A double-layer radial line helical array antenna with rectangular aperture is proposed. With rectangular aperture, the antenna can be assembled to form high gain antennas. The use of double-layer feed system ensures an equal-amplitude in-phase feed for all elements in an expected frequency band, which can improve antenna gain and aperture efficiency. This paper presents its design concept, derives pertinent design and performance, and a 16-element array antenna is simulated and measured. The experimental results show that in the range of 3.8 GHz to 4.2 GHz, the antenna gain is over 17.7 dB, aperture efficiency over 82%, antenna sidelobe level below -12.0dB, antenna axial ratio below 3.2 dB, and antenna VSWR below 1.52.
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