Different from traditional multiple-input and multiple-output (MIMO) radar, the frequency diverse array MIMO (FDA-MIMO) radar generates beampattern that is dependent on both range and angle, making it applicable for joint range–angle estimation of targets. In this paper, we propose a novel time reversal based FDA-MIMO (TR-FDA-MIMO) approach for target detection. Based on the time reversal theory, the TR-FDA-MIMO signal model is established, the TR transmitting–receiving and signal processing procedure are analyzed, and the resulting range–angle spectra for targets imaging are acquired by utilizing the multiple signal classification algorithm. Numerical simulations are carried out for both single and multiple targets cases. The imaging resolution and robustness to the noise of the proposed approach are investigated and results are compared with conventional FDA-MIMO radar. It turned out that by cooperating with TR, the performance of FDA-MIMO radar for target range–angle estimation is effectively enhanced, consequently improving its applicability in practical target-detecting cases.

This paper presents design of quintuple-mode wideband bandpass filters, implemented with off-centered perturbed metallic cylinders in a rectangular waveguide cavity. Three perturbation cylinders are placed at the bottom of the rectangular waveguide cavity, along with a pair of perpendicularly fed coaxial lines; excite five quasi-transverse magnetic modes to realize the desired passband. The height of the waveguide cavity and the shape of the perturbation cylinders are exploited to shift the resonant modes far away from the passband and achieve a good out-of-band rejection and sharp skirt selectivity. The filter operates at the center frequency of 2.68 GHz with a wide fractional bandwidth of 43%. The proposed filter is fabricated with aluminum. The measured and simulated results are in good agreement with each other.

This paper presents center frequency and bandwidth switchable substrate integrated waveguide filters loaded with PIN diodes. The diodes are added in the slot on the surface of the resonators to switch the resonant frequencies and the coupling coefficients. Although the introduction of the slot causes extra radiation loss, it is small and acceptable. The proposed center frequency switchable filter has four center frequency switchable states of 1.78, 1.82, 1.88, and 1.91 GHz, while the bandwidth only changes ±0.64%. The bandwidth switchable filter has two states with 3 dB bandwidths of 70 and 103 MHz at a center frequency of 2.08/2.09 GHz. The measured performance of the fabricated filters shows good agreement with the simulation.

A high-isolation diplexer based on a dual-mode substrate integrated waveguide (SIW) resonator is proposed in this paper. Based on the theory of the dual-mode resonator, the miniaturized diplexers are designed by using the SIW dual-mode resonators. The superior isolation of the diplexers is obtained because the two operating modes of the dual-mode SIW resonators are not directly coupled and there is no interference with each other. In order to further improve the isolation of the circuit, the number of the order of the diplexer is added. Equivalent circuits are given to analyze and design the dual-mode high-isolation diplexers. Detailed analyses are given according to the equivalent circuits. The dual-mode third-order and fourth-order diplexers are designed and fabricated. The measured results agree well with the simulated ones. The total sizes of the fabricated third-order and fourth-order diplexers are 1.78λg × 2.64λg and 1.79λg × 3.63λg, respectively.

This paper presents a simple topology of 180° hybrid coupler with non-interspersed inputs and outputs, as opposed to the conventional rat-race topology. Such hybrid coupler topologies with inputs located on one side and outputs at the opposite side simplify the design of signal routings and module packaging when integrated with other circuits and components. It also relaxes the necessity of using auxiliary compensating components such as tapers, jumpers, and crossovers. The proposed coupler topology is theoretically analyzed through a T-matrix approach. The analysis comes up with design equations and diagrams that help choose the structural parameters for the desired specification. The simplicity and also the immunity against intrinsic parasitic effects within the proposed topology make it an excellent candidate for operation over any RF and millimeter-wave frequency bands, for applications such as automotive radar or E-band backhaul radio. To this end, a coupler based on the proposed scheme is realized for operation over 77 GHz and assessed through on-wafer measurements. Good agreement of the measured results with the simulated and analytical counterparts demonstrates the superior performance of the proposed 180° hybrid coupler.

A novel planar dual-band bow-tie slotted patch antenna backed by high-impedance surface (HIS) is designed at 2.5 and 3.5 GHz for wireless application. The antenna employs coplanar waveguide fed patch and bow-tie slot as radiating elements. The bow-tie slot enables dual-band operation for the antenna. The HIS is made asymmetric in design to make it polarization dependent. This polarization-dependent HIS is eventually designed to reflect circularly polarized waves from linearly polarized incident waves.

This paper presents compact rectenna arrays for ambient RF energy harvesting on the 2.45 GHz ISM band. The arrays are based on four and nine series-connected rectenna cells. Each cell is composed of a stacked fractal antenna and an RF-to-dc conversion circuit. The antenna is a compact third Koch fractal shape, fed by a coaxial probe for more compactness. The conversion circuit is a full-wave rectifier with a differential output, each DC polarity is provided by a two-stage Dickson voltage multiplier. Measurement results show a significant increase of the output DC voltage for the one, four, and nine cells rectenna arrays. They provide, for power density of 1.7 μW/cm2, an output DC voltage of 0.9, 2.2, and 4.1 Volts, respectively. The 9 cells rectenna array is used in a remote supply experiment of a temperature and acceleration wireless sensor, where the data are transmitted via a Bluetooth low energy link to a distant smartphone every 1 min.

The present work describes a unique planar low-profile wideband circularly polarized Multiple-Input and Multiple-Output (MIMO) antenna operating in the X-band, with pattern and polarization diversity over the entire axial-ratio bandwidth (ARBW). The design is unique in the sense that a simple grounded stub introduced between two linearly polarized monopole antennas has been used to realize wideband circular polarization, pattern diversity, and high isolation between antennas. The ARBW of the MIMO antenna is 2.45 GHz (8.11–10.56) 3 dB and its impedance matching bandwidth is 3.52 GHz (8.07–11.59). The isolation is better than 20 dB. The antenna can be easily adapted to operate other frequency bands by simple frequency scaling. It has been fabricated on an FR-4 substrate and its performance has been compared against several existing available antennas.

A novel multiple-input and multiple-output (MIMO) fractal antenna excited by a coplanar waveguide was investigated in this study. A novel technique was used to improve the isolation of 20 dB between the dual radiating elements by inserting a strip line into the outer edges of the ground plane. A sunflower structure was used to configure the antenna in three steps. At each step, an additional sunflower structure was added with half the size of that used in the previous step to enhance the impedance bandwidth. The measured values of envelop correlation coefficient and total active reflection coefficient indicated that the proposed MIMO antenna has high-diversity performance between radiating elements. Wide dual operating bands of 2–2.9 and 5–10 GHz were obtained, which can support different wireless communications, such as 3G, LTE (2.6 GHz), WLAN (2.4 GHz/5 GHz), WiMAX (2.4 GHz/5GHz), ISM (2.4 GHz/5 GHz), 5G (5–6 GHz), and satellite communications (6–8 GHz). The MIMO fractal antenna with a small size achieved a maximum efficiency of 85% and a peak value gain of 6 dBi, low-channel capacity loss of 0.15–0.4 b/s/Hz, and high isolation between radiating elements is suitable for portable communication devices.

This paper presents the capacity of an S band circularly polarized patch antenna using a ferrite material for low-earth orbiting satellites (2025–2100 MHz for TeleCommand) to obtain a semi-isotropic radiation pattern. Indeed, circularly polarized antennas are generally large and bulky which is often incompatible for spatial applications especially for small satellites. A new antenna design is proposed with the following maximum dimensions: a diameter of 80 mm and a height of 12 mm. The structure presents an axial ratio lower than 6 dB (according to requirements) and a realized gain higher than −6 dB over a 4% bandwidth at the limit of coverage i.e. 90° irrespective of the azimuth angle (φ).

In this paper, a reduced-size dielectric resonator antenna with switchable diversity patterns is proposed. Ring- and linear-shaped slots are etched in the ground plane of the antenna so as to generate two modes $TE_{\delta 11}^x$ and $TE_{\delta 12}^x$ at a center frequency of 19 GHz. Moreover, two groups of PIN diodes are integrated into these slots to short one group of slots, and let the other group generates the required mode. Thus, the antenna is able to generate two switchable patterns with an envelope correlation coefficient of 0.4. Furthermore, the antenna size is reduced to half of its original size by placing a copper sheet over certain plane of the antenna structure. The antenna achieves wide bandwidths of 17.6–20.9 GHz (17.1$\percnt $) and 18.3–21.6 GHz (13.8$\percnt $) in cases of exciting $TE_{\delta 11}^x$ and $TE_{\delta 12}^x$ modes, respectively. The antenna also attainsa high gain of 7.1 and 3.2 dB at the center frequency.