front-to-back ratio for both polarizations, thus enabling a high peak gain of 10.3 and 11.3 dBi for VP and HP, respectively. A wide beam scanning range of ±50° with realized gain above 7 dBi is realized at 27 GHz for both polarizations. In virtue of compact size, wide bandwidth, high gain, and good spatial coverage, the proposed design strategy is promising for future 5G mm-Wave mobile phones. [ABSTRACT FROM AUTHOR]

A high-gain low-profile leaky-wave antenna (LWA) for vertically-polarized (VP) fixed-beam endfire radiation is proposed in this article. The LWA is composed of two back-to-back half-mode waveguides with grooves etched at the margins of both radiation edges. By etching grooves, the phase velocity of the traveling wave can be tailored to slower than the wave velocity in free space for fitting the Hansen–Woodyard condition. Compared with periodic LWAs, the proposed antenna continuously radiates at two symmetric apertures, contributing to a high endfire gain. A prototype is fabricated and tested to validate the concept. The measured and simulated results are in good agreement. A simulated gain of 14.29 dBi and a measured gain of 13.37 dBi are achieved within an antenna length of $3.05\lambda _{0}$. Moreover, a measured overlapping bandwidth of 2.74–3.17 GHz (14.3%) is realized with $S_{11} < - 10$ dB and gain variation less than 3 dB. To the best of the authors’ knowledge, the proposed VP endfire LWA realizes the highest aperture efficiency and shows great advantages in high gain, low profile, and wide bandwidth. [ABSTRACT FROM AUTHOR]

In this article, a dual-linearly polarized endfire antenna is proposed with high gain and stable radiation patterns. A double-sided slot array antenna (DSSAA) and a corrugated rectangular metal strip (CRMS) are employed to realize vertical and horizontal polarization (HP), respectively. Omnidirectional radiation pattern is obtained by etching transverse slots on both sides of the top and bottom metallic plates of a cavity. Based on the double-sided slot antenna, a DSSAA is proposed and fed by an air-substrate parallel strip line (PSL) for stable endfire radiation patterns. Then, the top plate of the DSSAA is reused as a CRMS for HP and compact size. To verify this concept, an all-metal prototype was fabricated and measured. Both the simulation and experimental results show that the DSSAA and the CRMS could offer the highest endfire gains of 12.5 and 12.9 dB, respectively, within a footprint of $3.47\lambda _{0} \times 0.72\lambda _{0} \times 0.13\lambda _{0}$ ($\lambda _{0}$ is the free-space wavelength at the center frequency). Moreover, an overlapping bandwidth of 7.6% is realized with $\vert S_{11}\vert < -10$ dB, gain variation less than 3 dB. This work may provide useful references for designing dual-polarized endfire antennas with stable radiation patterns, high gain, and compact size. [ABSTRACT FROM AUTHOR]

Future generations of wireless services will always need more efficient strategies for the use of the available frequency spectrum, given the ever-increasing demand for high-speed data services. One of such strategies is given by in-band full-duplex (IBFD) systems, also called simultaneously transmit and receive (STAR) systems, that are considered because spectrum efficiency is not maximized with the current practice of half-duplex communication, where a node either transmits or receives a signal in a single channel. In fact, all currently deployed wireless networks require two separate channels to achieve bi-directionality: Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are the two most commonly-used techniques. This channel separation prevents the self-interference that has been seen as an insurmountable technical problem. Promoted by the popularity of small cell wireless systems, recent works have provided experimental evidence and methodologies for IBFD systems, where a node transmits and receives signals at the same time and on the same frequency band ,. IBFD systems not only increase spectral efficiency, but also minimize overhead and timing constraints. This increases throughput and allows for the possibility of real-time sensing of spectrum occupancy for cognitive devices. Even though IBFD systems have their primary application for bidirectional communication topology (where there are two data flows: terminal A sends data to terminal B, and terminal B sends data to terminal A), a different body of recent works has focused on IBFD in other settings including relay, base-station and multi-hop topologies –. [ABSTRACT FROM AUTHOR]

This article presents a co-linearly polarized (LP) stacked two-port antenna with compact size and broadside radiation patterns for in-band full-duplex (IBFD) application. The proposed IBFD antenna consists of a patch antenna and a substrate integrated waveguide (SIW) cavity-backed slot antenna (CBSA). The patch antenna and the CBSA are used as the transmitter (Tx) and receiver (Rx), respectively. The Rx is placed above the Tx for compact size. Linear polarization and broadside radiation patterns are achieved in this article. A cavity-patch coupling (CPC) path is added for canceling the slot-patch coupling (SPC) by using four probes to connect the top layer of the SIW cavity and the metal ground. Parametric studies show that the two coupling paths can be adjusted independently. Experimental results illustrate that the measured highest isolation over 40 dB is obtained with a compact size of $30\times 30\times {4}$ mm3 ($0.63\lambda _{0} \times 0.63\lambda _{0} \times 0.08\lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at the center frequency of about 6.29 GHz). The measured −10 dB bandwidths of the Tx and Rx antennas are 430 and 100 MHz, respectively. The proposed stacked IBFD antenna is validated with the merits of identical polarization, same radiation direction, and compact size. [ABSTRACT FROM AUTHOR]

In this article, a compact co-polarized decoupled antenna for full-duplex application working at 1.71 GHz is proposed. The antenna is composed of two back-to-back air-dielectric planar inverted-F antennas (PIFAs) with zero edge-to-edge distance. The whole system is included in a compact volume of $0.45\lambda _{\mathrm {\mathbf {0}}} \times 0.45\lambda _{\mathbf {0}} \times 0.02\lambda _{\mathbf {0}}$. The proposed antenna is designed based on the common mode (CM)/differential mode (DM) cancellation theory and its design guideline is illustrated in detail. Isolation maximum of 42 dB, broadside gain of 6.7 dB, and efficiency of 90% have been achieved in the proposed structure. There is a 1.9 dB gain difference between the broadside gain and the maximum gain. The proposed antenna shows its potentials in the radio frequency front-end incorporate design. [ABSTRACT FROM AUTHOR]

In this article, a wideband and highly-integrated quad-element multiple-input multiple-output (MIMO) antenna is proposed for the first time, to adapt to the size-limited environment in fifth-generation (5G) smartphones. First, the wideband decoupling between two extremely closely-spaced open-slot antennas with face-to-face and back-to-back configurations are investigated. Then, based on the complementary antenna pairs, wideband integrated quad-element MIMO antennas are implemented by the ingenious combination of these antenna pairs. For validation, an 8 × 8 MIMO system, constituted by two sets of integrated quad-antenna configurations, is simulated, fabricated, and measured. Both the simulated and measured results show that the 8 × 8 MIMO system can provide isolation of better than 10 dB between any two ports and a total antenna efficiency of 52.8%–70.8%/40.5%–75.0% across 3.3–5.0 GHz. Compared with the existing integrated quad-antenna design schemes, the proposed solution can expand the bandwidth from less than 200 to 1700 MHz, covering the entire 5G N77, N78, and N79 bands. In addition to the integrated quad-antenna design, further extension to integrated multiantenna configurations is also discussed by the flexible combination of complementary antenna pairs, which paves the way for future higher-order MIMO system in smartphones. [ABSTRACT FROM AUTHOR]

In this article, an inductance-based decoupling scheme is proposed to reduce the mutual coupling between extremely closely spaced microstrip antennas. The original strong coupling can be effectively suppressed by simply inserting a lumped inductance in between. To offer a systemic design guideline for this decoupling strategy, a mode cancellation method, based on the synthesis of common mode (CM) and differential mode (DM), is proposed. The inserted inductance plays a role of tuning CM and DM impedances to a similar status, which has an equivalent decoupling effect according to the theory of microwave network. Alternatively, the lumped inductance could also be replaced by an inductive connecting strip for a concise topology. To validate the proposed decoupling concept, a prototype is simulated, fabricated, and measured. The experimental results show that the poor isolation of 5 dB is improved to better than 15.4 dB across the entire matched bandwidth of 2.394–2.530 GHz, with an extremely close edge-to-edge distance of 0.016 λ0 and center-to-center distance of 0.44 λ0. Furthermore, the validation of extending to large-scale 1-D and 2-D arrays is also discussed. Featuring simple structure, compressed dimension, strong-coupling suppression, and good radiation performance, the proposed decoupling scheme possesses promising potential for antenna array applications. [ABSTRACT FROM AUTHOR]

This communication proposes a wideband decoupling technique to suppress the strong coupling between two extremely closely spaced open-slot antennas and implements a wideband integrated slot antenna pair for fifth-generation (5G) metal-rimmed smartphones. By simply inserting a connecting line between two closely spaced open-slot antennas, a top slot structure can be constituted with odd- and even-mode resonances in the lower and higher bands, respectively, to cancel the original strong mutual coupling. In addition to the decoupling effect, the top slot can also expand the effective radiation aperture of the antenna pair for the bandwidth improvement. The proposed slot antenna pair, with a compact footprint of 28 × 7 ×1.8 mm3, shows good impedance matching, isolated and diversity performance across 3.3–5.0 GHz. Then, an 8 × 8 multiple-input multiple-output (MIMO) system, constituted by four sets of slot antenna pairs, is simulated, fabricated, and measured. Both the simulated and experimental results show that the 8 × 8 MIMO system can offer isolations of better than 10.8 dB and envelope correlation coefficients (ECCs) of less than 0.14 between all eight ports across 3.3–5.0 GHz. Also, a total efficiency of 55.0%–83.1% and 52.5%–83.1% is achieved for Ant1 and Ant2, respectively. Featuring wide bandwidth, compact size, high integration level, and metal rim compatibility, we forecast the proposed solution has great potential for future 5G smartphones. [ABSTRACT FROM AUTHOR]

In this article, a general decoupling method based on a new perspective of common mode (CM) and differential mode (DM) cancellation is proposed. For two closely spaced antennas, the mutual coupling effect can be analyzed and solved by exciting them simultaneously with in-phase (CM) and out-of-phase (DM) signals. It is theoretically proved that, if CM and DM impedances are the same, the mutual coupling effect between two separated antennas can be totally eliminated. Therefore, we can solve the coupling problem by CM and DM impedance analysis and exploit the unique field properties of characteristic modes to assist in antenna decoupling in a physical intuitive way. To validate the feasibility of this method, two practical design examples, including the decoupling between closely spaced dipole antennas and planar inverted-F antennas, are proposed. Both design examples have demonstrated that the proposed method can provide a systemic design guideline for antenna decoupling and achieve better decoupling performance compared to the conventional decoupling techniques. We forecast the proposed decoupling scheme, with a simplified decoupling procedure, has great potential for the applications of antenna arrays and multi-input multi-output (MIMO) systems. [ABSTRACT FROM AUTHOR]

In this communication, a special Fabry–Perot (FP) cavity antenna with a slender structure is proposed to realize an omnidirectional radiation pattern with horizontal polarization. In our design, a partially reflective strip consisting of periodical slots is placed over the aperture of a long and narrow cavity, thus generating an FP resonance with a uniform electric field distribution along this aperture. As the slot arrays are excited in phase and with a nearly identical amplitude, the proposed slender architecture can provide a horizontally polarized omnidirectional radiation pattern with an enhanced directivity. The proposed antenna is prototyped with a length of $2.95\lambda $ ($\lambda $ is the free space wavelength at the center frequency). In well agreement with the simulation results, the experiment verifies that an omnidirectional pattern is achieved over the operating band (2.41–2.5 GHz), with a realized gain of 7.1–8.52 dBi and a gain variation on the E-plane less than 2.3 dB. In virtue of the low-profile all-metal structure, high efficiency (>86%), and enhanced gain, the proposed slender FP antenna is promising for future wireless communication applications. [ABSTRACT FROM AUTHOR]

In this communication, a planar antenna array is proposed in millimeter-wave bands by using a modified bulk silicon micromachining technology. The proposed antenna array is designed and fabricated based on air-filled microstrip line structures. By periodically blocking the microstrip line, fringing electric fields along the microstrip lines are excited in phase, which contributes to achieving high gain performance with a relatively compact feeding structure. Moreover, the proposed antenna array is designed and fabricated by a modified bulk silicon micromachining technology. The process is quite suitable for designing low loss antennas in millimeter-wave bands. In the process, the silicon wafers are deeply etched and gold-plated. As a result, microstrip line structures in the proposed antenna array are filled with air medium, which mitigates dielectric loss and improves radiation efficiency. A prototype of the proposed antenna array is fabricated and tested to verify the design strategy. The dimension of the antenna array is $49.40\times 20.00\times0.75$ mm3. The simulated −10 dB bandwidth is 58.8–59.7 GHz and the measured bandwidth is 58.8–59.5 GHz. In the operating band, the maximum simulated gain of the proposed antenna is 19.8 dBi and the minimum simulated gain is 18.3 dBi. The measurement results illustrate that the maximum gain of the proposed antenna array is 18.8 dBi and the minimum gain is 17.0 dBi. [ABSTRACT FROM AUTHOR]

In this article, the guided-wave modes and leaky-wave characteristic of rectangular metal strips are investigated to realize endfire radiation. By strategically exciting the high-order TE1 leaky-wave mode in the metal strip, a transverse in-phase $E$ -field could be realized to generate an endfire radiation. An effective scheme is validated to manipulate the phase constant and leaky rate by altering the width of the TE1 mode metal strip. Then, to further tailor the phase constant and leaky rate for achieving an enhanced directivity, periodic grooves are etched on both edges of the metal strip to slowdown the phase velocity to a surface-wave mode. To verify this concept, an all-metal planar prototype of corrugated rectangular metal strip (CRMS) was fabricated and measured. Both the full-wave simulation and experimental results show that the CRMS could offer a high endfire gain of 14.2 dBi within a footprint of $3.0\times 0.5\lambda _{0}^{2}$. Moreover, a broad overlapping bandwidth of 27.7% is realized with $S_{11} < -10$ dB, gain variation less than 3 dB, and total efficiency better than 90%. The proposed design scheme paves the way for the traveling-wave antenna with simple structure, wide bandwidth, high gain, and good platform accommodation. [ABSTRACT FROM AUTHOR]

This article presents a pattern-reconfigurable saber-like antenna with low wind drag for the onboard communication application, under the requirement of upper hemispherical coverage. The compact antenna mainly consists of a segmented loop and a $\Pi $ -shaped polygon, which are closely integrated for compact size. By controlling the OFF/ON states of a p-i-n diode embedded in the segmented loop, the proposed antenna can be operated at dipole/monopole modes. An upper hemispherical coverage area can be achieved since broadside and omnidirectional patterns are obtained at dipole and monopole modes, respectively. To validate the design strategy, a prototype was fabricated and tested. A good agreement between the simulated and measured results is achieved. The measured −8 dB reflection coefficient common bandwidth of two modes is from 2.16 to 2.63 GHz with the relative bandwidth of 19.6% while keeping a smaller size of $30.5\times 32.5\times 0.6$ mm3 ($0.24\,\,\lambda _{0} \times 0.26 \lambda _{0}\times 0.005 \lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at the center frequency). We portend that the proposed design scheme, with merits of low wind drag, metal ground compatibility, and upper hemispherical coverage, has the potential for airborne application. [ABSTRACT FROM AUTHOR]

In this article, a novel self-decoupled multiple-input multiple-output (MIMO) antenna pair with a shared radiator is proposed for fifth-generation (5G) smartphones. In our approach, a radiator is directly excited by two feeding ports, and interestingly, the two ports are naturally isolated across a wide bandwidth without using any extra decoupling structures. To offer a deep physical insight of the self-decoupling mechanism, a mode-cancellation method based on the synthesis of common and differential modes is developed for the first time. The proposed self-decoupled antenna pair shows a good isolation of better than 11.5 dB across the 5G N77 band (3.3–4.2 GHz) with a radiation pattern diversity property. Based on the self-decoupled antenna pair, an $8 \times 8$ MIMO antenna system, constituted by four sets of antenna pairs, is simulated, fabricated, and measured to validate the concept. The experimental results demonstrate that the proposed $8 \times 8$ MIMO system can offer an isolation of better than 10.5 dB between all ports and a high total efficiency of 63.1%–85.1% across 3.3–4.2 GHz. With the advantages of self-decoupling, shared radiator, simple structure, wide bandwidth, and high efficiency, the proposed design scheme exhibits promising potential for the future highly integrated MIMO antennas for 5G smartphones. [ABSTRACT FROM AUTHOR]

In this communication, a millimeter-wave (mm-wave) air-filled slot antenna with a conical beam at 60 GHz is proposed. The antenna is suitable to be integrated with various IC systems mainly because it is designed and fabricated based on bulk silicon microelectromechanical systems (MEMS) technology. The whole structure is composed of an antenna part and a waveguide to substrate integrated waveguide (SIW) converter. In the antenna part, four meander slots are arranged annularly and excited by a square air cavity. The conical radiation pattern benefits from the out-phase ${E}$ -field excited on slots. An operating bandwidth of 2.17% for S11 < −10 dB and a conical beam peak gain of 3.21 dBi are achieved. The proposed antenna shows its potentials in mm-wave indoor communication. [ABSTRACT FROM AUTHOR]

This article presents a wideband orthogonal-mode dual-antenna pair with a shared radiator for fifth-generation (5G) multiple-input multiple-output (MIMO) metal-rimmed smartphones. The wideband decoupling property of the dual-antenna pair is realized by the combination of the orthogonal monopole/dipole modes in the lower band and the orthogonal slot/open-slot modes in the higher band. With the orthogonal-mode design scheme, the dual-antenna pair shows a wide impedance bandwidth of 3.3–5.0 GHz and a high isolation of more than 21.0 dB across the entire band without using any external decoupling structures. By arranging four such dual-antenna pairs at two side edges of the smartphone, an $8 \times 8$ MIMO system is fulfilled. Both the simulation and measurement results show that the proposed $8 \times 8$ MIMO system could offer an isolation of better than 12.0 dB and an envelope correlation coefficient of lower than 0.11 between all ports. The measured average antenna efficiencies are 74.7% and 57.8% for the two antenna elements of the dual-antenna pair. We portend that the proposed design scheme, with merits of shared radiator, wide bandwidth, and metal rim compatibility, has the potential for the application of future 5G smartphones. [ABSTRACT FROM AUTHOR]

This communication presents a dual-mode dipole antenna for impedance bandwidth improvement. In our approach, an electrically small loop (ESL) resonator is properly loaded to compress the operating frequencies of first- and third-order modes. With the existence of ESL, the third-order mode shifts to its first-order mode to obtain a wide bandwidth with stable polarization and radiation patterns, due to the fact that ESL is equivalent to an orthogonal magnetic dipole at the third-order mode. To validate the design strategy, a prototype was fabricated and tested. Omnidirectional radiation pattern in H-plane is achieved over the entire operating band from 2.94 to 3.89 GHz while keeping a smaller size of $0.45\lambda _{0}\times 0.17\lambda _{0} (\lambda _{0}$ is the wavelength in free space at the center frequency). Good agreement has been obtained between measurement and simulation. [ABSTRACT FROM AUTHOR]

In this communication, a circularly polarized (CP) reconfigurable 2 bit antenna array is proposed for beam-steering applications. The proposed antenna array utilizes a novel CP reconfigurable 2 bit patch antenna as radiation element. For a reconfigurable 2 bit element, there should be four phase states (i.e., 0°, 90°, 180°, and 270°). In the proposed radiation element, a corner truncated microstrip patch antenna is presented for CP radiation. Owing to the symmetry of the patch antenna, the 0° and 180° phase states are achieved by selecting feeding points of the patch instead of using complex feeding network. A 90° digital phase shifter is cascaded to realize the 90° and 270° phase states for the 2 bit reconfigurable element. Based on the novel 2 bit CP element, an eight-element CP reconfigurable 2 bit antenna array is fabricated and tested to verify the design strategy at 3.65 GHz. The measured results match well with simulation. By appropriately controlling the ON and OFF states of p-i-n diodes, the proposed CP array can be steered from −49° to +49°. [ABSTRACT FROM AUTHOR]

In this paper, a rectangular dielectric rod antenna fed by an air-substrate parallel strip line is proposed. The dielectric rod has a compact and regular structure, which is easy to fabricate. The matching of the antenna is mainly determined by the parallel strip line, which simplifies the design process. Fed by an air-substrate parallel strip line, the antenna obtains a relatively uniform magnitude distribution on the surface and a moderate phase constant. With the length of 6 wavelengths at the center frequency of 6 GHz, the antenna generates a high measured gain of 16.7 dBi. The measured 1 dB gain bandwidth is about 18.3%. Over the entire operating band from 5.4 to 6.6 GHz, a stable endfire radiation pattern and a good matching are achieved by the antenna. [ABSTRACT FROM AUTHOR]

A broadband and high-gain endfire antenna array fed by an air-substrate parallel strip line is proposed in this communication. It has a simple and planar structure consisting of multiple dipole radiating elements and one parallel strip line. By utilizing the center-fed dipole working between the first and third resonances as the radiating element, the antenna array could obtain stable radiating capability over a broad bandwidth. Fed by an air-substrate parallel strip line working on the TEM mode, the antenna array achieves a moderate phase constant, which is helpful for realizing stable endfire radiation. The radiating element separation is optimized to eliminate the high-order spatial harmonics, ensuring single beam radiation. Fabricated by thin copper plates, the merits of the antenna array include lightweight, low cost, and easy fabrication. According to the measured results, the antenna array has a good match and a stable endfire radiation pattern over the operating band from 4 to 10.5 GHz with the measured gain varying from 10.1 to 13.5 dBi. [ABSTRACT FROM AUTHOR]

This paper presents a planar omnidirectional multiple-input multiple-output (MIMO) antenna with pattern phase diversity. The profiles of conventional dual-polarized omnidirectional MIMO antennas are too high to be planar integrated due to the limited bandwidth of vertical-polarized element. Therefore, a dual-channel horizontal-polarized MIMO antenna is proposed for the planar integration of omnidirectional MIMO antenna. The antenna is formed with four-element folded dipole array, and a compact feed network with two sets of 90° progressive phases is employed to feed the two channels with shared elements, thus the antenna can be integrated into a sheet of substrate with an ultralow profile of 0.8 mm. Although the polarization and radiation pattern (amplitude) of the two colocated channels are the same with each other, a high isolation is still achieved as a result of the orthogonal pattern phase. A prototype was fabricated to validate the performance. The measured highest isolation between two channels is 31.7 dB, and −10 dB isolated bandwidth is 2.29–2.57 GHz (11.7%) with S11 < −14 dB and envelope correlation coefficient less than 0.05, which shows a good diversity performance. Therefore, the proposed dual-channel antenna gives a feasible solution for the planar integration of the omnidirectional MIMO system. [ABSTRACT FROM AUTHOR]

In this communication, a dual-polarized 36-element ($3 \times 6 \times 2$) magnetic current array antenna is designed without using complex feeding network. Instead, a pair of differential feeds is adopted to excite the overall dual-polarized array within a low profile. Such a simple feeding strategy is achieved by using the TM90 mode of a microstrip antenna, which is loaded by $3 \times 4$ identical half-wavelength slots for each polarization. In this configuration, an in-phase $3 \times 6$ element single-polarized magnetic current array is achieved for high gain, but with narrow bandwidth due to the high-order mode. By trivially 90° rotating and combining, the proposed dual-polarized, high-gain, and low-profile magnetic current array is designed, constructed, and measured. Within a total size of $2.92\lambda _{0}\times 2.92\lambda _{0}\times 0.085\lambda _{0}(\lambda _{0}$ is the free-space wavelength at the center frequency), the measured maximum gain is 15.5 dBi, also with the merits of low profile, low sidelobes, simple feeding, and high port isolation, exhibiting promising usage in MIMO and diversity applications. [ABSTRACT FROM AUTHOR]

This paper presents a novel high-gain endfire slot antenna array fed by air-substrate microstrip line. It has a simple and compact structure consisting of a copper strip and two copper sheets. Designed based on the slot antenna array, the antenna could be embedded into a large metallic platform. The surface wave above the antenna radiates the energy to free space. Fed by the air-substrate microstrip line, the antenna works on the TEM mode. Therefore, the antenna obtains a suitable phase constant to realize stable endfire radiation pattern and could be designed by a long structure to realize high endfire gain. Because all of the antennas are designed by metal, it has the characteristics of high strength, easy fabrication, and low cost. With the radiating aperture of 3.5 wavelength, the antenna achieves a measured endfire gain of 11.81 dBi. The measured 1 and 3 dB gain bandwidths reach 4.5% and 8.4%, respectively. Good matching and stable endfire radiation pattern are obtained over the entire operating band from 3.75 to 4.15 GHz. [ABSTRACT FROM AUTHOR]

This communication presents a novel single-fed compact circularly polarized (CP) dual-layer planar inverted-F antenna (PIFA). The proposed antenna is composed of two stacked orthogonal-orientated PIFAs which can be fabricated by elaborately folding one piece of metal plate. The two PIFAs can be excited synchronously with equally allocated power by a single probe, thus a pair of orthogonal polarizations is created. Moreover, the resonant lengths of the upper and lower PIFAs are slightly different from each other to acquire 90° phase shift between the two orthogonal polarizations. Consequently, a right-hand CP radiation is achieved, and the boresight realized gain is 3.8 dBic at the center frequency. The proposed dual-layer PIFA with two quarter-wavelength resonators provides a novel solution for the compactness of CP antennas. To validate the performance of the antenna, a prototype was fabricated and measured. The measured −10 dB impedance bandwidth and 3 dB axial ratio bandwidth are 7.4% and 1.1%, respectively, with a compact antenna volume of $0.26\times 0.22\times 0.05\,\,\lambda _{0}^{3}$. The proposed antenna is a good candidate for the 433 MHz CP active radio-frequency identification reader, and it has the merits of compact size, lightweight, high efficiency, and low cost. [ABSTRACT FROM AUTHOR]

In this communication, we propose a new type of compact and wideband coplanar waveguide (CPW)-fed microstrip antenna. Very different from traditional microstrip antennas, the proposed design has the quasi-periodic radiating aperture fed through the slot-to-CPW transition. By optimizing the geometry of the quasi-periodic aperture and the slot-to-CPW transition, the fundamental TM10 mode and antiphase TM20 mode can be efficiently excited, coupled, and matched over a broad bandwidth. We have fabricated and characterized the proposed antenna, and found a good agreement between measurement and simulation results. Our experimental results show that despite the antenna has a thickness of only $0.052\lambda _{0}$ ($\lambda _{0}$ is the free-space wavelength at the center frequency), it exhibits a −10 dB impedance bandwidth of 36.5% (3.96–5.73 GHz), a 3 dB gain bandwidth of 35.4% (4–5.72 GHz), and a maximum available gain of 10.45 dBi. Our findings may offer a feasible solution to realize wideband microstrip antennas using a single-layer substrate, with unique merits, such as low profile, low cost, and ease of fabrication. [ABSTRACT FROM AUTHOR]

This communication presents a microstrip-fed surface-wave antenna for endfire radiation. The antenna has the advantages of high endfire gain, wide endfire gain bandwidth, and stable endfire radiation pattern. It consists of a microstrip line with air media and a dielectric cuboid. The half-mode dielectric waveguide, which is made up by the dielectric cuboid and the air substrate introduced by the microstrip line, is the radiating structure of the antenna. Fed by a microstrip line, the half-mode dielectric waveguide couples the energy continuously with a moderate phase constant, which is helpful for the antenna to realize a high endfire gain and a stable endfire radiation pattern. With a low dielectric constant, the half-mode dielectric waveguide has a stable radiation performance within a wide bandwidth. With a length of 5.76 wavelength, the antenna achieves a maximum endfire gain of 13.3 dBi at 4.75 GHz. The 1 and 3 dB gain bandwidths of the antenna is 18.1% and 37.8%, respectively. Good matching is obtained from 3 to 6.5 GHz. [ABSTRACT FROM AUTHOR]

In this communication, the novel compact tightly arranged pairs are employed to form a $4 \times 4$ multiple-input-multiple-output (MIMO) system and an $8 \times 8$ MIMO system operating at 3.4–3.6 GHz for fifth-generation (5G) mobile phones. Each tightly arranged pair is composed of a bent monopole and an edge-fed dipole with a compact size of $7 \times 12$ mm2. The orthogonal-mode method is presented to mitigate the mutual coupling of the tightly arranged pairs without any external decoupling structure. With the help of the orthogonal mode, isolation performances across the desired band of the $4 \times 4$ MIMO system and the $8 \times 8$ MIMO system are better than 20 and 17 dB, respectively, with the elements closely spaced. The measured efficiencies are 51.7%–84.5%/49%–72.9%, and the measured envelope correlation coefficients are less than 0.06/0.07 for the $4 \times 4/ 8 \times 8$ MIMO system. The proposed MIMO systems provide a promising solution to compact 5G MIMO mobile phone antennas with good isolation and diversity performance. [ABSTRACT FROM AUTHOR]

This communication presents a periodic leaky-wave antenna array with the advantages of a high endfire gain at the center frequency and a stable endfire radiation beam over the entire operating band. The antenna has a narrow width and consists of multiple monopoles located on the microstrip line. It can be fabricated by line-cutting copper strips. The antenna has a high endfire gain because it is a leaky-wave antenna designed based on the Hansen–Woodyard condition. With a length of $5.5\lambda _{0}$ and a width of $0.33\lambda _{0}$ , a measured endfire gain of 12.9 dBi is achieved at the center frequency of 5 GHz. Good matching and stable endfire radiation beams are achieved over the entire operating band from 4.5 to 5.5 GHz. Good agreement between the measurement and the simulation has been obtained. [ABSTRACT FROM AUTHOR]

A planar air-filled terahertz antenna array based on channelized coplanar waveguide (CCPW) is proposed and fabricated using silicon bulk micromachining technology. Planar air-filled four-unit array composed of two silicon layers is verified at 300 GHz. Periodical field blocking stubs are loaded in the gaps of the CCPW alternatively to break the fringing field symmetry, generating an in-phase radiating aperture. The fabrication technology has two features: first, the silicon dielectrics are completely plated with gold, generating a purely air-filled antenna with high performance and second, the hierarchical micromachining method enables a single silicon layer to be divided into two sublayers, resulting in reduced antenna layers. The numerical result shows a simulated −10 dB impedance bandwidth of 71.1 GHz (25.95%, 238.4–309.5 GHz) and stable fan-shaped beams with broadside gains and radiation efficiencies higher than 10.6 dBi and 87.9%, respectively. Experiment of the fabricated prototype exhibits a bandwidth that totally covers the simulation and stable beams with broadside gains higher than 8.6 dBi. To the best of author’s knowledge, this is the first time that a planar air-filled terahertz antenna array is proposed. Moreover, confirmation of the strategy demonstrates the antenna’s distinguished suitability to realize antenna-in-package solution. [ABSTRACT FROM AUTHOR]

In the massive multiple-input multiple-output systems, the antennas should have the ability of beamforming. Under this requirement, the sliding aperture technique is adopted. In this paper, the principle of the sliding aperture technique is first demonstrated. This technique refers to the customization of each beam by part of the aperture, resulting in low scan loss over a wide angular range. Then, with the help of II-shaped transmit element, a linear multibeam transmitarray antenna based on the sliding aperture technique is designed. To the best of the authors’ knowledge, the linear transmitarray is reported for the first time. The fabricated prototype shows that the proposed antenna can generate eight beams simultaneously covering the ±42° azimuth area with low scan loss and the beam intersection level better than −4 dB. The proposed antenna has been simulated, fabricated, and measured, and the experimental results are in good agreement with the simulation results. [ABSTRACT FROM AUTHOR]

A low-profile endfire leaky-wave antenna is proposed in this paper. The proposed low profile antenna utilizes microstrip line to excite planar inverted-L antenna (PILA) elements. Air media is utilized in the proposed array to realize the endfire property. By using air media, the proposed leaky-wave array maintains phase constant slightly larger than the wavenumber in free space and achieves endfire radiation. PILA elements are periodically spaced to radiate and the spacing period is designed as \lambda 0 /4 ( \lambda 0 is the wavelength in free space) to achieve a relatively wide impedance bandwidth. A prototype of the proposed endfire antenna is fabricated and tested. The measured results are consistent with full-wave simulation results. The length of the whole leaky-wave array is about 6\lambda 0 , and the measured gain in the endfire direction is 11.5 dBi at 5.1 GHz. [ABSTRACT FROM PUBLISHER]

In this paper, the scanning performance at a fixed frequency of the 2-bit linear array is presented. Then, for the first time, a microstrip line that supports a traveling wave is used as bit phase sources with any bit number to construct reconfigurable fixed-frequency beam steering bit arrays. Using the \text TM1n0\,\,(n = 0.5 ) open cavity as the radiating element, a ten-element reconfigurable 2-bit fixed-frequency beam steering array operating at 5 GHz is designed and fabricated to verify the idea. By sampling two feed points with a separation distance of a quarter wavelength on one side of the microstrip line, the phase states of 0° and −90° are obtained; other two counterpart points on the other side of the line can offer the phase states of −180° and −270°, producing the required 2-bit unit. Five prototypes representing five array configurations show a measured scan range of 94° between −48° and +46° at 5 GHz, and the measured five beams show good quality and consistency: the peak gains range from 10.01 to 12.33 dBi with a gain variation of only 2.32 dB and the 3 dB beam widths vary between 13° and 17°. [ABSTRACT FROM PUBLISHER]

A fixed-beam leaky-wave slot antenna with an air-filled back cavity is proposed in the millimeter-waveband. The antenna is fabricated based on the bulk silicon microelectromechanical systems micromachining technology. This micromachining process can etch silicon wafers and plate them with gold. Because all surfaces of the proposed antenna are plated with gold, electromagnetic waves can transmit in the air medium without any other dielectric. The air-filled structure contributes to achieving a leaky-wave slot antenna with fixed beam as operating frequency varies. A prototype of the proposed antenna is fabricated to verify the design strategy. The measured results show that the impedance bandwidth of the proposed antenna is 55–67 GHz and within the operating frequency band, the main beam direction only varies from 48° to 53°, which confirms the fixed-beam characteristic. The measured peak gain varies from 9.1 to 10.5 dBi in the operating frequency band. [ABSTRACT FROM AUTHOR]

A low-sidelobe air-filled slot array is proposed and fabricated using silicon micromachining technology for millimeter-wave application. As is well known, the high-permittivity silicon prohibits the silicon-based antenna from having good performance. Here, this problem is addressed through manufacturing air-filled antenna on silicon substrate. An eight-way unequal power divider is adopted to excite eight air-filled slots and the excitation coefficients present a taper. At the end of the power divider, an air-filled substrate integrated waveguide cavity is used to improve antenna performance and suppress wave leakage, and eight silicon cuboids are stretched into the cavity beneath each slot to acquire good impedance matching. The input interface is selected as the ground-signal-ground structure to facilitate antenna-in-package solution that requires the integration of the antenna and chips or integrated circuits. The three-layered antenna is fabricated using through-wafer etching, gold-plating, optical alignment, and wafer bonding processes. The measured −10 dB impedance bandwidth is 1.27 GHz, and a measured fan-shaped beam at 58.5 GHz with a broadside gain of 11.95 dBi and a sidelobe level of 18.6 dB is obtained. The 3-dB beamwidth is about 14° in H-plane and 8-dB beamwidth is more than 175° in E-plane. [ABSTRACT FROM PUBLISHER]

An air-filled long slot leaky-wave antenna (LWA) based on folded half-mode waveguide (FHMW) fabricated using silicon substrate is proposed for millimeter-wave application. As is well known, the high-permittivity silicon dielectric is not suitable for antenna design. Thanks to the through-wafer dry etching and gold-plating processes deriving from the silicon bulk micromachining technology, three purely air-filled structures including the vertical part of the FHMW that also acts as the leaky-wave long slot in the top layer, horizontal part of the FHMW and matching section in the middle layer, and coupling slot in the bottom layer constitute the high-performance air-filled long slot LWA. To the best of the authors’ knowledge, this is the first time that an FHMW is adopted for antenna design. Compared with the conventional half-mode waveguide, the profile is lowered, the required silicon layer number is fixed to three, and the design can be more flexible. Experiment of the fabricated prototype shows that the main beam can be scanned from 41° to 49° with a gain variation between 13.15 and 15.41 dBi in the frequency range from 56 to 64 GHz. Moreover, confirmation of the design strategy provides the feasibility to realize the system-in-package solution. [ABSTRACT FROM AUTHOR]

A new antenna structure is proposed to show that a single-layer magnetic current array based on the microstrip line structure can achieve a relatively uniform and large in-phase radiating aperture together with high realized aperture usage rate (RAUR). By alternatively and periodically loading the microstrip line with blocking stubs, the equivalent magnetic currents along one direction are suppressed while the currents along the other direction are left, forming a relatively uniform and in-phase 1-D magnetic current array. The electromagnetic wave is radiated into space while traveling forward along the line. By in-parallel concatenating a series of the 1-D arrays with adjacent arrays presenting mirror relations, a large in-phase radiating aperture is built with a relatively high RAUR performance. Moreover, the shorting walls at the interconnecting planes of the neighboring 1-Darrays can be removed attributing to the antenna arrangement style and the required feed network. Eight rows of the 24-element 1-D magnetic current array fed using a substrate integrated waveguide-based eight-way power divider fabricated using a single substrate board is simulated and built to demonstrate the design strategy. The prototype shows a measured bandwidth of 4.37% and a peak gain of 25.1 dBi at 20 GHz. The RAUR is 8.66/ \lambda 0^{2} , which is higher than most of the single-layer high-gain antennas. [ABSTRACT FROM PUBLISHER]

A new structure of horizontally polarized (HP) antenna array is presented for azimuthal omnidirectional coverage. The proposed antenna array is engineered by breaking the fringing field symmetry along the typical parallel strip-lines, which is implemented by loading field-block structures alternatively and periodically. In physics, the blocking structures constrain the fringing fields inside, producing a series of rectangular open cavities operating at the \rmTM1n0 ( $n = 0.5$ ) mode. Counterintuitively, these asymmetric open cavities are able to generate an omnidirectional radiation pattern in the $E$ plane with horizontal polarization, similar to the linear magnetic current array. In order to verify the design strategy, a five-unit center-fed cascaded cavity array is fabricated and tested. The measured bandwidth is 120 MHz (5.83–5.95 GHz) with reflection coefficient lower than −10 dB. The HP omnidirectional pattern at 5.9 GHz provides a gain variation of 1.94 dB ranging from 6.7–8.64 dBi. [ABSTRACT FROM PUBLISHER]

A triangular-cavity-cascaded antenna is proposed and analyzed for wideband applications. By alternatively and periodically shorting a metallic plate on its edges with small slices, isosceles-triangular field patterns are generated and cascaded for wideband antenna design. Synchronous electric edge fields which contribute to radiation occupy almost all the areas of both sides of the plate. Compared with a typical microstrip antenna with a rectangular cavity, the triangular-cavity-cascaded antenna is a relatively open structure with less shorting boundary around. As a result, the quality factor is smaller, and the impedance bandwidth is wider. In order to prove the design strategy, a 1-D cascaded antenna with $1 \times 5$ triangular cavities and a 2-D cascaded antenna with $2 \times 5$ triangular cavities are fabricated and tested. Experimental results show that the impedance bandwidths of 1-D and 2-D antennas are 35.66% and 21.43%, while the peak gains are 13.27 and 16.47 dBi, respectively. [ABSTRACT FROM AUTHOR]

In this communication, an all-metal standing wave antenna array is proposed based on the microstrip transmission line. By alternatively blocking the microstrip line every half wavelength, the symmetrical equivalent magnetic currents are broken for effectively radiating. By properly tuning the blocking positions along the microstrip line, a broadside pattern is achieved with high gain. The proposed antenna is scalable along one direction to a certain level to acquire the directivity as high as about 15 dBi. More importantly, the proposed antenna array has the potential to be built only by metal without any dielectrics. The all-metal structure has the merits of being low-cost, light weight, easily fabricated, and having no dielectric losses. Based on this idea, a five-segment linear array with a center feed is designed and tested. The measured bandwidth is from 7.83 to 8.81 GHz (980 MHz, 11.78%), and the measured gain is ranging from 9.1 to 12.47 dBi. [ABSTRACT FROM AUTHOR]

This communication proposes a compact single-feed dual-mode antenna for active radio-frequency identification (RFID) tag application. The proposed antenna consists of a slotted patch on an FR4 substrate, a feeding microstrip line, and a foam spacer with a volume of only 60\;\textmm \times 50.3\;\textmm \times 6\;\textmm ($0.48{{\lambda}_0} \times 0.40{{\lambda}_0} \times 0.048{{\lambda}_0}$, where ${{\lambda}_0}$ is the wavelength at the center frequency). It operates as a dipole in free space and as a patch when placed on a metallic surface. A prototype that can provide an impedance bandwidth of 390 MHz (2.28–2.67 GHz) in free space and 470 MHz (2.32–2.79 GHz) on a metallic surface is built and measured. The measured radiation patterns, peak gains, and radiation efficiencies in these two scenarios are provided. [ABSTRACT FROM PUBLISHER]

A six-beam switched beam antenna (SBA), whose beam patterns are shaped in the azimuth plane, is designed and fabricated. A synthesis method, which is based on a four-elements array and can generate a flat-top beam pattern with 46° flat gain region (gain fluctuation within \pm \mathbf0.5\,\mathbfdB), 60° 3-dB beam width, and less than - \mathbf20.1\,\mathbfdB side lobe level (SSL), is proposed. By combining six of those beams together, the proposed SBA can provide near consistent gain and good interference rejection in the azimuth plane. Meanwhile, an interleaving array architecture, which minimizes the size of the proposed array, is also implemented. To extend the beam coverage, a series-fed in-phase array with tapered amplitude distribution is introduced in the elevation plane. The peak gain of the beam reaches 16.6 dB, and the SLL is controlled under - 23.2\,\mathbfdB in this plane. [ABSTRACT FROM PUBLISHER]

A wideband printed dipole with V-shaped ground branches is proposed, which is designed for multiple-input multiple-output (MIMO) antennas. It is based on a dipole with an integrated balun, and V-shaped ground branches are introduced to improve the impedance matching. The bandwidth of this element reaches 62.3% (2.30–4.40 GHz), which covers all three WiMAX bands (2.30, 2.50, and 3.30 GHz) and the 2.40 GHz WLAN band. Based on this element, a quad-element MIMO antenna is designed and fabricated. By reusing the V-shaped ground structure between adjacent elements, the size of this quad-element antenna is only $0.31 \lambda \times 0.31\lambda \times 0.01\lambda $. Meanwhile, a bandwidth of 60.6% (2.30–4.30 GHz) is achieved, in which the ${{\rm{S}}_{11}} < - 10\;{\rm{dB}}$, ${{\rm{S}}_{12}} < - 10\;{\rm{dB}}$, and ${{\rm{S}}_{13}} < - 13\;{\rm{dB}}$. Directional radiation patterns with 2.1 dBi average gain are attained, which are very stable throughout this band. This antenna system can be suitable for multielement MIMO devices such as wireless routers and adapters. [ABSTRACT FROM PUBLISHER]

It is a challenging and tough task to achieve and tune multiple frequencies for handset antennas in a small area. In this communication, we have proposed a novel modes controlling method to build and tune the handset antenna. By combining different modes of an open slot and different monopole branches, a hepta-band, covering GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500, handset antenna is achieved in a small area of 8\,\times\,60 mm^2. The most essential merit of the proposed antenna is that the related modes can be added step by step, according to the operating bands. The modes for the lower and upper bands can be easily tuned and optimized. We have also built a prototype of the proposed antenna to validate the design strategy. The tested results include reflection coefficient, radiation patterns, efficiency, and gain. [ABSTRACT FROM PUBLISHER]

This communication proposes a wideband differential-fed slot antenna that works in its dominant mode (\lambda/2 mode). The antenna has a near omnidirectional radiation pattern with vertical polarization, which is designed for near-ground wireless sensor node applications. A compact integrated T-slot balun and a loop to T-slot feeding structure are introduced to realize the differential feeding; the bandwidth is expanded to 30.7% (2.2 GHz–3.0 GHz) with a 3.8 dB peak gain. A detailed matching method without using lumped elements is proposed, which can be applied to various types of RFIC chips. A prototype is built and measured. Its differential impedance, antenna patterns, and gain are consistent with the simulation results. [ABSTRACT FROM PUBLISHER]