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In this paper, an Archimedean spiral antenna has been developed to flexibly radiate orbital angular momentum (OAM) wave with tunable modes. The operation mechanism for generation of the OAM wave is elaborated in terms of phases of the currents flowing on the spiral arm of the Archimedean structure. Two pairs of feeding probes are introduced to excite the Archimedean spiral antenna. With the signals with different phase differences fed into the spiral antenna, the OAM waves with the modes of l=±1, l=±2, and their all mixed modes are radiated, respectively. Simulated and measured results are given to demonstrate that the proposed Archimedean spiral antenna radiates the tunable OAM waves from the sole mode to the mixed four modes in an overlapped band of 5~6 GHz.INDEX TERMS Archimedean spiral, orbital angular momentum (OAM), tunable, mixed four modes.

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In this paper, a composite planar spiral antenna consisting of an eight-arm equiangular spiral antenna and eight Archimedean spiral antennas has been designed to radiate electromagnetic wave carrying tunable angular momenta in a wide band. A tunable eight-way Wilkinson power divider network is used to offer three kinds of feeding modes for the equiangular spiral antenna, and thus the composite antenna can radiate the electromagnetic waves with angular momenta of the modes l=1, 2, and 3, respectively. The Archimedean spiral is introduced to improve the gain of the composite antenna in the case of the angular momentum of l=3. By analyzing axis ratio (AR) of the proposed antenna, the generated angular momentum of l=1 is spin angular momentum (SAM), and the angular momenta of both l=2 and 3 include SAM and orbital angular momentum (OAM). Simulated and measured results are given to demonstrate good performance including tunable modes, good purity and wide band.

This article presents a novel pattern-reconfigurable antenna based on theory of characteristic mode for body area networks (BAN) communication. The radiation pattern of the antenna is able to switch between the omnidirectional pattern and broadside pattern, which can satisfy on-body and off-body communication links simultaneously. An array of 4 Â 2 rectangular plates with the off-body communication mode at BAN frequency band is used as the main radiator, and four copper posts between the radiator and the ground are introduced to generate the on-body mode. A simple feeding network is designed to selectively excite the two modes. The footprint of the proposed antenna is 0.43 λ 0 Â 0.43 λ 0 Â 0.05 λ 0 . For demonstration, the proposed antenna has been fabricated and measured both in the free space and on the phantom. The measured results agree well with the simulated ones. Furthermore, the issue of the specific absorption rate (SAR) is studied. The peak SAR values of the proposed antenna in two communication modes are far below the Federal Communication Commission standard. In addition, for practical application, the link budget of the on-body link is investigated. The stable impedance and radiation properties, the low SAR values and the reliable communication ability make the proposed antenna be a suitable candidate for BAN communications.