In this article, an inductance-based decoupling scheme is proposed to reduce the mutual coupling between extremely closely spaced microstrip antennas. The original strong coupling can be effectively suppressed by simply inserting a lumped inductance in between. To offer a systemic design guideline for this decoupling strategy, a mode cancellation method, based on the synthesis of common mode (CM) and differential mode (DM), is proposed. The inserted inductance plays a role of tuning CM and DM impedances to a similar status, which has an equivalent decoupling effect according to the theory of microwave network. Alternatively, the lumped inductance could also be replaced by an inductive connecting strip for a concise topology. To validate the proposed decoupling concept, a prototype is simulated, fabricated, and measured. The experimental results show that the poor isolation of 5 dB is improved to better than 15.4 dB across the entire matched bandwidth of 2.394–2.530 GHz, with an extremely close edge-to-edge distance of $0.016~\lambda _{0}$ and center-to-center distance of $0.44~\lambda _{0}$ . Furthermore, the validation of extending to large-scale 1-D and 2-D arrays is also discussed. Featuring simple structure, compressed dimension, strong-coupling suppression, and good radiation performance, the proposed decoupling scheme possesses promising potential for antenna array applications.

Decoupling Between Extremely Closely Spaced Patch Antennas by Mode Cancellation Method

A novel and efficient parasitic decoupling network (PDN) is proposed for closely coupled antennas in this paper, which provides a new perspective and approach to design DNs based on the parasitic decoupling concept. The PDN is composed of sections of transmission lines (TLs) and reactive components, allowing additional power-traveling paths. With the TL lengths and reactive-component reactances being precisely determined, the traveling power from one antenna to another could cancel out the undesired power wave induced by mutual coupling, resulting in the high port isolation. The decoupling theory is rigorously derived with the design procedure of the PDN systematically described in connection with two examples of the two- and three-element monopole arrays. The measurement results show that good impedance matching for each antenna port, isolations of more than 25 dB, total efficiencies of more than 64%, and envelop correlation coefficients (ECCs) of less than 0.07 are attained simultaneously for both examples using the proposed PDN. The results verified the decoupling theory and proved the PDN concept.

Novel and Efficient Parasitic Decoupling Network for Closely Coupled Antennas

The active impedance of each element in a phased array changes substantially with scan angle due to mutual coupling. Therefore, a key challenge in the design of wide-angle scanning phased array is to achieve wide-angle impedance matching (WAIM). In this communication, an efficient decoupling network is developed to reduce the mutual coupling between adjacent elements for this purpose. A complete scattering matrix analysis of the decoupling network is given and the design procedure is summarized. A ${1} \times {16}$ linear microstrip phased array has been designed and fabricated for experimental verification. Good agreement is obtained between measured and simulated results. The measured mutual coupling between adjacent elements is reduced to lower than $- {35}\; \text{dB}$ at the center frequency. Due to the reduced mutual coupling, the array can experimentally scan to 66° with a gain reduction of only 3.04 dB.

Wide-Angle Scanning Phased Array Using an Efficient Decoupling Network

In this paper, an effective approach for mitigating the near-field coupling between four-port circularly polarized (CP) antennas in a 30-GHz multiple-input, multiple-output (MIMO) system is suggested and investigated. This is obtained by incorporating a two-layer transmission-type frequency selective surface (FSS) superstrate based on planar crossed-dipole metal strips. This paper presents a comparison between the mutual coupling when the patches are radiating in free space and in the presence of the FSS layers. The simulated results, when the FSS layers are applied, show an average of 6–12-dB improvement in the isolation between four adjacent CP-MIMO antennas. In addition, an accurate study is carried out on the insignificant reflections produced by the FSS layers to redirect those and also prevent any interference. The proposed $2\times 2$ CP-MIMO antenna along with the superstrate is implemented and tested to validate the simulation results. Experimental results of the coupling and reflection coefficients and axial ratio show an acceptable agreement with the corresponding simulated ones.

Spatially Decoupling of CP Antennas Based on FSS for 30-GHz MIMO Systems

A general and systematic approach is the first time proposed in this paper to design the neutralization line (NL) for isolation enhancement in multiple-input multiple-output (MIMO) antenna arrays. Each NL consists of a metal strip and a reactive component attached at its middle position. It is connected with every two antenna radiators to generate an additional coupling path against the original antenna coupling for isolation enhancement. A general network model that leads to a systematic design procedure is developed to meet the required isolation criteria. Three practical examples are presented and used to verify the proposed NL-based decoupling technique (DT). The measurement results align well with simulations. For all these examples, good impedance matching, isolation over 25 dB, total radiation efficiencies over 70%, and very low envelop correlation coefficients (ECCs) can be achieved simultaneously after adopting the proposed NL DT. Compared with previous decoupling works using parasitic elements, decoupling networks or NLs, the proposed method can achieve either a much wider decoupling bandwidth or a much smaller antenna separation distance while keeping a low design complexity along with systematic design procedure.

A General and Systematic Method to Design Neutralization Lines for Isolation Enhancement in MIMO Antenna Arrays

An efficient decoupling feeding network is proposed in this letter. It is composed of two directional couplers and two sections of transmission line for connection use. By connecting the two couplers, an indirect coupling with controlled magnitude and phase is introduced, which can be used to cancel out the direct coupling caused by space waves and surface waves between array elements. To demonstrate the method, a two-element microstrip antenna array with the proposed network has been designed, fabricated and measured. Both simulated and measured results have simultaneously proved that the proposed method presents excellent decoupling performance. The measured mutual coupling can be reduced to below $-58 ~\hbox{dB}$ at center frequency. Meanwhile it has little influence on return loss and radiation patterns. The decoupling mechanism is simple and straightforward which can be easily applied in phased array antennas and MIMO systems.

An Efficient Decoupling Feeding Network for Microstrip Antenna Array

The invention discloses a frequency-control wave beam/focal point scanning plane reflective array/reflector. The basic structure of the plane reflective array and the basic structure of the reflector respectively comprise a printed patch array, a primary feed source and a corresponding supporting structure. The plane reflective array is irradiated by spherical waves, focusing of wave beams is realized in a far field, and the plane reflective array can be used in a modern wireless communication system and a modern radar system. The reflector is irradiated by plane waves or cylindrical waves or spherical waves, can realize wave beam focusing in a near field and can be applied to an imaging system. The plane reflective array and the reflector respectively enable the phase value actually obtained on each frequency point to meet the theoretically required value through selection of the size and freedom degree parameters of each unit, therefore, in-phase superposition, in different directions, of the array at different frequency points is realized, and the function of wave beam or focal point scanning is achieved. The frequency-control wave beam/focal point scanning plane reflective array/reflector has the advantages that electric wave beam/focal point scanning and high gain are realized through the simple array surface structure, and the cost, the loss and the complexity of a current scanning array/imaging system are greatly reduced.

Frequency-control wave beam/focal point scanning plane reflective array/reflector

The invention discloses a terahertz medium-filled metallic channel waveguide. The terahertz medium-filled metallic channel waveguide structurally comprises a metallic channel and a medium filled in the channel. An electromagnetic wave is propagated along the longitudinal direction of the channel, the fundamental mode thereof is a transverse electric mode, the higher mode is a transverse magnetic mode, and the cut-off frequency is decided by the depth of the channel. The invention gives the characteristic equation of the propagation constant of the fundamental mode. The terahertz medium-filled metallic channel waveguide disclosed by the invention is simple in structure, low in loss, and easy to integrate. The invention enumerates various passive devices capable of being based on the waveguide structure. In particular, the terahertz medium-filled metallic channel waveguide disclosed by the invention is used as a series feed network for an antenna array, and capable of realizing a terahertz antenna array which is simple to feed, high in gain, with a planar structure, and easy to integrate. The terahertz medium-filled metallic channel waveguide disclosed by the invention is capable of being used for a terahertz high-speed wireless communication and radar system.

Terahertz medium-filled metallic channel waveguide

In this letter, a novel three-element linear cavity- backed antenna (CBA) array with high aperture efficiency is proposed. First, in order to reduce the loss and cost caused by the feeding network, a multiport CBA acting as a radiator as well as a power divider is investigated. Then, based on the multiport CBA, a three-element antenna array with a wide operating frequency band (19–21 GHz) is designed. The simulated broadside gain is from 9.7 to 11.1 dBi over the whole operating band, and the aperture efficiency of the proposed CBA array ranges from 99.1% to 123.7%. The proposed idea has potential applications in low-cost antenna arrays due to the simplified feeding network.

Novel Multiport Cavity-Backed Antenna for Low-Cost Array Applications

In this letter, a microstrip phased array adopting an overlapped feeding network in order to achieve wide-angle impedance matching is proposed. A 1 × 16 phased array prototype has been designed and fabricated for experimental verification. Coupling coefficients and active element patterns for all the radiating elements have been measured to calculate the active reflection coefficients and to synthesize the array radiation patterns. Simulated and measured results show that the investigated method is effective to improve the impedance matching over a wide scan range compared to conventional impedance-matching techniques. Due to the improved impedance matching especially at large angles, the array can experimentally scan to ±60
 °
 with a gain decrease lower than 3 dB.

Experimental Investigation of Wide-Angle Impedance Matching of Phased Array Using Overlapped Feeding Network

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