Wearable Antennas for Off-Body Radio Links at VHF and UHF Bands: Challenges, the state of the art, and future trends below 1 GHz

Abstract

Abstract-An in-depth survey of wearable antennas for offbody radio links in the VHF and UHF frequency bands (below 1 GHz) is presented. The review encompasses distinctive specifications, design criteria and challenges, material characterization procedures, antenna topologies and technologies, multi-antenna systems, dedicated testing procedures, applications and future trends.Index Terms-Wearable antennas, body-worn antennas, textile antennas, embroidered antennas, VHF antennas, UHF antennas, antennas for body-centric communications, antennas for body area networks, antennas for personal area networks, antennas for off-body wireless communications. I. INTRODUCTIONhere are several existing and forthcoming wireless applications that either require or can benefit from one or more antennas that are directly stitched on a piece of clothing or a garment, or integrated into a personal accessory (such as shoes, glasses, buttons, helmets). These antennas are usually referred to as body-worn antennas, textile antennas, BAN (Body Area Network) antennas, antennas for body-centric communications, wearable antennas. The last term is the one that is going to be used in this paper, as it denotes any antenna that is small and light enough to be worn or carried on one's body.In the last decades, hundreds of scientific papers on wearable antennas have been published in the open literature [1]- [8]. Therefore, for the sake of completeness and clarity, a valuable analysis of the state-of-the-art on wearable antennas should be limited to antennas for a specific application, a given frequency range, or a chosen antenna technology. In this context, the present review focuses on wearable antennas operating in the VHF band and part of the UHF frequency band (below 1 GHz). Choosing 1 GHz as an upper boundary for this review is highly relevant, as the operating frequency is usually the main parameter to discriminate among available antenna topologies, technologies and characterization techniques. Moreover, we limit our discussion on antennas for off-body radio links, also excluding antennas integrated into mobile terminals/cases and operating close to the body. Specifically, only wearable antennas that can be used separately from the radio unit are of interest here.VHF/UHF wearable antennas are mostly used in professional mobile radio communications for soldiers, emergency operators and law-enforcement personnel. VHF As far as VHF/UHF wearable antennas are concerned, many of their distinctive features are related to the fact that the corresponding free-space wavelength (with 30cm<<10m when 30MHz<f<1GHz) is larger than or at most comparable to the size of the human body sections the antenna can be attached to (torso, back, legs, arms, shoulders, head). These features impose some serious challenges to the wearable antenna designer, who is concerned with meeting two conflicting design requirements. On the one hand, he/she aims for maximum antenna performance in proximity of the human body. On the other hand, he/she must ensure that the wea...

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