A parabolic reflector antenna with an azimuthally deformed Cassegrain subreflector is proposed to effectively generate arbitrary orbital angular momentum (OAM) modes. The Cassegrain dual-reflector antenna was fabricated for 18 GHz and measured in the near-field range. The nearfield to far-field transformed radiation phase around a full azimuth shows that the proposed antenna generates fields with the l = 1 OAM mode. Simulated and measured feed reflection coefficients are below −10 dB for 15.1-21.2 GHz, even though the distance between a horn feed and a deformed subreflector is 15.6 mm, thus it is expected that the proposed structure can be used for a broadband low-profile antenna.Introduction: Various researches based on orbital angular momentum (OAM) modes have been extensively studied in optics [1]. Recently, communication systems analogous to the optical one were demonstrated in radio frequency [2][3][4]. To utilise OAM characteristics for radio communications, generating and detecting arbitrary OAM modes is essential. In [2], a helicoidally parabolic reflector, which is similar to [5], is used as the main reflector to transmit electromagnetic waves with a fundamental OAM mode (l = 1). From the standpoint of reflector antenna design, fabricating the main parabolic surface as a helicoid [2, 5] is expensive and inefficient, because ordinary main reflectors should be modified for the generation of non-zero OAM modes. Therefore, we need a simple and efficient method to generate arbitrary OAM modes while keeping the main reflector unchanged. In this Letter, we propose an alternative structure [6] in which a Cassegrain subreflector is azimuthally deformed instead of the main parabolic reflector. A parabolic reflector antenna with an azimuthally deformed Cassegrain subreflector enables us to process small area of a subreflector without modifying a large main reflector. Owing to this merit, we can adopt a variety of ordinary main reflectors for the efficient generation of arbitrary OAM modes. In addition, the proposed antenna has very good reflection characteristics due to the fact that a deformed subreflector radiates higher-order OAM modes which cannot be received by a normal feed horn.
In this paper, we propose a convenient microwave orbital angular momentum (OAM) mode generation and multiplexing method operating in the 18 GHz frequency band, based on a 2×2 uniform circular array and a 4×4 Butler matrix. The three OAM modes −1, 0, and +1 were generated and verified using spatial S‐parameter measurements; the measured back‐to‐back mode isolation was greater than 17 dB in the full 17 GHz to 19 GHz range. However, the radiated OAM beam centers were slightly dislocated and varied with both frequency and the mode index, because of the non‐ideal characteristics of the Butler matrix. This resulted in mode isolation degradation and transmission distance limitations.
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