In this article, a new low-profile decoupling structure originated from the phase shift concept for the patch antenna array is proposed. To clearly illustrate the operation principle, the phase of the signal transmitted from Patch 1 to Patch 2 has been initially studied and the decoupling condition for two closely spaced patch antennas in H-plane has also been obtained. Afterward, the decoupling element concisely composed of a half-wave microstrip line and a shorting pin is developed. Attributing to the introduction of additional signal path by the new decoupling structure, mutual coupling between two adjacent patches is effectively suppressed. To verify the feasibility of the proposed design scheme, demonstrators of two-element patch antennas with and without decoupling structure are, respectively, implemented and analyzed. Results indicate that compared with the coupled array, the isolation between two patch elements is enhanced from 7 to 18 dB at the center frequency of 3.16 GHz under the edge-to-edge separation of only 0.027 $\lambda _{0}$ . Besides, owing to the single layer layout, the profile of the whole antenna structure is as low as 0.02 $\lambda _{0}$ . Ultimately, the proposed decoupling scheme has been applied to the three-element counterpart, so as to demonstrate and validate its effeteness of isolation enhancement for multielement patch array.

New EM structures are demonstrated to realize low radar-cross-section (RCS) antennas by making effective use of frequency selective absorber (FSA). According to the well-known reciprocity principle, the two-layered FSA can be considered as an actual receiving antenna and then transformed to a circularlypolarized (CP) antenna. At the radiation state, a truncated patch resonator on the bottom layer is fed by a coaxial probe so as to produce CP wave, and it further excites the slots on the upper layer, resulting in its radiation towards free space; when the antenna at stealth state, the detective incident wave can be effectively absorbed outside the radiation band, thus achieving RCS reduction. The design strategies are then explained and verified with the aid of the corresponding equivalent circuit models. Two examples of 2x2 and 4x4 antenna arrays were designed to validate the flexibilities of the proposed design method. In final, the 4x4 antenna array was fabricated and measured, and reasonable agreement is achieved.

A co-design of a wideband circularly polarized (CP) patch antenna is proposed in this paper. The patch antenna is seamlessly integrated with two bandpass filter networks consisting of one and two λ/2 resonators, respectively, which are designed to excite two orthogonal modes with equal magnitude and quadrature phase over a widened bandwidth. In co-design of this filtering antenna, the patch antenna is equivalently treated as the last-stage resonator and the output port of the filter network. In analysis of its far field, axial ratio (AR) of CP wave is mapped to its magnitude and phase responses. By simultaneously synthesizing the magnitude ratio and phase difference of the two filter networks' transmission coefficients to vary around 1° and 90°, respectively, wideband AR response with three minima is realized. Based on this design method and the theoretically derived results, a CP patch antenna prototype with a height of 0.028λ ₀ is fabricated and measured. It is experimentally demonstrated that around 8.8%, bandwidth can be achieved with VSWR <; 2, AR <; 3 dB, and 3 dB gain variation.

This paper presents a patch and SIW cavity hybrid antenna with wide bandwidth and high-selectivity filtering response. The proposed antenna is a compact single layer structure with a height of 0.037 free-space wavelength and composed of one circular patch resonator, one square SIW cavity, and nine metallic pins. The circular patch is internally embedded in the square SIW cavity with a surrounded ring slot and the feeding probe is located in the SIW cavity. Here, the dominant resonant mode of the circular patch resonator (TM11) and first two modes of the SIW cavity (TE11 and TE21) are employed to achieve effective radiation from the ring slot under these three resonances. Among them, TM11 and TE21 both have broadside radiation performances and thus be used to achieve wide bandwidth, while TE11 is utilized to generate a radiation null in the lower stopband. Meanwhile, the mixed coupling occurs between these two distinct resonators can introduce another radiation null in the upper stopband. Besides, nine metallic pins can further enhance the frequency selectivity in the lower stopband. For demonstration, a prototype antenna is designed, fabricated, and measured. All the expected and simulated results have been satisfactorily confirmed in experiment.

In this paper, a compact microstrip patch antenna (MPA) with enhanced bandwidth and filtering response is proposed and implemented solely by loading pairs of slots on the conventional square patch radiator. Initially, the dominant mode TM10 of the square patch radiator is theoretically investigated, revealing its broadside radiation pattern and its current flows in the consistent direction. After that, based on the regularities of field distribution, two pairs of slots are etched in the square patch so as to generate two extra modes without destroying both resonant and radiating properties of TM10 mode. By virtue of these two excited modes, two extra resonances are produced in proximity to its dominant one, so as to widen the bandwidth under triple resonant modes. Meanwhile, owing to the inverse equivalent magnetic currents (EMCs) along the radiation edges of the patch and the out-of-phase electric fields at the two sides of each slot under these two excited modes, radiation cancellation towards broadside direction and efficiency reduction occur simultaneously. Hence, radiation nulls are achieved in all direction and particularly the radiation null towards broadside direction are deeper than others. After our intensive investigation is done on the entire slots-loaded patch radiator, a wideband filtering MPA with three in-band resonances and two out-of-band radiation nulls has been finally constructed. For demonstration, a prototype antenna at a center frequency of 3.5 GHz is designed, fabricated and measured. The measured results are found in good accordance with the simulated ones, demonstrating an impedance bandwidth of 9.14% with stable broadside realized gains as well as two radiation nulls at 3.12 and 3.83 GHz, respectively, in addition to its low profile of 0.029 free-space wavelength. Notably, the presented antenna has occupied the almost same area as the conventional square MPA, maintaining its good size compactness.

In this paper, a class of bandpass frequency selective structures (FSSs) with improved out-of-band rejection is proposed by using stacked slotlines. The proposed FSS consists of a periodic array of slotlines etched in pieces of single-layer printed circuit boards, which are vertically placed along the periodic plane and then stacked together with a certain distance. Due to this special arrangement, slotline resonators are constructed based on the desired propagation modes, thus leading to a stable bandpass filtering response in a wide frequency range. To better understand the operating principle, design procedure of this class of FSSs with uniform slotline unit cells, inductor-loaded uniform slotline unit cells, and stepped-impedance slotline unit cells is presented step by step. In particular, the equivalent transmission line models are established and the even–odd mode analysis method is introduced to intensively analyze the resonant and filtering mechanisms. As two examples, a single-polarized FSS and a dual-polarized one, both based on stepped-impedance slotline unit cells, are designed, fabricated, and measured. Measured results indicate that the proposed FSSs feature miniaturized unit-cell size, wide upper rejection band, and stable frequency filtering response under a large variation of incident angles.