# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Biomedical Devices
#### 2.1.5 Magnetoelectric Antennas  
A novel class of antennas, particularly suitable for low-frequency (LF) applications where compact size and device integration are critical has been gaining attention. For example, the absorption of LF magnetic fields in human tissue is notably lower, enhancing the suitability of antennas generating dominant magnetic fields for use in or near the human body. These antennas exploit magnetoelectric (ME) coupling and electromechanical resonance (EMR) to address the limitations traditionally associated with conventional electric field antennas, particularly in terms of efficiency and miniaturization, for very high frequency (VHF) and ultrahigh-frequency (UHF) applications. Unlike conventional EM antennas that resonate based on electromagnetic properties, LF ME antennas operate by resonating at the mechanical frequency of their physical structure, which allows for a significant reduction in their overall size. In ME or multiferroic or mechanical antennas, a low-frequency AC magnetic field (\(H_{AC}\)) interacts with a magnetostrictive layer, causing a periodic rotation of magnetic domains and a shift in domain boundaries [43]. This interaction generates a mechanical strain, which, when transferred to an acoustically coupled piezoelectric layer, results in electrical polarization (or the reverse process, depending on the operation mode). Acoustically driven antennas can operate at resonant wavelengths that are up to \(10^{5}\) times smaller than their electrical counterparts, providing a substantial advantage in terms of miniaturization and efficiency for specific applications. It is important to note that the term high efficiency is denoted for ME when compared with the same dimensions of electric current-based antennas at the same frequency. This means that the efficiency may not be high as compared to conventional antennas, but their size is smaller while keeping the efficiency at acceptable levels. One of the standout features of acoustic antennas is their real resonant impedance, while electrically small antennas of similar size present high reactance [43]. Notably, high real input impedance means that the antenna can accept and then radiate a significant amount of power. On the contrary, high input reactance causes the reflection of the power fed to the antenna. This unique operation mechanism allows LF ME antennas to achieve remarkable miniaturization and higher efficiency compared to embedded traditional EM antennas. Moreover, ME antennas can possess the dual functionality of wireless energy harvesting and sensing at different frequencies, expanding their utility beyond data transfer [44]. This dual capability, combined with their compact size and acceptable efficiency (0.5%), positions ME antennas as a promising solution for advancing the functionality and performance of a wide range of low-frequency applications, including those in the medical field. Despite the above benefits, it is important to note that the radiation efficiency of acoustic antennas can vary widely depending on how the acoustically driven antenna is implemented [45]. While the impedance characteristics are a common trait among most acoustic antennas, leading to enhanced matching efficiency, the specific design and fabrication of an acoustically driven antenna play a crucial role in determining its overall radiation efficiency. A groundbreaking development in ME antenna technology involves a self-biased, miniaturized LF ME antenna, as proposed in [46]. This antenna operates at an electromechanical resonant frequency of 49.9 kHz, and its volume has been significantly reduced to only 1.75 mm\({}^{3}\), making it substantially smaller than comparable EM antennas. The design features a piezoelectric layer sandwiched between two magnetostrictive layers, effectively leveraging the magnetoelectric effect for signal generation and reception.