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For the first time, liquid metal is incorporated with an elastomer platform for designing an electromagnetic bandgap (EBG)-backed wearable antenna that can be placed on human body. Here, the liquid metal (EGaIn) is injected into a soft and stretchable Ecoflex 0030 substrate for making a fluidic slot antenna and a liquid EBG (LEBG) structure. The LEBG is then placed at the back of the slot antenna for mitigating the effects of human body and improving the antenna performance. It has an antenna gain of 5.80 dBi in the boresight and a low specific absorption rate (SAR for 1 g U.S. standard) of 1.25 W/kg. Since liquid metal is highly dense, the ground plane of the LEBG is separated into six segments, which are connected through narrow bridges. Due to the use of such design, the wobbly effect of the dense liquid metal can be reduced significantly. The measured impedance bandwidth (3.23–3.59 GHz) of the proposed antenna is very stable and not affected much by its own structural deformation on human body. Since the elastomer platform is waterproof, it makes the antenna characteristics very stable in humid environment, which is a very desirable feature for wearable antennas. [ABSTRACT FROM AUTHOR]

The analysis, modeling, design, simulation, and experimental evaluation of a 400 GHz on-chip antenna is presented, with a novel combination of metastructures, a microstrip patch, a quartz-based dielectric resonator, and a diamond-based antireflex layer—all integrated on a 35 nm InGaAs metamorphic high-electron-mobility transistor (mHEMT) technology. The said combination represents a first-time implementation for all submillimeter-wave-capable semiconductor technologies. Circumventing a substrate-thickness limitation of $4.98 \mu \text{m}$ , a state-of-the-art broadband, efficient, and to-the-broadside radiating on-chip antenna solution is realized. It achieves a measured impedance bandwidth of 100 GHz, 25.6%, spanning from 340 to 440 GHz. A consistent pattern bandwidth of 75 GHz is recorded, with an efficiency of 50%–66% and a directivity of up to 10.4 dBi—or 27 dBi, with the utilization of a polypropylene-based dielectric lens. The theoretical analysis of the proposed on-chip antenna is presented, and two modeling approaches are shown and compared. Between the analytical and the 3-D electromagnetic (EM) model, the latter is chosen as it offers greater precision in defining the metastructure unit cell and enables the inclusion of the remaining components of the proposed antenna setup. The measured reflection coefficient and far-field patterns are compared to simulations via the utilized model, and a strong agreement is observed. These far-field patterns are acquired with the on-chip antenna inserted within a broadband 400 GHz transmitter submillimeter-wave monolithic integrated circuit, processed on the 35 nm mHEMT technology. [ABSTRACT FROM AUTHOR]

Based on the newly suggested wideband circularly polarized (CP) aperture-coupled magneto-electric dipole (MED), a millimeter-wave (mm-wave) frequency-scanned array (FSA) is proposed in this article. The MED is printed on two substrate layers. The electric dipole is two metal patches with perturbation structure, while the magnetic dipole is two rows of metal vias passing through the top substrate layer. The electric and magnetic dipoles, fed by a coupling slot, excite CP wave. By loading an arc-shaped vias row (AVR) at both ends of the magnetic dipole, the CP bandwidth with axial ratio (AR) less than 3 dB is extended to 40.9%. A new compact wideband transition from the rectangular waveguide (RWG) to the substrate integrated waveguide (SIW) is suggested as the feed structure for the CP FSA. A slow wave structure is etched on the ground of SIW to widen the beam scanning range. A 16-element array is designed and measured. The measured results show that the beam scanning range is 56° over 23–33 GHz band with a peak gain of 19.5 dBi. The CP mm-wave FSA has the advantages of wide scanning angle, low profile, and easy fabrication. [ABSTRACT FROM AUTHOR]

A novel dual-beam fixed-beam leaky-wave antenna (LWA) is proposed in this article. This antenna has a very simple structure, which is basically a grounded coplanar waveguide (GCPW) printed on a specially shaped printed circuit board (PCB). Its working principle is that, for a GCPW with no ground vias (leaky GCPW), there will be a parallel plate waveguide (PPW) mode between its two ground layers, which couples with its GCPW mode. Because the GCPW mode has a phase velocity higher than that of the PPW mode, leaky-wave radiation will be generated from the GCPW to the PPW. This radiation is then radiated into the free space through a metal flare. Since the GCPW is nearly non-dispersive, the beam direction of this radiation is inherently not frequency scanning. Besides its simplicity, the proposed antenna has a pretty good performance. Measured results show a 3 dB realized gain bandwidth and −10 dB impedance bandwidth of larger than 26–40 GHz, a peak gain of 17.5 dBi, and a negligible beam scanning range of only 2.5° (±1.25°) across 26–40 GHz. [ABSTRACT FROM AUTHOR]

In this communication, a high-order-mode (HOM)-fed open-ended rectangular waveguide antenna with high gain and broadband is proposed for fifth-generation (5G) applications. A miniaturized HOM cavity with four slots introduces hybrid modes of $TE_{340}$ and $TE_{140}$ , which act as a low loss feeding network. Furthermore, an open-ended waveguide is loaded above the HOM cavity to widen the impedance bandwidth. Compared with $TE_{340}$ HOM, hybrid modes of $TE_{340}$ and $TE_{140}$ HOM provide more compact size and higher gain. For the proof of concept, a $4\times $ 4 antenna array is designed and fabricated. The measured impedance bandwidth is 28.1% from 32.85 to 43.6 GHz. The measured peak gain is 21.7 dBi. Great compatibility between simulated and measured results is achieved. [ABSTRACT FROM AUTHOR]

This communication presents a novel bifunctional metasurface (MS) combining the reflectarray (RA) and transmitarray (TA) with dual-band orthogonally polarized dual-beam characteristics. The proposed reflect-transmit-array (RTA) is capable of reflecting the $x$ -polarized incidence without changing its polarization state around 10 GHz while transmitting the $y$ -polarized incidence and converting it into the circularly polarized (CP) waves around 14 GHz. To completely cover the working frequency bands of both modes, an antipodal Vivaldi antenna (AVA) is introduced as the source feed due to its intrinsic ultrawideband characteristics. As a proof of concept demonstration, a prototype consisting of 20 $\times20$ elements is also designed, fabricated, and measured. We observe a good agreement between the measurement and simulation results that shows a peak gain of 26.2 dBi and 27.3 dBic, 1 dB gain bandwidth (BW) of 9.8–11.3 GHz (14.2%) and 12.8–14.2 GHz (10.4%), and peak aperture efficiency (AE) of 48.7% and 35.1% for the reflection and transmission modes, respectively. [ABSTRACT FROM AUTHOR]

A recently developed material technology at Corning Inc., namely, Alumina Ribbon Ceramic (ARC), is investigated as a potential substrate candidate for fifth generation (5G) and millimeter-wave (mm-wave) applications. In this article, we characterize an 80- $\mu \text{m}$ -thick ARC substrate material in the frequency range of 3–50 GHz using microstrip ring resonator (MRR) method and extract the loss of various planar transmission lines, such as microstrip and coplanar waveguide (CPW) lines on ARC substrate up to 50 GHz as well. Moreover, different test structures designed on an 80- $\mu \text{m}$ -thick ARC substrate are utilized to extract the electrical parasitics associated with a single-grounded through-alumina via (TAV) and transmission properties of ground–signal–ground (GSG) TAV of 40 $\mu \text{m}$ diameter over the frequency range of 0.1–50 GHz. The semiadditive patterning (SAP) process is employed to metallize the top and bottom layers of ARC substrate to form the copper traces for the designed structures. Combined with the previous characterization results of a 40- $\mu \text{m}$ -thick ARC substrate in 30–170 GHz, the dielectric constant of ARC was extracted to be ~10.12 over 3–170 GHz, while its loss tangent varies in the range of $6.6\times 10^{-5}-1.3\times 10^{-3}$ over the same frequency band. The average measured insertion loss of CPW lines varied from 0.026 to 0.24 dB/mm over 3–170 GHz. For the microstrip lines, the average measured insertion loss over the same frequency range was extracted to be between 0.019 and 0.293 dB/mm. Furthermore, the parasitic inductance and resistance of a single-grounded TAV are extracted to be 24.41 pH and $0.987~\Omega $ , respectively, at 50 GHz, and the insertion loss per a GSG-TAV extracted from CPW-TAV daisy chain measurements is found to be 0.056 dB at 50 GHz as well. In addition to an excellent performance of ARC-based interconnects, the TAV in ARC is shown to have lower transmission loss up to 50 GHz, while exhibiting low parasitics. [ABSTRACT FROM AUTHOR]

This article discusses three integration and packaging techniques for gap waveguides and monolithic microwave integrated circuits (MMICs) suitable for multilayer waveguide applications. Two vertical transitions between microstrips and ridge gap waveguides (RGWs) are presented. The first vertical transition connects RGW to a microstrip line from the top where a rectangular patch has been used. Measured results of the transition in a back-to-back structure show that the reflection coefficient is better than −10 dB from 75 to 83 GHz, and the insertion loss for a single transition over the frequency band is 0.65–0.85 dB. The second vertical transition connects RGW to a microstrip line from the back by a slot in the ground plane. Measured results of the transition in a back-to-back structure show that the reflection coefficient is better than −10 dB from 69 to 86 GHz, and the insertion loss for a single transition is 0.65–1 dB over the frequency band. Commercially available E-band MMIC amplifiers are integrated with RGWs using the two proposed transitions. Moreover, for the very first time, the integration of MMIC with inverted microstrip gap waveguide (IMGW) is realized by a compact packaging structure that utilizes bond wires and capacitive pads. All the three active integrations are consistent with the passive measurements in terms of operational bandwidth, losses, amplifier gain flatness, and unwanted resonance suppression. [ABSTRACT FROM AUTHOR]

This article presents shear horizontal surface acoustic wave (SH-SAW) devices with excellent temperature stability and low loss on ultrathin Y42-cut lithium tantalate film on sapphire substrate (LiTaO3-on-sapphire, LTOS). The demonstrated resonators exhibit scalable resonances from 1.76 to 3.17 GHz, effective electromechanical coupling coefficients between 5.1% and 7.6%, and quality factors (Bode-Q) between 419 and 3019. The filter with a center frequency of 3.26 GHz features a suppressed spurious passband, a 3-dB fractional bandwidth (FBW) of 3%, and a minimum insertion loss (IL) of 2.39 dB. In addition, coplanar waveguides (CPWs) and SH-SAW resonators built on LTOS and LiTaO3-on-insulator (LTOI) substrates were compared over a temperature range of 25 °C–150 °C. Due to the extremely high resistivity of the sapphire and the excellent thermal stability of the LiTaO3/sapphire interface, the IL of the CPW and the impedance ratio (in addition to Bode-Q) of the SH-SAW on the LTOS are maintained well even at 150 °C, while those on the LTOI seriously deteriorate. Of these, the impedance attenuation of LTOS-SAW at the antiresonant frequency is only 3.7 dB at 150 °C, whereas that of LTOI-SAW reaches 9.6 dB, demonstrating excellent temperature stability of the LTOS substrate’s radio frequency (RF) performance. Overall, the SAW devices on LTOS substrates show great potential for temperature-sensitive and low-loss applications in RF wireless communications. [ABSTRACT FROM AUTHOR]

A wideband metasurface antenna operating at millimeter-wave spectrum in substrate-integrated waveguide (SIW) technology is proposed. Coupled by a slot etched on an SIW section, the proposed metasurface antenna achieves a wide 10 dB impedance bandwidth in simulation from 19.2 to 42.4 GHz, indicating a fractional bandwidth of 75%. The development of the proposed metasurface antenna can be divided into four stages, i.e., a slot antenna, dielectric loading, adding covered metasurface, and connecting to a transition. The input impedance and the resonance of these stages are elaborated in detail to explain the working mechanism. Parametric study is carried out to inspect into the design process. By analyzing the $E$ -field distributions, the proposed metasurface antenna is characterized by TM10 mode, antiphase TM20 mode, and TM30 mode. Compared with other similar designs in the open literatures, the proposed metasurface antenna displays a widest impedance bandwidth. The dimension of the proposed metasurface antenna is $0.80\,\,\lambda _{0} \times 0.72\,\,\lambda _{0} \times 0.24\,\,\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at 30 GHz. The maximum gain within the interested frequency spectrum is 7.0 dBi. [ABSTRACT FROM AUTHOR]

We report, here, the design and experiments of dual-wideband dual-circularly polarized (dual-CP) shared-aperture reflectarrays (RAs) operating at the K-/Ka-band utilizing a single functional substrate. By coupling the adjacent but separated resonances of the H-shaped aperture and circular patch, and utilizing the shorted microstrip phase delay lines, the proposed RA cells can offer wideband performance. Then, these RA cells are interleaved in a shared aperture for the dual-band operation. By combining the dynamic phase and Berry phase compensation techniques, the RA cells can independently control the reflective phase delay of orthogonal CP beams at both of the targeted bands, providing four degrees of freedom for beamforming. The diffractive property of an infinite periodic gradient array of the interleaved K- and Ka- band cells is investigated, which verifies the wideband performance of the proposed cells. Two RAs with a thin panel thickness of $0.27\lambda _{K}$ ($\lambda _{K}$ is the free-space wavelength at the K-band) are fabricated and characterized, generating symmetrically and asymmetrically distributed dual-wideband dual-CP beams at the K-/Ka-band, respectively. The measured bandwidths for the two RAs are wider than 14.1% for the K-band right-handed CP (RHCP)/left-handed CP (LHCP) beams and about 11% for the Ka- band RHCP/LHCP beams with both the gain variation and axial ratio of smaller than 2 dB. [ABSTRACT FROM AUTHOR]

In this article a wireless passive temperature sensor is designed, fabricated, and tested. The sensor uses a contactless air-filled substrate-integrated waveguide (CLAF-SIW) resonator that consists of an air cavity surrounded by a low-impedance electromagnetic band gap (EBG) structure. The contactless feature of the sensor mitigates the fabrication complexity issue of air-filled SIW (AF-SIW) while it benefits from the high-quality factor of an air-filled resonator. The resonant frequency of the CLAF-SIW resonator is dependent on the permittivity of the substrate and the dimensions of the structure which are both temperature-dependent. Change in temperature results in a variation in the resonant frequency of the sensor. As such, we measure temperature by measuring the resonant frequency of the sensor. The developed CLAF-SIW sensor has a measured unloaded quality factor of 1340 that allows for long-range wireless measurements. The resonant frequency of the sensor is measured using a ringback-based wireless interrogation system. The interrogation system energizes the sensor using radio frequency (RF) pulses and determines the frequency of the ringback signals radiated by the sensor using frequency domain analysis. The sensor has a sensitivity of 300 kHz/ $^\circ \text{C}$ and an interrogation-to-sensor distance of 80cm. [ABSTRACT FROM AUTHOR]

In this communication, a broadband substrate integrated waveguide (SIW)-fed circularly polarized (CP) cross-dipole antenna is proposed for millimeter-wave (MMW) applications. Its radiation parts consist of a cross dipole and reflecting ground, which are etched on two substrates. The antenna is fed by an SIW through a double-sided parallel-strip line (DSPSL). In addition, zigzag edges and L-shaped etches are utilized to expand the impedance bandwidth (IBW) and axial-ratio bandwidth (ARBW), respectively. By these methodologies, the antenna prototype possesses a measured IBW of 34.6% from 22.5 to 31.9 GHz, an ARBW of 23.1% from 24 to 30.25 GHz, and a peak gain of 8.8 dBic at 29.5 GHz. In comparison to general coaxial-line-fed cross-dipole antennas with a nonplanar topology and high profile of $0.25\lambda _{0}$ ($\lambda _{0}$ is the free-space wavelength at the center frequency), the proposed antenna with PCB process maintains broadband and owns a low profile of $0.16\lambda _{0}$ , which makes it more desirable for MMW integrated circuits. [ABSTRACT FROM AUTHOR]

This article presents a differential-fed magneto-electric (ME) dipole with integrated balun based on ball grid array (BGA) packaging. The electric dipole consists of four metal patches on the top layer, and the magnetic dipole consists of two rows of plated through-holes (PTH) and ground on the bottom layer. The electric and magnetic dipoles are excited simultaneously by double L-shaped metal probes. In addition, the integrated balun is implemented by a simple 180° delay line that provides a pair of differential signals to the double L-shaped metal probes. The miniaturized size is obtained by the integrated balun and BGA packaging. Antenna elements and $1 \times 8$ fixed beam arrays are carefully designed, manufactured, and validated. The proposed antenna element achieves a compact size of 5 mm $\times 5$ mm $\times1.8$ mm ($0.47\,\,\lambda _{0} \times 0.47\,\,\lambda _{0} \times 0.017\,\,\lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at the central operating frequency of 27.9 GHz). The measurement results demonstrate that the antenna element has a reflection coefficient below −10-dB bandwidth of 17.2% and covers 25.5–30.3 GHz. The antenna element and $1 \times $ 8 array achieve a realized gain of 8.3 and 14.8 dBi, respectively. The proposed antenna elements can be widely applied to the 5G millimeter-wave n257 (26.5 −29.5 GHz) and n261 (27.5 −28.35 GHz) bands. [ABSTRACT FROM AUTHOR]

This work studies the influence of substrate loss on the performance of acoustic resonators and on-chip inductors and investigates the effective substrate resistivity of seven commonly used substrates in silicon-based devices. The substrates include X-cut lithium niobate (LiNbO3) film with two different thicknesses (400 nm and 1.6 $\mu \text{m}$) on high-resistivity Si (HR-Si) and amorphous Si wafers, SiO2 film with two different thicknesses on HR-Si, and bare HR-Si. The effective resistivities of these substrates are extracted using coplanar waveguides (CPWs) over a frequency range from 1 to 40 GHz. Using the effective resistivity approach, the efficiency of two substrate loss reduction techniques—Si wafer removal and amorphous Si—in reducing substrate loss is quantified. Comparison of the extracted substrate resistivities of the suspended and un-suspended dielectric-on-Si structures and comparison of LiNbO3 on HR-Si and amorphous Si are carried out. Substrate loss reduction techniques are more advantageous for a thinner dielectric film and at a lower frequency range due to the higher filling factor of the electric field in the Si wafer. Finally, by comparison of the effective substrate resistivity of SiO2 film on an HR-Si with bare HR-Si, thick plasma-enhanced chemical vapor deposition (PECVD) SiO2 film is found to be a good insulation layer to reduce substrate loss. [ABSTRACT FROM AUTHOR]

In this article, a flexible, stretchable, and dynamically tunable attenuator based on coplanar waveguide (CPW) and graphene is proposed. Soft silicone elastomer and serpentine metal structure are used to realize the flexible, stretchable, and biocompatible characteristics of the CPW. A graphene sandwich structure (GSS) is placed on the upper surface of the CPW to dissipate the electromagnetic wave, achieving the dynamically adjustable characteristics of the stretchable attenuator. When the surface impedance of the GSS is tuned by applying a proper bias voltage, the simulated and measured results reveal that the attenuation tuning range is dynamically tuned from 4–11 dB with a low reflection below −10 dB. In addition, the fabricated attenuator is simultaneously optimized to work at conditions of the normal state, 180° bending and 130% stretching, with acceptable changes between insertion loss and return loss up to 6 GHz. The attenuator covers the medical frequency band of 2.4–2.483 GHz and the 5th generation (5G) communication frequency band of 3.5–3.8 GHz, which demonstrates that the attenuator has potential applications in stretchable electronic systems. [ABSTRACT FROM AUTHOR]

In this work, the localized backside etching (LBE) structure is introduced as a strategy for total ionizing dose (TID) irradiation hardening. Immunity to TID-irradiation-induced radio frequency (RF) property degradation is observed for the first time in SOI substrates with the LBE structure. Identical co-planar waveguide transmission lines (CPW TLines), crosstalk characterization structures, and planar spiral inductors are designed and fabricated on different 8-in SOI wafers to characterize the RF properties of substrates. Electrical characterization in the frequency ranges from 100 MHz to 40 GHz reveals that the LBE structure is beneficial to suppressing the parasitic surface conduction (PSC) effect, which will lead to better RF properties of SOI substrate. It is shown that under 60Co $\gamma $ -ray irradiation, RF losses are significantly increased in the case of conventional substrates. In contrast, LBE substrates are almost insensitive to TID irradiation due to suppression of PSC. A developed physical model is proposed to characterize the behavior of inductors under PSC and TID effects, and the improvement by LBE structure on RF TID tolerance is separated through model parameter extraction. [ABSTRACT FROM AUTHOR]

This brief describes a transition from conductor-backed-coplanar waveguide (CB-CPW) with substrate-integrated coaxial line (SICL) to substrate integrated waveguide (SIW). The transition is designed for the C-band frequency range. The CB-CPW slot lines play the main role in widening the bandwidth. These CPW slot lines are providing excitation of both even and odd mode waves in SIW, which improves the impedance bandwidth. Using the proposed concept, the measured single-mode fractional impedance bandwidth of 63.34%, the minimum insertion loss of 0.16 dB, and the overall loss below 25% are achieved. The simulated results are found to be in good agreement with the experimental ones obtained for the prototype developed in the laboratory. [ABSTRACT FROM AUTHOR]

A dual-polarized endfire array is presented in this article for millimeter application. The substrate integrated mode-composite transmission line is introduced to exciting the dual-polarized radiation. Specially, the horizontal polarization is excited by the transverse electromagnetic (TEM) mode, whereas the vertical polarization is based on the TE10 mode. Two metal blocks sandwich the substrates to form a sandwich structure, and an open-ended slot is designed on the block edge to construed complementary sources radiation. Benefiting from the wideband feature of complementary sources, the proposed dual-polarized endfire array achieved a considerable bandwidth performance of 26.9% (24.8–32.5 GHz) based on the standard that both the voltage standing wave ratios of the two ports are less than 2. In terms of the key factor of dual-polarized antenna and isolation, measured results show a high port isolation of over 36 dB and a cross-polarization discrimination rate of over 30 dB. The excellent dual-polarized performance gives the proposed endfire array the potential of application in millimeter-wave mobile communications. [ABSTRACT FROM AUTHOR]

An indium phosphide (InP)-based E-plane transition for monolithically integrating terahertz photodiodes with standard rectangular waveguide (WR)-outputs is presented for all standard WR-frequency bands from 0.22 THz to 2.2 THz, i.e., from WR3 to WR0.51. The integration concept comprises a modified uni-travelling carrier photodiode (MUTC-PD) chip, an E-plane transition and a stepped impedance low-pass filter (LPF), which are all monolithically integrated on an InP substrate. The E-plane transition converts the quasi-TEM coplanar waveguide (CPW) mode of the MUTC-PD output to the dominant TE10 mode of the WR. To our knowledge, this is the first frequency-scalable monolithic integration concept that enables packaging of photodiodes with standard WR-outputs up to 2.2 THz. The recommended thickness of the InP substrate and the proposed E-plane transitions’ design parameters are investigated by numerical analysis to achieve minimum insertion loss (IL) and a wide operational bandwidth (BW). The presented optimized transitions exhibit a maximum IL of 1.4 dB, a return loss (RL) better than 10 dB and a minimum 1 dB IL BW of 92.23% for all WR-bands up to 2.2 THz. To prove the proposed monolithic integration concept, a MUTC-PD is integrated with a CPW-to-WR3 E-plane transition (220-320 GHz) on a 95 µm-thick InP substrate. At 300 GHz, the maximum achieved RF output power of the fabricated MUTC-PD chip is −12.4 dBm at a photocurrent of 18.5 mA. For experimental characterization, the MUTC-PD chip with the integrated E-plane transition has been mounted on a 1 mm-thick soda-lime glass substrate as carrier and it has been manually aligned within a WR3 together with an adjustable back-short. Due to non-perfect alignment of the chip and the back-short as well as the additional losses and substrate modes due to the thick glass carrier, the calculated average IL is increased to 5.3 dB. Experimentally, an average IL of 8.6 dB is measured within the WR3-band from 220 GHz to 320 GHz. Integration of the chip in a real package without misalignment and without the glass carrier is expected to improve the IL by ∼4.8 dB. [ABSTRACT FROM AUTHOR]

This communication presents a Fabry–Pérot cavity antenna with a thin polarizer loaded inside the cavity. The antenna is excited by a linearly polarized magneto-electric dipole. To realize self-circular-polarizing characteristics, a polarizer with the meander line is placed between the partially reflective surface and the reflective ground plane. Sharing the same radiating aperture, left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves can be generated by separate input ports. After analyzing the mechanism of the circular polarization, simulation and measurement are carried out to verify the proposed design. The operating bandwidth is 4%, from 27.4 to 28.5 GHz, covering fifth-generation (5G) 28 GHz band. The peak gain is 16.85 and 17.20 dBi for LHCP and RHCP, respectively. [ABSTRACT FROM AUTHOR]

A magnetoelectric antenna (MEA) that uses coplanar waveguide (CPW)-to-parallel-strip-line transition as feed to obtain a wide bandwidth is presented for millimeter-wave (mm-wave) bands. The balanced feed is utilized to feed a modified magnetoelectric dipole. The modified structure consists of a simple two-metal-strips MEA loaded with metallic loops. The resonant modes study shows that a new resonant mode is contributed by the loop when the loop length is nearly a wavelength, thereby improving the impedance bandwidth (IBW) and realized gain. A prototype is fabricated in-house with printed circuit board technology. Measured results validate a fractional IBW (VSWR $\leq$ 2) of 75.76% with realized gain being 8.6 $\pm$ 1.8 dBi for 14–29 GHz. The radiation pattern measurements indicate a stable radiation pattern across the band with low broadside cross-polarization levels in both E- and H- planes. The simulated total efficiency is 86–96%. [ABSTRACT FROM AUTHOR]

A polylithic integration technology is demonstrated for seamless stitching of chiplets of various functions, including RF, analog, and digital chiplets. In this technology, stitch-chips with compressible microinterconnects (CMIs) are used for low-loss, low-parasitic, and dense interconnection between chiplets. The modeling, fabrication, assembly, and characterization of a set of testbeds using fused-silica stitch-chips with integrated CMIs are demonstrated in this article. A 500-μm-long stitch-chip signal channel is measured to have less than 0.42-dB insertion loss up to 40 GHz with minimal parasitics (4.68 Ω at 28 GHz, 4.99 Ω at 39 GHz, 359 pH up to 40 GHz, and 62 fF up to 40 GHz). Moreover, CMIs’ S-parameters are extracted by the L-2L deembedding technique up to 40 GHz. CMIs exhibit less than 0.2-dB insertion loss up to 40 GHz with better than 16.2-dB return loss. Furthermore, benchmarking to other interconnections is presented in terms of loss and parasitics. Compared with conventional silicon-based interconnection, the data suggest that the fused-silica stitch-chip-based interconnection reduces the insertion loss by approximately 3.8-dB at the 28- and 39-GHz frequency bands. The stitch-chip interconnection has a similar insertion loss as glass interposer, organic substrate, and integrated fan-out (InFO) with the benefit of simple rework. [ABSTRACT FROM AUTHOR]

The microwave frequency band typically used for wireless communications will soon become saturated and will no longer be able to fulfill the high bandwidth demands of modern communication networks. Terahertz (THz) communication has appeared as a highly attractive, future-generation wireless technology that offers higher spectral bandwidth and, therefore, higher data rates. However, the full exploitation of THz technologies is contingent upon the availability of energy-efficient sources and devices. In this letter, we present a fabrication and measurement of a microscale planar inverted cone antenna array made of gold. Using an ungrounded coplanar waveguide (CPW) feed, the microfabricated structure provides a bandwidth of 37.9% with the resonant frequency of 0.925 THz. Given the cost of microfabrication is reduced substantially with rapid technological advancements, the results of this letter suggest that high-speed THz communications can be realized for wide-scale applications. [ABSTRACT FROM AUTHOR]

This article presents the accurate modeling results of the nonlinear behavior of a wide range of silicon-based substrates at RF. The TCAD-based model includes carrier inertia effects and captures the transient nonequilibrium phenomena in the semiconductor substrate regions in response to a large-amplitude RF signal. The model is applied to coplanar waveguide (CPW) lines measured on 19 different silicon-based samples that have key differences in their material and interface parameters, such as different values for nominal doping, interface oxide charge, the presence or not of a trap-rich passivation layer, and characteristic dimensions. Furthermore, the CPW line’s distortion levels are measured from 25 °C up to 175 °C and at five fundamental frequencies under large-signal excitation from 50 MHz to 5.4 GHz. An excellent model to experiment correlation is achieved under all of these conditions, and the impact of the material and excitation parameters are discussed with strong physical insight provided by the simulation tool. [ABSTRACT FROM AUTHOR]

The design and experimental results of millimeter-wave bandpass filters (BPFs) implemented in 55-nm BiCMOS technology and operating around 120 GHz are presented in this article. Slightly modified coupled line filters’ structures are used in the designs. It allows generating the transmission zero at the upper stopband, leading to the selectivity of enhanced filters. The coupled line structure is implemented in two technologies, microstrip coupled lines and coupled slow-wave (SW) coplanar waveguides (CS-CPWs). The latter shows more design flexibility, allowing to reach a better filtering performance than the microstrip filters, i.e., better in-band flatness and higher selectivity. Good agreement is achieved between simulation, modeling, and measurement results. Also, the overall length of the filter based on CS-CPWs is smaller as compared to the filters based on the microstrip lines, thanks to the SW propagation. [ABSTRACT FROM AUTHOR]

This article explains the frequency limitation in designing microstrip circuits based on a complementary split-ring resonator (CSRR) and reports a novel technique for increasing its operating frequency, which makes the CSRR suitable for high-frequency applications. This study helps in synthesizing the dimensions of symmetric CSRR (SCSRR) and asymmetric CSRR (ACSRR) circuits, which shows the applicability of the proposed technique in designing mm-wave circuits. It is observed that the upper frequency limit of the microstrip-coupled SCSRR is mainly due to weak electric coupling. The weak coupling implicates radiation loss in the structure, and the frequency at which the SCSRR has maximum loss due to radiation is found to be much less than the cutoff frequency of the higher order mode of the microstrip line, thus effectively limiting the operating frequency range of the circuit. In this article, the type of coupling and associated electrical parameters, e.g., reflected power, transmitted power, radiated power, losses, quality factor, voltage standing wave ratio, and gain, are investigated in detail. The allowable bandwidth of the fundamental mode of the microstrip line can be utilized to design CSRR with improved coupling. It is found that the ACSRR enhances the coupling coefficient using the cross-coupling effect. At higher frequencies, where the electric coupling is becoming too weak for the SCSRR, it is possible to design a magnetically coupled ACSRR of the same dimensions as the SCSRR. Finally, the design profile for both the SCSRR and ACSRR is studied in detail, and the radiating condition is discussed as a function of the height and dielectric constant of the substrate. For practical-verification purposes, two circuit prototypes are fabricated on a 1.5-mm-thick Rogers RT5880 substrate, and the measured results are found to be in good agreement with the proposition. It is found that, for a given microwave substrate, the maximum allowable resonating frequency of the ACSRR structure is nearly twice that of the SCSRR structure. [ABSTRACT FROM AUTHOR]

A novel slow wave (SW) substrate-integrated waveguide (SIW) with enhanced SW factor (SWF) and broadened bandwidth is proposed by simultaneously increasing the effective permeability and permittivity. The effective permittivity is increased by the enhanced capacitance between the top plane of the waveguide and the loaded patches which are shorted to the SIW bottom plane by the blind via-holes. The effective permeability is increased by lengthening the current path by adding patches on the blind via-holes. The proposed SW-SIW with simultaneous increased effective permeability and permittivity has two obvious advantages over the reported SW-SIW with only effective permeability or permittivity increased: 1) smaller longitudinal and lateral dimensions resulted from higher SWF and lower cutoff frequency, respectively, and 2) wider bandwidth due to stable wave impedance along the periodical SW structure. The lateral dimension of the proposed SW-SIW is decreased by 53% compared with the normal SIW with the same cutoff frequency. The phase velocity is decreased by 73%, which means the longitudinal length is reduced by 73% consequently. The operation band is 7.0–18.8 GHz (91.5% fractional bandwidth) covering X- and Ku-bands. A prototype of the proposed SW-SIW is manufactured and measured, and the measured results agree well with the simulations. [ABSTRACT FROM AUTHOR]

A cost-effective ball grid array (BGA) packaged surface-mount off-center-fed dipole antenna element and its array are proposed for 5G millimeter-wave new radio (NR) applications. The proposed antenna element exhibits a compact size of 0.61 λ0 × 0.61 λ0 × 0.16 λ0 (λ0 is the wavelength in free space at the center operating frequency) and is realized by only one FR4 layer to minimize cost. Plated through holes (PTHs) connect the dipole patch on the top layer to the bottom layer. Furthermore, tin-lead solder balls are mounted under the bottom layer to form the BGA packaging. The surface-mountable feature enables the antenna element to be integrated with other surface mount devices without any waveguide or coaxial connectors. The evaluation board uses a 50-Ω grounded coplanar waveguide (GCPW) line to test the performance of the antenna element and array. The antenna prototype and evaluation board have been manufactured and measured. The measurement results show that the −10-dB impedance bandwidth of the proposed antenna element achieves 35.7% in the range of 25.1–36 GHz. The measured peak gains of the antenna element and the 1 × 8 arrays are 6.72 and 13.53 dBi, respectively. [ABSTRACT FROM AUTHOR]

A new class of wideband microstrip (MS)-to-microstrip (MS) vialess vertical transition with function of filtering performance is proposed and demonstrated on coplanar waveguide (CPW) resonator in this brief. Initially, a higher unloaded quality factor (Qu) in the proposed CPW resonator than a conventional slotline resonator shows that the CPW resonator is a better choice for design of the proposed transition than its counterpart on slotline resonator. Meanwhile, to give an insight into the Qu of the CPW resonator, the underlying working mechanism how the proposed CPW resonator is provided with higher Qu than the slotline resonator is elaborated. Then, resonant mode in the proposed CPW resonator will be properly investigated. After that, to facilitate design process, a synthesis design method is introduced for design of the proposed transition. Finally, one design example, i.e., a three-pole vertical transition, is designed, constructed, and experimentally verified. The three sets of results, i.e., expected, simulated and measured results of the proposed transition match well with each other. [ABSTRACT FROM AUTHOR]

In Part I of this two-part work, we have presented the theoretical foundation and physical mechanism of the mode-selective transmission line (MSTL). This part is devoted to its excitation scheme, practical transition design, and experimental verification. The MSTL excitation is first explained by comparison with a conductor-backed coplanar waveguide (CBCPW). To produce the specific mode excitation, an ultrabroadband transition between CBCPW and MSTL is proposed. It has a tapered structure operating as a tapered-mode coupler, which supports smooth evolution from the fixed CPW mode of CBCPW to the frequency-dependent dominant mode of MSTL. This modal evolution along the transition is analytically treated based on a local normal mode theory. Couplings between the local normal modes are negligible as a result of gradual tapering, thereby effectively suppressing higher order modes in MSTL and sustaining a mode purity over an ultrabroad frequency range. For experimental demonstration purposes, back-to-back CBCPW-to-MSTL transition circuits are physically realized. The measured results are in agreement with the prediction, showing a return loss of 15 dB from 3 to 109 GHz; the single transition introduces a relatively low insertion loss of 1.84 dB at 100 GHz. The propagation characteristics of MSTL are also extracted from the measured S-parameters, which provides an excellent validation of the proposed structure. [ABSTRACT FROM AUTHOR]

In this work, a rigorous approach for the design of a differential coupled-line directional coupler (DCLDC) implemented in double-side coplanar-waveguide (CPW) technology is presented. Due to the double symmetry of the structure, it is possible to obtain the coupling level and port characteristic impedances of the coupler in a simple and efficient way. In addition, due to the broadside coupling of the involved differential CPW lines, a wide range of coupling levels can be covered. Design curves relating the geometrical parameters of the structure to the electrical parameters of the DCLDC are provided, and a 3-dB coupler with an impedance of $50~\Omega $ at the terminal ports has been designed, fabricated, and measured. Common-mode (CM) transmission to both through and coupled output ports has been reduced by adding a simple and compact CM filter at the input port. The good agreement between the results obtained with EM simulation, transmission line models, and measurements confirms the benefits of the proposed structure and the suitability of the design procedure. [ABSTRACT FROM AUTHOR]

In this article, a novel waveguide-to-microstrip transition with a built-in dc/IF return path is proposed, in which a traditional E- plane probe is directly connected to the backshort by a thin metal wire along the axis of the waveguide. The proposed waveguide-to-microstrip transition implements an additional dc/IF return path while realizing mode conversion. Moreover, the presence of the dc/IF return path has little influence on the performance of the original E- plane probe. The operating principle is analyzed by theoretical analysis and full-wave simulation, and the design procedure of the transition is also presented. To implement the proposed dc/IF return path on different substrates, two approaches, including direct integration with planar circuits or with wire bonding technology, are developed. Furthermore, two back-to-back transition prototypes working in the millimeter and submillimeter bands were designed and characterized. The measured results are in excellent agreement with the simulations. Finally, as an application example, a 70-/140-GHz unbalanced frequency doubler adopting this structure for both dc biasing and dc grounding was designed and fabricated. Measured results show that the conversion loss of the fabricated doubler is less than 13 dB in the range from 125 to 145 GHz, and the minimum conversion loss is 11 dB at 140 GHz with 100-mW input power. [ABSTRACT FROM AUTHOR]

In this letter, a new compact filtering power divider (FPD) on the square substrate integrated waveguide (SIW) with wideband isolation is proposed. As the isolation network is embedded in a square SIW cavity, good in-band isolation, compact structure, and low insertion loss can be simultaneously attained. The proposed FPD consists of the coplanar waveguide feeding line of Port 1, the capacitor, a resistor, and the square SIW cavity. TE101, TE201, and slot-line modes can realize third-order filtering response. Center frequency ($f_{0}$) and bandwidth of the filtering response can be independently adjusted. In order to achieve out-of-band isolation, Port 2/3 is placed in the specific position to suppress the high-order modes according to the electric-field distributions of these modes. Finally, a prototype with in-band return loss of 20 dB across 5.59–6.4 GHz is designed at 5.99 GHz. The bandwidths of isolation higher than 21 dB and upper stopband attenuation higher than 20 dB are extended to $2.79f_{0}$ and $2.83f_{0}$ , respectively. [ABSTRACT FROM AUTHOR]

A balanced substrate-integrated waveguide (SIW) bandpass filter (BPF) is designed and fabricated based on through-quartz-via (TQV) technologies in this article. Sixth-order filtering response centered at 41.4 GHz with equal source and load termination is achieved by the transformation of the Chebyshev polynomial. By exploiting cross-coupled resonators, the 3-dB fractional bandwidth (FBW) achieves 12.1%, and the out-of-band rejection is better than 30 dB with frequencies up to $2f_{0}$. High CM suppression can be achieved up to $2.2f_{0}$ with the help of two slots on the bottom metal layer. Based on the theory of the proposed SIW BPF, asymmetric diplexer with different resonance orders for two channels is studied with high isolation and wide stopband. The mixed dual-mode (TE101 and TE301) and single-mode (TE101) junction resonators provide a transition between the input port and two channels. Diplexer with lower and upper channels centered at 20.2 and 37.5 GHz, respectively, is fabricated and measured to verify the proposed structure, which exhibits high isolation better than 40 dB over the whole band and a wide stopband up to $3f_{0}$ of the lower channel. Measured results are in good agreement with the simulations. [ABSTRACT FROM AUTHOR]

In this work, a new ultrawideband grounded coplanar waveguide (GCPW) to substrate integrated waveguide transition is presented. The proposed design improves the performance of the triangular tapered slot transition, by providing a return loss level better than 28 dB in the $X$ -band frequency range (with enhancements of up to 29 dB at higher frequencies in such a band) and a flatter response for the insertion losses. The analysis of the new transition is performed using a dielectric material with high permittivity (${\varepsilon _{r}=9.8}$), which leads to an extremely compact implementation. In order to validate the proposed design, several back-to-back transitions with different lengths were manufactured, measured, and compared with the simulated data. Further experimental results, after performing an additional thru-reflect-line calibration for deembedding undesired effects of involved connectors, are also shown. [ABSTRACT FROM AUTHOR]

A new beam controllable Fabry–Pérot (FP) antenna utilizing a liquid-metal partially reflective surface (PRS) is investigated. The proposed PRS can be divided into three different liquid-metal-filled areas. By injecting the liquid metal into specific zones, the antenna beam direction and 3 dB beamwidth can be controlled separately, providing an easy control method for the radiation pattern reconfigurable FP antenna. The experimental results demonstrate that the proposed antenna works at 5.1 GHz. Five beam directions in the elevation plane ($\varphi = 90^{\circ }$) and four antenna horizontal beam widths can be obtained. The FP antenna is simple in design and is easily fabricated as well. It will be a good candidate antenna for future wireless communication systems. [ABSTRACT FROM AUTHOR]

In this communication, a dual-polarized magneto-electric (ME) dipole antenna is implemented in the printed ridge gap waveguide (PRGW) technology. The antenna is composed of a cross-shaped ME-dipole backed by an artificial magnetic conductor (AMC) sidewall cavity. This cavity is fed by two orthogonal PRGWs implemented in two separate layers to achieve a dual-polarization with −28 dB isolation level at the center operating frequency (30 GHz). The proposed antenna achieves a 23.4% relative impedance bandwidth at 30 GHz with isolation better than 20 dB over the whole operating frequency band. In addition, a gain enhancement technique is used to increase the gain to 10 dB over the operating bandwidth. A prototype is fabricated and there is a good agreement between simulated and measured results. [ABSTRACT FROM AUTHOR]

This article provides guidelines to design porous silicon (PS) layers regarding optimization of small-signal properties in passive structures for radio frequency (RF): insertion loss and crosstalk. Results are based on high-frequency measurements on 200-mm wafers and electromagnetic simulations up to 40 GHz. Using substrate resistivity below 1~Ω cm allows the best tradeoff with minimized PS thickness. [ABSTRACT FROM AUTHOR]

We have used a neon focused-ion-beam to fabricate both nanoscale Nb Dayem bridges and NbN phase-slip nanowires located at the short-circuited end of quarter-wavelength coplanar waveguide resonators. The Dayem bridge devices show flux-tunability and intrinsic quality factor exceeding 10 000 at 300 mK up to local fields of at least 60 mT. The NbN nanowires show signatures of incoherent quantum tunneling of flux at 300 mK. [ABSTRACT FROM AUTHOR]

This article studies about the structure of high-impedance wires (HIWs) based on ferrite substrate coplanar waveguide (CPW). Slotted planar stubs are etched on one or two ground planes of the CPW line. Several prototype measurements have been performed showing excellent performance to implement narrow-band microwave isolators. These HIW components can work at low bias-field giving an easier implementation in microwave devices. [ABSTRACT FROM AUTHOR]

This work presents the development of a coplanar waveguide based split ring resonator, integrated with a microfluidic channel suitable for liquid sensing applications. The sensor was fabricated via photolithography on a 0.5 mm borosilicate glass substrate with patterned gold. The microfluidic channel was aligned with the sensing area and bonded to the substrate to ensure consistent liquid volume ($0.784~\mu \text{L}$) over the sensing region. The microwave sensor was $1.5\times 3$ cm2 and had a resonant frequency between 19.3 and 19.4 GHz, offering the advantage of measuring extremely small sample volumes. The performance of the sensor was validated through simulations and a series of experiments using a $\vert \text{Z}\vert $ -probe station. The sensor demonstrated a resonant amplitude sensitivity of -0.085 dB/vol.% for isopropyl alcohol in water (0-100 vol.%), 0.00023 dB/[mg/dL] for various glucose concentrations (50-1200 mg/dL), and 0.00056 dB/[mg/dL] for NaCl in water (50-1200 mg/dL, with equivalent Na+ 9 - 226 mEq/L solutions); measured under room ambient conditions (24 ± 1°C and RH: 36 ± 1 %). [ABSTRACT FROM AUTHOR]

A strategy aimed at decreasing dielectric loss in coplanar waveguides (CPWs) and qubits involves the creation of trenches in the underlying substrate within the gaps of the overlying metallization. Participation of contamination layers residing on surfaces and interfaces in these designs can be reduced due to the change in the effective dielectric properties between the groundplane and the conductor metallization. Although finite element method approaches have been previously applied to quantify this decrease, an analytical method is presented that can uniquely address geometries possessing small to intermediate substrate trench depths. Conformal mapping techniques produce transformed CPW and qubit geometries without substrate trenching but a nonuniform contamination layer thickness. By parametrizing this variation, one can calculate surface participation through use of a 2-D, analytical approximation that properly captures singularities in the electric field intensity near the metallization corners and edges. Examples demonstrate two regimes with respect to substrate trench depth that capture an initial increase in substrate-to-air surface participation due to the trench sidewalls and an overall decrease in surface participation due to the reduction in the effective dielectric constant and are compared with experimental measurements to extract loss tangents on this surface. [ABSTRACT FROM AUTHOR]

A novel $4 \times 4$ Butler matrix fabricated on ridged half-mode substrate integrated waveguide (RHMSIW) technology is designed and realized. The integration of the proposed Butler matrix with tunable phase shifters fabricated using the same technology is also demonstrated to achieve semi-continuous beam steering for fifth-generation (5G) applications. This article discusses the design considerations of the various ridged half-mode subcomponents of the Butler matrix in detail and presents two novel interconnects, namely 90° waveguide bends and a half-mode open-edge-to-open-edge transition. This article shows that the capacitive ridge on the open edge of one waveguide can be directly connected to the open edge on another with little compromise on the performance over a reference line of the same length. The simulated and measured results of each Butler matrix component are also presented and are all in good agreement. The proposed Butler matrix has a 5% operational bandwidth centered at 3.51 GHz with less than ±22° error in the progressive differential phase shift and less than ±1.2-dB coupling imbalance. Finally, it is shown that the proposed Butler matrix achieves over 70% board area miniaturization in comparison to a similar design fabricated on half-mode substrate integrated waveguide technology. [ABSTRACT FROM AUTHOR]

In this article, the dielectric properties of a biosourced polymer are investigated. To that end, coplanar transmission lines were fabricated on a 600- $\mu \text{m}$ thick cellulose palmitate substrate by using a photolithographic process and a transfer technique. The ${S}$ -parameters of the transmission lines were measured in a broad microwave frequency band spanning from 10 to 67 GHz in order to determine the propagation constant. From these data, the complex dielectric permittivity was extracted by using two analytical methods (RLCG method, based on resistance, inductance, capacitance and conductance of transmission lines and conformal mapping method) and a numerical one [finite element method (FEM)]. The three methods allow distinguishing the dielectric losses from the total measured losses of the transmission lines. Finally, the dielectric constant and the loss tangent of this biopolymer were compared to the data of common and commercially available substrates: polyimide and polyethylene naphthalate. [ABSTRACT FROM AUTHOR]

substrate-integrated gap waveguide (SIGW)-fed metasurface antenna (metantenna) array with nonuniform excitation is proposed for wideband operation and sidelobe-level (SLL) reduction at millimeter-wave Ka-band. To achieve wideband and low profile, a $2 \times 2$ square patch-based metasurface is introduced based on characteristic mode analysis (CMA). The ridged microstrip-fed slot is employed to excite the metasurface, while the mushroom-like patches are positioned around feeding lines to operate as the SIGW for suppressing the undesired modes. Due to the compactness of the proposed SIGW, the amplitude of each element is controllable using unbalanced T-junctions. A nonuniformly excited $4 \times 4$ antenna array is designed and fabricated for validation. The measured results show an impedance bandwidth ($\vert \text{S}_{11}\vert < -10$ dB) of full Ka-band with a 3 dB gain bandwidth of 28–40 GHz (35.3%) and SLLs less than −12.1 and −15.6 dB in the E- and H-planes, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a flexible microwave sensor for material dielectric characterization is presented. The sensor is composed of a two-turn complementary spiral resonator (CSR) in coplanar waveguide (CPW) technology developed on a flexible polyethylene terephthalate (PET) substrate. The sensing principle is based on the measured transmission coefficient of the designed structure, where the resonant frequency and its magnitude are in function of the dielectric properties of loaded materials under test (MUTs). A prototype of the proposed sensor is fabricated and experimentally validated by extracting the dielectric constant and loss tangent of the reference samples loaded above the region of the CSR. Specially, due to the flexibility, the fabricated sensor can be conformally attached to the palm of a dexterous robotic hand to function as robotic electronic skin for material dielectric characterization, more specifically, to obtain dielectric properties of the objects grasped by the dexterous robotic hand. Moreover, the sensing functionality of the sensor structure under different bending levels is validated by calculating the dielectric property of loaded objects with non-planar surfaces. [ABSTRACT FROM AUTHOR]

In this communication, a dual-polarized cross-slot antenna is proposed with a relative small dimension. Orthogonal polarizations with high port isolation are provided by the cross-slot structure. A dual-mode colocated feeding network is properly integrated with the cross slot, without occupying extra area. Therefore, the overall dimension of the proposed antenna is only $0.293\lambda _{0} \times 0.293\lambda _{0} \times 0.008\lambda _{0}$ ($\lambda _{0}$ is the free space wavelength at center frequency). A prototype is fabricated and measured to prove the design strategy. In such a compact antenna volume, the −10 dB impedance bandwidths for both orthogonal polarizations are over 6%, with the port isolation better than 30 dB and radiation efficiency over 68%, exhibiting potential applications in space-limited systems, such as access points and portable base stations. [ABSTRACT FROM AUTHOR]

An effective method of designing low-profile cavity-backed multi-band antennas is introduced based on half-mode substrate-integrated waveguide (HMSIW) technique. By loading U-shaped strips outside the radiating aperture of an HMSIW cavity, additional resonances can be produced, thus leading to the realization of multi-band capability. An inset $50~\Omega $ microstrip feedline is employed to energize the HMSIW cavity, whereas the loaded strips are excited through proximity coupling from the radiating aperture of the HMSIW. The proposed antenna is constructed on a single-layered substrate with a low profile of 0.024 free-space wavelength. To validate the multi-band responses, prototypes of the single-band HMSIW antenna as well as dual-band and triple-band antennas are fabricated and measured, respectively. The experimental results show that the proposed antennas possess advantages of stable radiation patterns with a gain of 5–5.9 dBi as well as acceptable front-to-back (FTB) ratio of larger than 10 dB over different frequency bands. [ABSTRACT FROM AUTHOR]