A Low-RCS and Low-Profile Circularly Polarized Double-Folded Transmitarray Based on Absorptive Frequency-Selective Polarization Converter

In this letter, a low-profile circularly polarized double-folded transmitarray antenna (CP-DFTA) at Ka band with low radar cross-section (RCS) performance is proposed by introducing a novel absorptive frequency-selective polarization converter (AFSPC). The proposed AFSPC comprises an upper lossy layer and a lower linear-to-circular polarization converter with an air gap between them, which achieves not only in-band linear-to-circular polarization conversion but also out-of-band absorption performance with a low profile of only 0.058$\lambda _{L}$ ($\lambda _{L}$ is the free-space wavelength of the lowest absorption frequency). Then, a double-folded transmitarray antenna is designed and integrated with the AFSPC to achieve the low profile of the antenna. A prototype of the low-RCS CP-DFTA is finally fabricated and measured. The measured results show that the proposed antenna achieves a peak CP gain of 25.85 dBic at 31 GHz, a 3 dB gain bandwidth of 20.7% covering 26 GHz to 32 GHz, and a 3 dB axial ratio bandwidth of 15.9% ranging from 26 GHz to 30.5 GHz. Simultaneously, an ultrawide 8 dB RCS reduction band of 105.2% covering 6.8 GHz to 21.9 GHz is achieved. The measured results are in convincing agreement with the simulated ones.