A compact Ka-band monopulse Cassegrain antenna based on the reflectarray elements is presented in this letter. The proposed antenna consists of a main-reflectarray, a sub-reflectarray, a SIW comparator and a monopulse feed to achieve good sum-difference characteristics. Compared with traditional monopulse Cassegrain antenna, the total length of the proposed antenna is shrinked to 67.85%. The results of measurement agree with that of simulation very well. The measured SUM beam gain of the proposed antenna is 29.4 dBi by eliminating the loss of Ka-band comparator, the 3-dB beamwidth is 3.85°, the first sidelobe level is under -12 dB, the cross-polarization level is better than -20 dB, and the gain ratios between the SUM and DIFF are about 3 dB and 5 dB in azimuth and elevation plane, respectively. The measured results show that the proposed antenna is a good candidate for the low-cost multi-mode tracking system with limited space, especially for that working at high frequency.

A photonic scheme that can simultaneously estimate the microwave Doppler-frequency shift (DFS) and angle-of-arrive (AOA) is demonstrated. In the proposed system, the transmitted signal is independently mixed with two echo signals by a dual-channel microwave photonic mixer. By measuring the frequency of the intermediate frequency (IF) signal output from the two channels and the phase difference between them, the DFS (with direction identification) and AOA parameters can be obtained. In a proof-of-concept experiment, the errors are less than ± 0.08 H z for the DFS measurement within a range of ± 100 k H z and less than ± 1 . 3 deg for the AOA measurement ranging from 0° to 90°, respectively.

A novel mode-transducing antenna based on metal-only transmitarray surfaces is proposed in this paper. It provides a compact, low-cost planar antenna structure. To clearly demonstrate the performance of the proposed mode-transducing antenna, a prototype consisting of a TM0,1 mode generator, a corrugated waveguide taper and several transmitarray surfaces was designed, fabricated and measured at X-band. The simulated and measured resuts show that the circular waveguide TM0,1 mode can be converted into a mode with linear polarization using the proposed mode-transducing antenna. Furthermore, a gain of 23.4 dBi at the center frequency of 10.3 GHz and a 1-dB gain bandwidth of 9.7% have been achieved. The measured polarization purity is 98.57% at 10.3 GHz.

A novel polarization rotation element (PRE) is employed to design a wideband transmitarray antenna (TA). The PRE comprises three metallic layers. The middle layer features double C-shaped patches, while the upper and bottom layers consist of two grating polarizers. By adjusting the opening sizes and directions of the double C-shaped patches, full phase coverage within -1 dB transmission magnitude can be achieved. Moreover, the polarization conversion ratio (PCR) is more than 97% from 9.2 to 15.2 GHz. Based on the proposed PREs, a horn-fed TA with a 256×256 mm2 aperture area have been designed, fabricated and measured. The measured results of the TA show that the peak gain of 27.3 dBi and the aperture efficiency of 61.9% are achieved at 9.7 GHz. The maximum sidelobe level (SLL) and cross-polarization level at this frequency are -18.14 dB, and -37.36 dB, respectively. Furthermore, the 1-dB and 3-dB gain bandwidth of the TA is 33.9% and 46.9%, respectively. The measured results are in reasonable agreement with the simulated ones.

Metasurfaces offer exciting opportunities that enable precise control of microwave propagation, phase, and polarization, which has been recently studied to realize the versatility of such surfaces. However, there have been few attempts to realize mode conversion using them. In this communication, we demonstrate a novel method for mode conversion that consists of a mode generator, a wavefront transforming mirror, and a metasurface. The measured polarization content can reach 98.3% at the center frequency of 94 GHz using the proposed method, indicating that the circular TE _0,1 mode has been successfully converted into linear polarization through the metasurface. The proposed method will be an important step forward in the advancement of millimeter communication and radar systems.