In this article, a low-profile, compact, quad-port super-wideband (SWB) multiple-input–multiple-output (MIMO) antenna is presented for the internet of things (IoT) applications. The proposed antenna comprises four identical sickle-shaped resonating elements, which are excited by tapered coplanar waveguide (CPW) feed lines. The antenna elements are arranged in rotational symmetry (mutually orthogonal to each other) to achieve high port isolation. A complementary slot, which matches the sickle-shaped radiator, is etched from the ground of the proposed monopole antenna element to achieve massive bandwidth. The MIMO antenna possesses a resonating bandwidth ( $\vert S_{11}\vert \le -10$ dB) of 1.3–40 GHz and a bandwidth ratio of 31:1. In addition, an L-shaped slit and a complementary split-ring resonator (CSRR) are introduced in the sickle-shaped radiator to reject Bluetooth (2.4 GHz), WLAN (5.5 GHz), and downlink of X-band satellite communication (7.5 GHz) signals from the SWB. The proposed MIMO antenna is fabricated and experimental results are found in agreement with the simulated results.

A compact, low-profile, coplanar waveguide (CPW)-fed quad-port multiple-input–multiple-output (MIMO)/diversity antenna with triple band-notched (Wi-MAX, WLAN, and X-band) characteristics is proposed for super-wideband (SWB) applications. The proposed design contains four similar truncated–semi-elliptical–self-complementary (TSESC) radiating patches, which are excited through tapered CPW feed lines. A complementary slot matching the radiating patch is introduced in the ground plane of the truncated semi-elliptical antenna element to obtain SWB. The designed MIMO/diversity antenna displays a bandwidth ratio of 31:1 and impedance bandwidth (|S₁₁| ≤ − 10 dB) of 1.3–40 GHz. In addition, a complementary split-ring resonator (CSRR) is implanted in the resonating patch to eliminate WLAN (5.5 GHz) and X-band (8.5 GHz) signals from SWB. Further, an L-shaped slit is used to remove Wi-MAX (3.5 GHz) band interferences. The MIMO antenna prototype is fabricated, and a good agreement is achieved between the simulated and experimental outcomes.

A planar, microstrip line-fed, quad-port, multiple-input-multiple-output (MIMO) antenna with dual-band rejection features is proposed for ultra-wideband (UWB) applications. The proposed MIMO antenna design consists of four identical octagonal-shaped radiating elements, which are placed orthogonally to each other. The dual-band rejection property (3.5 GHz and 5.5 GHz corresponding to Wi-MAX and WLAN bands) was obtained by introducing a hexagonal-shaped complementary split-ring resonator (HCSRR) in the radiators of the designed antenna. The MIMO antenna was etched on low-cost FR-4 dielectric substrate of size 58 × 58 × 0.8 mm³. Isolation higher than 18 dB and envelope correlation coefficient (ECC) lesser than 0.07 was observed for the MIMO/diversity antenna in the operating range of 3–16 GHz. The presented four-port UWB MIMO antenna configuration was fabricated, and the experimental results validate the simulation outcomes.

This article presents a compact, planar, quad-port ultra-wideband (UWB) multiple-input–multiple-output (MIMO) antenna with wide axial ratio bandwidth (ARBW). The proposed MIMO design consists of four identical square-shaped antenna elements, where each element is made up of a circular slotted ground plane and feed by a 50 Ω microstrip line. The circular polarization is achieved using a protruding hexagonal stub from the ground plane. The four elements of the MIMO antenna are placed orthogonally to each other to obtain high inter-element isolation. FR-4 dielectric substrate of size 45 × 45 × 1.6 mm³ is used for the antenna prototype, and a good agreement is noticed among the simulated and experimental results. The proposed MIMO antenna shows 3-dB ARBW of 52% (3.8–6.5 GHz) and impedance bandwidth (S₁₁ ≤ −10 dB) of 144% (2.2–13.5 GHz).

A small size, low-profile, ultra-wideband (UWB) antenna with (WLAN/Wi-MAX) band notch behaviour is proposed for microwave imaging applications. The designed UWB antenna consists of a microstrip line feed of 50 $\Omega$, rhombus-shaped monopole radiator, and a rectangular ground surface. In order to achieve a miniaturized antenna configuration, the two edges of the ground plane are modified by using the Koch fractal shape of second-order. Further, in the radiating element of the UWB antenna, elliptical-shaped complementary split-ring resonator (CSRR) is loaded to notch the interfering Wi-MAX (3.5 GHz) and WLAN (5.5 GHz) signals. The designed antenna covers a bandwidth $\left(S_{11} \leq-10 \mathrm{dB}\right)$ of 3-12.8 GHz (9.8 GHz) with two notches at 3.5 GHz (3.3-3.7 GHz) and 5.5 GHz (5.3-5.7 GHz).