Dual-band shared-aperture Fabry-Pérot cavity (DS-FPC) antennas with single-layer partially reflective surface (PRS) are inherently limited in the achievable frequency ratio. This paper analyzes such antennas to derive the feasible range of frequency ratio and thereafter proposes a dual-band shared-aperture antenna that can fill the frequency ratio gap. The proposed antenna integrates a short backfire antenna (SBA) for the low band (LB) and a Fabry-Pérot cavity (FPC) antenna for the high band (HB), utilizing a shared single-layer PRS and a parasitic element. In LB, using a parasitic element that is transparent for HB, the SBA works well despite its low profile. In HB, the FPC antenna works with the first-order FPC mode. A prototype working at 5.5GHz and 9GHz was fabricated and measured. The overall height of the antenna is 0.36 and 0.59 wavelength in LB and HB, respectively. The prototype achieves 7.3% and 6.7% of measured overlapping 10dB impedance bandwidth and 3dB realized gain bandwidth, as well as peak realized gains of 10.3 and 14.6dBi, in LB and HB, respectively.

A reconfigurable partially reflective surface (PRS) antenna can offer low-cost and low-complexity beam steering, traditionally implemented using a phased array with a bulky feeding network. This paper utilizes ray-tracing to prove analytically that an inductive nonuniform PRS facilitates larger beam deflection and lower sidelobe level (SLL) than a capacitive one, given the same reflection phase difference. With a suitable tradeoff in the beam deflection angle and gain variation between the reconfigurable states, an inductive reconfigurable PRS is designed for a ±13° steering range. To enhance the steering range with minimal complexity, two parasitic elements loaded with PIN diodes are added to the feeding source, resulting in the overall steering range of ±30°. The fabricated prototype verified that the pencil-shape beam can be steered towards 0º and ±30º with SLLs of less than -19dB at 5.5GHz. The antenna achieved peak realized gains of 9.5-10.4dBi for the three states with gain variation of less than 0.9dBi. The measured overlapped impedance band covers 5.41-5.63 GHz for all states.

This paper introduces a systematic design method for decoupling elements, which can significantly improve the isolation between two co-located antennas, e.g. between transmit and receive antennas of an in-band full-duplex system. The design method applies the theory of characteristic modes for controlling the phase and amplitude of the scattered fields of the decoupling element, in order to optimally cancel the original incident fields which couple to the receiving antenna. We describe concisely the effects that characteristic angle, modal near-field, and modal excitation of the decoupling element have on the antenna isolation. For validating the proposed method, a planar wavetrap is designed and the isolation improvement verified with full-wave simulations. When we use the proposed method to optimize a wavetrap that is placed between two co-located patch antennas, we obtain an improvement of the isolation between the antennas by 33 dB at the centre frequency of their operational frequency band, and at least 12-dB improvement across the whole 142-MHz operational bandwidth of the two antennas. As a benchmark, the wavetrap is replaced by an absorber occupying 10 times the volume of the wavetrap. The absorber gives only 6 dB of isolation improvement, substantiating the effectiveness of the proposed wavetrap method.

Multiple-input multiple-output (MIMO) is a key enabler for high data rates in mobile communications. However, it is challenging to design MIMO terminal antennas for LTE bands below 1 GHz, due to the conventional chassis offering only one resonant characteristic mode (CM). Recently, it was shown that minor structural changes can yield up to two additional resonant modes for designing two-port MIMO antennas. Nonetheless, the resulting bandwidth for the second port is relatively small. To simultaneously meet bandwidth and other practical requirements (including low profile and no off-ground clearance), a step-by-step approach for structural changes and feed design is applied in this work to exemplify the use of physical insights from CM analysis to achieve a competitive wideband two-port solution. The main novelty is that an entirely new mode is identified and appropriately tuned by structural modification for creating an additional resonance below 1 GHz. Moreover, two simple probe-feed ports are designed to jointly excite different subsets of four modes over frequency. In addition, far-field pattern orthogonality is guaranteed by the different phase shifts of the characteristic electric fields at the port locations. Furthermore, bulkier self-resonant antenna elements are avoided. To show design flexibility, a three-port version is also demonstrated.

In recent years, the screen-to-body ratio of mobile handsets has been increasing. Today, the screen nearly fills up the entire front side. Conventionally, the screen is mainly seen as a metallic object that adversely affects antenna performance. In this paper, the large screen is used for the first time to facilitate an additional uncorrelated MIMO port in a tri-port design, for several LTE bands below 1 GHz. To this end, the design procedure explicitly considers the screen together with the terminal chassis, which can be simply modelled as two metal plates. In particular, characteristic mode analysis of the double-plate model enables a sufficient number of resonant modes to be created, tuned and selectively excited to yield three uncorrelated MIMO ports in the low band. Simulation and measurement results are found to be in good agreement. The measured bandwidths of the three ports are 23%, 17% and 21%, respectively. Within the operating band, the measured isolation is above 13 dB, envelope correlation coefficient below 0.16 and average total efficiency above 72%.