A high-gain circularly polarized (CP) filtering stacked patch antenna with independently controllable radiation nulls is designed without an additional filtering circuit/network in this letter. The lower radiation patch consists of a square outer loop and a three-quarter circular inner ring to realize CP radiation, with the inner ring connected to two adjacent edges of the outer loop. In addition, the upper stacked patch provides improved impedance matching and CP characteristics within the passband. Furthermore, a U-slot and square annular slot are etched on the lower and upper patch to generate two radiation nulls and obtain a filtering characteristic in the lower and upper stopband, respectively. A prototype is fabricated and measured, and the measured results show that the proposed antenna can provide an impedance bandwidth of 15.2% (1.83-2.13 GHz), an axial ratio bandwidth of 8.2% (1.87-2.03 GHz), and a gain higher than 7.0 dBi.

A low-specific absorption rate (SAR) four-antenna multiple-input–multiple-output (MIMO) antenna array is designed for fifth-generation (5G) mobile phones based on the theory of characteristic modes (TCM) of the composite perfect electric conductor (PEC) and lossy dielectric structures. First, the TCM of composite PEC-lossy dielectric structures is reviewed briefly. Then, the application of the theory for low-SAR antenna design is demonstrated through a four-antenna MIMO antenna array. Finally, the proposed MIMO array is fabricated, and its performance is measured. Its −10 dB impedance bandwidth can cover the desired frequency band from 3.4 to 3.6 GHz. The isolations between different antenna elements are higher than 15.4 dB. Their total efficiencies range from 81.3% to 89.9%. The envelope correlation coefficient (ECC) is lower than 0.07. The 10 g spatial-average body SAR corresponding to the situation that four antennas simultaneously radiate is 0.47 W/kg. Compared with the state-of-the-art 5G MIMO antenna arrays, the proposed MIMO antenna array has the lowest peak SAR value, satisfactory efficiency, isolation, and ECC. This article provides a general methodology for the design of low-SAR mobile phone antennas.

A low specific absorption rate (SAR) multiple-input and multiple-output (MIMO) antenna array is designed for 5G mobile phones based on the theory of characteristic modes (TCMs). First, a geometrically symmetric dipole antenna is analyzed using TCM. The modal currents with symmetrical and reverse distributions are selected and excited to obtain the low-SAR antenna element. Then the antenna element is further optimized, and eight antenna elements constitute an MIMO array. The −6 dB impedance bandwidth of the proposed MIMO array is from 3.4 to 3.6 GHz. The isolations between different antenna elements are higher than 16 dB. Their total efficiencies range from 48% to 67%. Its envelope correlation coefficient (ECC) is lower than 0.08. The 1 g spatial-average head SAR and 10 g spatial-average body SAR of the proposed antenna are simulated and measured.

A low specific absorption rate (SAR) and high on-body efficiency tri-band smartwatch antenna is designed utilizing the theory of characteristic modes (TCM) of composite perfect electric conductor and lossy dielectric structures. The antenna works in GPS frequency band, WLAN 2.4GHz/Bluetooth frequency band and 5G frequency band. After analyzing the characteristic modes of antenna and human body in the above three frequency bands, the characteristic modes with small modal coupling coefficients are selected and excited. Due to the small mutual coupling between the antenna and the human body, the proposed antenna has good performances on both SAR and on-body efficiency. Its measured 10 g average body SAR is lower than 1.3 W/kg. When it is placed 0 mm above the body, its on-body efficiency is 9.3%-9.88% in GPS frequency band, 12%-14.2% in WLAN 2.4GHz/Bluetooth frequency band and 17.3%-20.1% in 5G frequency band, respectively.

A novel technique, namely periodic characteristic mode analysis (PCMA) method, is proposed for the solution of characteristic modes (CMs) of large-scale finite periodic arrays. This technique first adopts a small set of the CMs of the reference element of the array with larger mode significance (MS) to construct the entire-domain basis functions defined on each array element and then applies them to expand the characteristic currents of the entire array. Through using these newly constructed CM based entire-domain basis functions, the proposed PCMA method can achieve great reduction of the matrix dimension of the resultant generalized eigenvalue problem governed by the entire array structure, thereby significantly reduce the computational time and memory requirement for solving CMs of the entire large-scale finite periodic array. Several typical examples are presented to demonstrate the efficiency and accuracy of the proposed method.

This letter presents a stacked patch antenna for circularly polarized (CP) filtering without any additional filtering structures. The lower patches consist of a square patch with truncated corners, two sets of parasitic patches, and a 50 Ω microstrip line with a quarter-wavelength impedance matching line at the front end. The upper patch consists of a square patch with a corner-truncated and etched crossed slots. Two CP antenna axial ratio (AR) minima are generated by the stacked patches with corner-truncated and crossed slot, respectively. Three radiation nulls are generated by the parasitic patches on both sides of the lower main patch, crossed slot, and parasitic patches on both sides of the quarter-wavelength impedance matching line, respectively. The proposed antenna allows for independently controllable radiation nulls. The fabricated antenna has been measured, and the measurement results indicate that the impedance bandwidth is 17.72% (2.16-2.58 GHz), the AR bandwidth is 9.32% (2.25-2.47 GHz), and the peak gain is approximately 7.07 dBi.