This study presents a new design of a four-element multiple-input multiple-output (MIMO) antenna, which is suitable for X-band applications. The circular polarization (CP) operation in this work is attained by using Tai Chi-shaped patches and L-shaped feeds. The proposed four-element MIMO antenna achieves two resonant frequencies, the first one at around 7.75 GHz and the second one with CP radiation is at approximately 10.15 GHz frequency. The measured isolations between ports with 46.7 × 46.7 mm² total dimensions are <−20 dB. Due to the experiments, the envelop correlation coefficient for the orthogonal and parallel ports of the proposed MIMO antenna is <0.003 and 0.005, respectively. Additionally, the presented antenna has a circularly polarized performance in the frequency band of 9.75–10.41 GHz and the average realized peak gain almost 2 dBic. The investigated antenna in this work is fabricated and tested in which the results are in a good agreement.

A unique coplanar‐waveguide (CPW)‐fed circularly polarised square slot antenna with enhanced impedance bandwidth (IBW) is presented. The antenna structure includes a pair of rectangular‐shaped notches located at two opposite corners of the slot for achieving a significantly enhanced IBW of 12.06 GHz (2.76–14.82 GHz), and a pair of reverse L‐shaped ground arms in the slot for realising circularly polarised radiation with 1.86 GHz (4.27–6.13 GHz) bandwidth. This proposed technique has the advantages of covering the whole of the ultra‐wideband spectrum (3.1–10.6 GHz), an average gain of 3 dBi and a reduced antenna size of 25 × 25 × 0.8 mm ³ .

A dual-functionality printed dipole array antenna is proposed in this study with a fan-beam property. An integrated balun structure cooperation with the dipole arms creates a resonance in the X-band for the antenna. Adding Single-Ring Split-Ring Resonator (SR-SRR) next to the feed line allows another resonance in the C-band. This way, a dual-band dipole array antenna with identical radiation properties is accomplished. Considering the distance between the two resonances of the antenna, the main contribution of this work is the similar radiation pattern in the two frequency bands of the antenna. According to the experimental results, the introduced antenna can cover the frequency band of 4.52-5.83 GHz for C-band applications (such as; satellite communication and microwave relay) and 9.08-9.66 GHz for X-band applications (such as; weather mapping and detecting and long-range tracking radar) with peak gains of 9.05 dBi and 11.30 dBi respectively and similar fan-beam radiation patterns in two frequency bands.

This paper presents a linear-to-circular polarization converter (LCPC) used for multi-band functionality. The LCPC surface features symmetrically printed copper strips on a single-layer substrate and is surrounded by a square substrate integrated waveguide (SIW) cavity arranged periodically in two dimensions. The study successfully utilizes the LCPC surface to achieve a desirable axial ratio (AR) band width (BW) below 3 dB in multiple frequency ranges, specifically: 1) 4.51-4.80 GHz, 2) 6.51-6.80 GHz, 3) 9.01-9.20 GHz, 4) 10.90-12.40 GHz, and 5) 17.05-17.49 GHz. The LCPC induces a change in polarization as the wave propagates through it. In the first ( $f_{31}$ ) and fourth ( $f_{34}$ ) frequency bands, the initially linearly polarized (LP) wave transforms into a left-hand circularly polarized (LHCP) wave. Conversely, in the second ( $f_{32}$ ), third ( $f_{33}$ ), and fifth ( $f_{35}$ ) frequency bands, the wave converts into a right-hand circularly polarized (RHCP) wave. Measurement results indicate that the LCPC provides circularly polarized (CP) wave bandwidths of 12.72%, 4.8%, 2.4%, 11.39%, and 3.15% across frequency ranges of 3.9 - 4.43 GHz, 6.21- 6.52 GHz, 8.9-9.08 GHz, 10.51-11.78 GHz, and 16.85-17.39 GHz, respectively. This characteristic renders the LCPC suitable for utilization in the C (4-8 GHz), X (8-12 GHz), and Ku (12-18 GHz) frequency bands.