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 single-layer novel windmill element is designed to broaden the bandwidth of reflectarray antenna in this paper. To obtain the enough phase range, a windmill ring patch and a circular ring patch are utilized to realize the multi-resonant state. As a result, a linear phase shift curve which covers 473.6° phase range is achieved by optimizing the parameters. Based on that, a large proportion of phase shift curve keeps parallel from 27.5 GHz to 42.5 GHz, it indicates the proposed element features broadband property. To this end, a single-layer offset-fed prototype with a square aperture of 80mm×80mm which is composed of 400 elements is designed, measured, and tested at Ka-band. Through detailed measurements, a peak gain of main beam is 27.86 dBi with an aperture efficiency of 51.7% has achieved at 37 GHz. The sidelobe level and cross-polarization are also lower than -17 dB and -25 dB, respectively. More importantly, the degradation of the peak gain from 27.5GHz to 40 GHz (35.71%) is less than 3 dB. The proposed antenna which features wide bandwidth with good radiation performance and simple structure will be suitable for 5G millimeter-wave communication.

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.

In this article, a broadband W-band TE□₁₀-TE◯₀₂ mode converter is proposed and verified. To improve the output performance of the TE□₁₀-TE◯₀₂ mode converter, a TE□₁₀-to-TE□₄₀ intermediate mode conversion section with a Y-type power divider structure and a TEx₄₄-like-to-TE◯₀₂ intermediate mode conversion section with a novel cross-sector-shaped transition structure are adopted in the design (the superscript symbol □ represents the rectangular waveguide, ◯ represents the circular waveguide, x represents the cross waveguide). The simulation results show that the transmission coefficient (S₂₁ parameter) of the designed TE◯₀₂ mode converter is better than -0.18 dB in the whole W-band (75-110 GHz), and the corresponding mode conversion efficiency is larger than 96%. Cold test results show that the transmission coefficient is higher than -1.9 dB and the reflection coefficient (S₁₁ parameter) is less than -15 dB in the frequency range of 75-110 GHz.

To realize multiband mode conversion in the high-power microwave (HPM) application system, a novel compact dual-band mode converter based on a universal method is proposed in this article. The proposed converter has a hybrid circular-embedded coaxial structure, and it integrates the conventional plate-inserted coaxial TEM-TE₁₁ converter and serpentine circular TM₀₁-TE₁₁ converter simultaneously. Its outer and inner waveguides have the same curvature distributions along a longitudinal direction. Both the numerically calculated results and simulations exhibit that its maximum conversion efficiencies at two central frequencies are over 99% with a structure length of 240 mm. Simulated results indicate that the bandwidths reach 390 MHz (at X-band) and 4.2 GHz (at Ka-band), respectively, when the conversion efficiencies are over 95%. A prototype is fabricated and measured. The far-field results demonstrate that coaxial TE₁₁ and circular TE₁₁ modes with high purity are detected at the output port of converter, and the S-parameter measurements illustrate that high conversion efficiencies at 8.4 and 30.5 GHz are obtained. The measurement results have a good agreement with the theoretical predictions and simulations.