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This letter presents a compact triple-band antenna composed of a spiral radiator with an open-ended slot at the ground for human head-implantable wireless communications. It operates simultaneously at a Medical Implant Communications Service (MICS) band (402–405 MHz), a Wireless Medical Telemetry Service (WMTS) band (1427–1432 MHz), and an Industrial, Scientific, and Medical (ISM) band (2400–2480 MHz). The proposed antenna has a thin profile of 0.5 mm with a volume of 197 mm3. Semisolid phantoms are used for the design and experimental validation, and detailed parameter studies are carried out to determine the working principles of the antenna. Based on simulations and measurements, it was found that the proposed antenna covers all the desired frequency bands with comparably high boresight gains given its compact dimensions.

A novel design of an electrically small, dual-band quasi-isotropic antenna with a microstrip balun is presented in this letter. A quasi-isotropic radiation pattern is realized from the total current flow over the proposed structure at two different frequency bands. The electrical size ka of the antenna is 0.49 with an extremely thin profile of 0.0014λg. The antenna measurements reveal good uniformity of the radiation pattern with a maximum gain deviation of 5.83 dB at 942 MHz and 8.24 dB at 1.71 GHz with 57% and 95% radiation efficiency, respectively. The radiation performance of the antenna is also verified through wireless power receiving experiments in an office non-line-of-sight (NLOS) environment.

A 3D printed, low-profile, electrically small antenna with a quasi-isotropic radiation pattern is presented herein. It is composed of an electric meandered dipole antenna, and the extended arcs from the meander line mimicking the current flow of the loop antenna. A quasi-isotropic radiation pattern is achieved from the total current flow over the proposed structure. Modern stereolithographic 3D printing and nano-polycrystalline copper coating technologies are used to build a prototype. The measured antenna exhibits a good uniformity in terms of the radiation pattern with a maximum gain deviation of 4.5 dB at 959 MHz and a radiation efficiency of 81 %, close to the computed expectations. The electrical size of the antenna ka is 0.48, and its height is $\lambda _{0}$ /82.32.

An electrically small, extremely thin Huygens source antenna with an endfire radiation characteristic is presented. It is composed of planar electric and magnetic dipoles that are simultaneously fed by a single feedline. The proposed antenna shows a directivity of 4.66 dB, very close to the value achievable by the magnetoelectric dipole antenna configuration, with a low profile of λ 0/103 and an electrical size ka of 0.47. The radiation efficiency is 71.1%. The prototype is built, and the simulated results are successfully verified experimentally.