A multiplexed Cassegrain reflector antenna with a 2 × 2 open-ended rectangular waveguide (OERW) matrix feed and an orbital angular momentum (OAM) mode mux is proposed for the simultaneous generation of three OAM modes (l = 0, ±1). The OAM mode mux (OMM) was designed using sequential combinations of quadrature hybrids, crossovers, and phase shifters to multiplex and demultiplex three OAM modes at the same time. The 2 × 2 OERW matrix feed and the OMM were separately measured and their performances were verified according to proposed theories. A near-field antenna measurement for a multiplexed Cassegrain reflector antenna was conducted to obtain the far-field magnitude and phase patterns around polar elevation angle θ and azimuthal angle ϕ, thus confirming that our antenna can produce three OAM modes simultaneously. We also measured the communication link characteristics of two identical multiplexed antennas. The measurement results show that the channel isolation of three OAM modes is more than 12.7 [dB] and 17 [dB] for fixed and compensated receiver positions, respectively, indicating that the proposed antenna system can be used for independent communication links with the same frequency and polarisation.
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.
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