This paper presents a novel method of designing the compact dual-sense circularly polarized (CP) microstrip antenna/array with improved impedance bandwidth, axial ratio (AR) bandwidth, and interpolarization isolation. The left-hand and right-hand CP characteristics are achieved simultaneously by conceiving a vertically coupled resonant structure for the first time. Different from traditional methods, the antenna shows a compact size but exhibits the wide impedance and 3 dB AR bandwidths. For each input, two coupling paths are purposely designed to excite the TM10 and TM01-mode of the patch, respectively. By composing different coupling units in the two paths, an intrinsic ±90° phase difference over a broadband can be obtained, resulting in the broadband CP characteristics. In addition, a novel method of using the neutralization line between the two feeds is investigated to improve the isolation between the polarizations. Based on the dual-sense CP element, a $1 \times 4$ dual-sense CP array is composed to exemplify its potential applications in large arrays. Compared with traditional CP antennas, the proposed antennas have the advantages of compact size, simplified feed, broad impedance and 3 dB AR bandwidths, and improved isolation. Experimental results verify the predictions.

An interleaved shared-aperture dual-band dual-polarized base station array antenna is proposed in this paper. The lower-band (LB) element is realized by using a multiple folded-dipole antenna (MFDA) and four parasitic loops. To interpret the working principle of the MFDA, a double folded-dipole antenna (DFDA) is firstly analyzed by using the transmission line model. Then, by combining two bended DFDAs and introducing four parasitic loops, a low cross-band scattering LB element with a high out-of-band rejection level of 16 dB is obtained to cover 2.3-2.7 GHz. The higher-band (HB) element with a wide impedance bandwidth of 42.5% (3.0-4.6 GHz), a high roll-off rate (RoR) of 249.2 dB/GHz, and a high out-of-band rejection level of 17 dB is obtained by introducing a meander line loop, a rectangular loop and V-shaped strips near the dipole arms. By combining the proposed low scattering low-pass LB element and the high RoR high-pass HB element, a novel interleaved shared-aperture dual-band dual-polarized array antenna with a small frequency ratio of 1.46 and a high cross-band isolation level of 25 dB is realized. Due to the low-scattering characteristic and filtering response of the LB element, the radiation patterns of the wideband HB sub-arrays are almost unaffected.

A dual-polarized filtering antenna with steep cut-off and compact size is developed for base-station applications. In this design, four controllable radiation nulls are obtained by utilizing split rings, slotted T-shaped branches, a single-stub tuner, and a parasitic loop. Split rings are first used as the dipole arms to obtain the first radiation null at upper out-of-band. Four T-shaped branches working as DGS are printed under the crossed dipoles to achieve the second radiation null. The connected outer conductors of the differential feed structure acting as a single-stub tuner can provide the third radiation null to further enhance the upper band rejection. Finally, a parasitic loop is incorporated around the split rings, and the out-of-band rejection of the lower band is further enhanced by the fourth radiation null. More importantly, the impedance bandwidth of the antenna can be expended with two newly introduced resonant modes. As a result, a compact filtering antenna with a wide operational bandwidth of 1.7–3.01 GHz (56%) is realized for $\vert \text{S}_{dd11}\vert < -15$ dB with the isolation higher than 38 dB. The out-of-band suppression is higher than 18.4 dB in 3.1–4.5 GHz and more than 47 dB in 0.8–1.1 GHz.

In this research, Airborne broadband (maximum 400 MHz bandwidth) C band circularly polarized synthetic aperture radar (SAR) is proposed and developed for further study on airborne compact polarized synthetic aperture radar (CP SAR) system using circular polarization. This paper explains the scattering characteristic of circular polarization as the concept of Circularly Polarized SAR, system configuration, RF system and antenna, ground test, and flight test of Circularly Polarized SAR in Hinotori-C2 (Firebird-C2) mission onboard CN235MPA aircraft on 14-15 March 2018 at South Celebes, Indonesia. The result of the flight test depicts multipath full polarimetric circularly polarized images that show good performance of Circularly Polarized SAR and matched well to the result of the ground test of a multi polarized single pulse. The Circularly Polarized scattering clarification using trihedral, cylindrical, and Omni corner reflectors (TCR, CCR, and OCR), assessment of Circularly Polarized image analysis, and image classification using the conventional axial ratio (AR), ellipticity (ε), and polarization ratio (ρ) is discussed. The proposed Circularly Polarized SAR will enrich the existing CP SAR systems and could be employed in further study of CP SAR calibration technique, also applications development for the environment and disaster monitoring using CP SAR.