One-dimensional electromagnetic bandgap (1-D EBG) and split ring resonator (SRR) structures were inserted between two closely located monopole antennas to suppress mutual coupling. The 1-D EBG and SRR structures in these planar multiple antennas function as a reflector and wave trap, respectively. With the effect of these two structures, the mutual coupling between the two antennas is reduced by more than 42 dB and the back lobes are reduced by 6 dB. Thereby, the radiation efficiency of the antenna is also improved. The two fabricated antennas with 0.19λ ₀ spacing exhibit mutual coupling (S ₂₁ , S ₁₂ ) of less than -30 dB from 2.43 to 2.54 GHz. A minimum correlation coefficient of 0.002 and maximum radiation efficiency of 82% are also demonstrated.

This paper presents a compact millimeter-wave (mm-Wave) phased array incorporating inverted-L antenna (ILA) with high-impedance electromagnetic surfaces (HIS) based on single-layer FR-4 PCB for user equipment (UE). In order to realize mm-Wave UE in low production cost, the design methodology of phased array antenna in single-layer FR-4 PCB is proposed without any additional packaging process. The presented antenna consists of low-coupled antenna element and low loss feeding networks in compressed linear array configuration with 0.39λ ₀ inter-element spacing. First, in order to realize broadband characteristics, the ILA is integrated with one-dimensional electromagnetic bandgap (1-D EBG) structures featuring HIS via slow-wave behavior. Due to high-impedance properties of the embedded via wall structures in common ground plane between each antenna element, the mutual coupling path through surface-current can be reduced. Second, in order to minimize leakage power caused by surface-waves, low loss coplanar waveguide ground (CPWG) feeding networks with via wall structures and island-shape pads are realized. The fabricated 4-elements and 8-elements phased array antennas including antenna element and feeding network based on FR-4 PCB achieve wide 3 dB beam coverage while featuring low beam squint in the Ka-band. In order to investigate over-the-air (OTA) system performance at three beam steering scenarios (boresight and ±60° scan angle), the measured maximum error vector magnitude (EVM) in QPSK modulation scheme is 16.6%. The coverage efficiency beyond 50% for the realized gain of −1.5 dBi from 26 to 36 GHz is demonstrated.

This communication presents a concept of a sub-6 GHz antenna-on-display (AoD) that can eventually be integrated into the transparent area of a display panel of various wireless devices for the first time in the literature. The transparency of the proposed microwave AoD is realized using metal mesh conductive film and features optical transparency, which is compatible with high-resolution liquid crystal display (LCD)/organic light-emitting diode (OLED) display panel requirements. To maximize the radiation efficiency despite the extremely thin substrate ( $0.0048\lambda _{0}$ ), a traveling-wave antenna topology is inspired and further modified to incorporate the inherent structural limitations of modern-day high-resolution display panels. Eventually designed to operate in a leaky-wave mode, a novella dedicated feeding mechanism is introduced to minimize undesired coupling between the antenna structure and the display panel. The composite right-/left-handed (CRLH) unit cell employs a single shunt inductor topology to maximize the radiation efficiency. The measured 6 dB impedance bandwidth of the proposed microwave AoD is from 2.36 to 6.17 GHz, which is consistent with the fast wave region of the proposed unit cell. The measured realized gain of the microwave AoD consisting of 13 unit cells is 2.68 dBi at 5.5 GHz with a radiation efficiency of 62.5%. This is the first demonstration to ascertain that an AoD topology can be applicable at microwave applications while reducing the antenna-to-ground distance by approximately a factor of 20 compared to previous studies.

This paper describes a compact inverted-L antenna (ILA) topology that can enhance antenna performance by utilizing one-dimensional electromagnetic bandgap (1-D EBG) ground structures and two-stage beam directors. The antenna is fed through the coaxial connector and the microstrip line, and the 1-D EBG structures are embedded on the ground plane edge of the microstrip line that is closer to the planar ILA. The 1-D EBG ground structures serve as high impedance surfaces with slow-wave behavior controlling both reflection phase in plane-wave illumination and surface-wave on the horizontal-direction of the ground plane edge. The proposed antenna with 1-D EBG ground structures exhibits a wide impedance bandwidth of 810 MHz between 2.24 GHz and 3.11 GHz within a compact size of 0.21λ ₀ × 0.32λ ₀ . In addition, the endfire radiation performances in horizontal-polarization of the designed antenna can be enhanced without any supplementary circuits by the integration of 1-D EBG structures within compact real-estate and by placing beam directors. As the criterion for improvement of the radiation pattern, the horizontally polarized end-fire gain of 4.0 dBi and radiation intensity difference between co-polarization and cross-polarization of 14.0 dB toward the end-fire direction are experimentally ascertained.

This article first reports a wideband asymmetric fed interdigital coupling antenna (AFICA) with the fan-out wafer-level package (FOWLP) process. The asymmetric fed structure is designed to realize a more compact array size. The interdigital-shaped coupling method between the elements achieves wideband impedance matching with 70- $\Omega$ feeding line. In addition, the exemplified 1 $\times$ 4 AFICA array package demonstrates a wide bandwidth from 60 to 90 GHz and an extremely low profile of 0.044 $\lambda_{{0}}$ at 60 GHz. Furthermore, the prototype module is fabricated and integrated with frequency quadrupler CMOS chip. The measured results exhibit a wide bandwidth from 60 to 90 GHz and a peak gain of 9.3 dBi. The measured antenna efficiency is higher than 80%. Both fabricated package and module feature a wide scanning angle of ${\pm}50^{\circ}$ and low cross-polarization levels ( ${\le}\!-\!18$ dB) at operating bands, respectively. The power-based de-embedding method is proposed to evaluate the performance of real-life modules and systems. The simulated and experimental results of the fan-out (FO) interconnection loss are less than 1 dB, respectively. As a result, the proposed co-planar wideband AFICA array with endfire mode design is fully compatible with various chips, such as 5G/6G communication, radar, and sensing applications.

An ultra-thin linear-to-linear polarization converter exhibiting a high polarization conversion ratio (PCR) in an ultra-wide bandwidth is fabricated on a printed circuit board. It is based on a bi-layer slot structure that is electrically connected by via holes. Front- and back-sided slot structures are arranged in mutually orthogonal directions and function as receivers and radiators, respectively. The via holes provide a route for the transfer of electromagnetic energy received by the front-sided slot structures to the back-sided slot structures, and radiate an electromagnetic wave having polarization orthogonal to that of the input electromagnetic wave. Moreover, two slots are employed for each unit cell design, and the locations of the via holes are carefully determined to achieve broadband polarization conversion. The via-hole positions that change the surface current distribution around the slot apertures enable the control of multiple resonance frequencies. Via holes are positioned to make 1 ^st and 2 ^nd resonance frequencies that are blue-shifted toward the 3 ^rd resonance frequency, which broadens the operational band of the perfect polarization conversion. The 0.8-mm-thick polarization converter exhibits PCR greater than 99% in the frequency band from 24.5 to 47 GHz.