A single radiator that is a part of four-port diversity Multiple-Input Multiple-Output (MIMO) antenna design is composed of four octagonal rings embedded between the two opposite sides of a T-shaped conductive layer surrounded by inverted angular edge cut L-shaped and E-shaped structures. The radiators are placed at the four corners with common ground at the center of a smartphone to form a four-element mobile MIMO antenna. The printing of the antenna is carried out on the flexible polyamide substrate (dielectric constant = 3.5 and loss tangent = 0.0027) with dimensions of 70 × 145 × 0.2 mm³. A wide impedance bandwidth of (84.12%) 2.39 to 5.86 GHz is achieved for all four radiators. The compact size of the radiators along with their placement enables the proposed MIMO antenna to occupy much less area while preserving the space for 2G/3G/4G antennas. The placement of the antennas results in self-isolation between antenna elements by achieving isolation greater than 17.5 dB in the desired operating bands. Furthermore, besides showing a high efficiency of 85% and adequate gain above 4 dBi, good diversity performances such as Envelope Correlation Coefficient (ECC) of less than 0.05, Diversity Gain (DG) of above 9.8 dB, Mean Effective Gain (MEG) of −3.1 dB, Channel Capacity of 21.50 bps/Hz, and Total Active Reflection Coefficient (TARC) of below −10 dB are achieved by the flexible MIMO smartphone antenna. The effect of bending along the X and Y-axis on the performance of the proposed MIMO antenna is also analyzed where decent performance is observed. This makes the proposed flexible four-element MIMO antenna a potential candidate to be deployed in future smartphones.

A tapered symmetrical coplanar waveguide (S-CPW) fed monopole antenna is initially studied. To achieve multiband characteristics, the radiating element of this monopole antenna is loaded with multiple narrow slots and multiple slotted stubs (MSS). The designed slot-loading monopole is further transformed into a two-antenna MIMO type with a gap distance of only 0.12λ (at 5 GHz), and thus it has a small overall size of 32 × 20 × 0.8 mm³. By deploying five concentric ring elements between the two adjacent antenna elements, the desirable isolation of better than 20 dB is yielded. As the low band and high band operation of the proposed two-antenna MIMO is 81.08% (3.3–7.8 GHz) and 40% (8.0–12.0 GHz), respectively, it can therefore satisfy the Sub-6 GHz 5G New Radio (NR) n77/78/79, IEEE 802.11ac/ax, X-band/C-band wireless and satellite applications. Furthermore, it has shown a desirable gain of above 3 dBi and a radiation efficiency greater than 69% throughout the two bands of interest.

A high-gain phase-graded digital metasurface lens (PG-DMSL) antenna enabled with active beamsteering based on reconfigurable feed source for millimeter-wave applications is proposed. An end loaded cross dipole slotted structure is proposed as a transmission-type unit cell (UC). The designed UC achieves a transmission magnitude better than –3 dB with 2-bit phase tuning at 28 GHz. Further, PG-DMSL is devised by arranging four sets of proposed UCs in an array of 25 × 25 resulting in an aperture size of 7 λ ₀ × 7 λ ₀ and a thickness of 0.23 λ ₀ . On switching the states of the 3 PIN diodes, the radiating feed antennas (FAs) kept beneath the designed PG-DMSL at an optimum focal distance of 20 mm can be switched resulting in steered beams along 0º and ±30º. The proposed design is experimentally validated showing a measured broadside gain of 19 dBi with an augmented gain of 14.9 dB and a scan loss of 1 dB.

A novel technique-self-injection-locked and phase-locked loop-using an optical feedback is proposed for phase noise reduction in microwave oscillators. Phase noise reduction of 13 dB at 10-kHz offset has been demonstrated by applying this technique to a 10-GHz dielectric resonator oscillator, achieving -125 dBc/Hz at 10-kHz offset. Phase noise modeling of the feedback technique is also presented, and corroborates with experimental results. Analysis using the modeling has shown that the current limit of the phase noise is due to the system residual noise. Phase noise of -145 dBc/Hz at 10-kHz offset is predicted when the relative intensity noise level is reduced to -170 dB/Hz and the detected photocurrent is increased to 10 mA.