# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Smart Home Appliances
#### 2.2.1 Antenna Specifications  
To ensure connectivity among various sensors and actuators within a household, the development of novel multifunctional antennas is critical. Given the requirement for all devices within a home to be interconnected, there are primarily two antenna design strategies to consider, as illustrated in Figure 4. One approach involves antennas that generate multiple beams, adaptively steering their main beam toward other IoT devices. Alternatively, antennas can be designed with omnidirectional or nearly isotropic radiation patterns to ensure comprehensive coverage across the entire household space. Examples of omnidirectional and isotropic radiation patterns are depicted in Figure 5, for which the radiation is emitted almost in every direction. However, in such cases the antenna gain is minimized, decreasing the covering range of the antenna, so an appropriate analysis must be performed to ensure the entire coverage of a home. While a hybrid of these approaches is also feasible [56], this review will focus on the latter strategy. This decision is based on the assumption that many wireless sensors may lack the size or power capabilities to support multi-beam functionality. Figure 4: Typical smart home architecture denoting the antenna utilization. Arrows denote some of the plethora of possible communication lines. Regarding operating frequencies, most smart home applications use widely adopted communication protocols such as Wi-Fi, Bluetooth, and Zigbee, which operate in the ISM bands. The prevalence of these technologies is due to their mass production capabilities, cost-effectiveness, and ease of programming for IoT devices. Additionally, these protocols support high data rates within the small, controlled environment of a home network. Consequently, the majority of IoT antennas currently operate within the Wi-Fi bands. However, there have been research efforts to explore mmWave wireless systems, which offer higher data rates and reduced interference from crowded Wi-Fi bands, as well as miniaturized sizes [57]. Despite these advancements, commercial availability of mmWave systems remains limited. In this review, we will not limit our discussion to Wi-Fi bands but will also cover the entire sub-6 GHz band and some millimeter-wave (mmWave) bands. This broader scope aims to provide readers with a comprehensive view of emerging technologies, extending beyond commercially available bands. The primary goal of this review is to highlight multifunctional antennas that could potentially be used in future IoT devices. Additionally, many of the presented devices can be appropriately tuned to operate in the desired frequency bands, even if those frequencies are not currently licensed for use. Non-directive antennas, therefore, are required to offer multi-polarization capabilities to cope with the polarization mismatch due to the highly reflective environment of modern homes. Notably, wave reflection is expected to alter the orientation of the electric field, thus causing a change in its polarization. This is very well understood in free space propagation, where a horizontally polarized wave may generate both horizontal and vertical fields when it is reflected at an obstacle. Thus, the multi-polarization feature is essential for maintaining reliable communication. Additionally, it is usually preferred for frequency-agile antennas to operate in different frequency bands of 5G because wideband behavior is difficult to achieve [15]. Such flexibility in antenna design is crucial for achieving comprehensive connectivity within smart homes, enhancing the integration and functionality of IoT devices. Incorporating antennas with versatile polarization capabilities into communication systems necessitates meeting two key criteria: (i) the antenna must possess a structure capable of supporting multiple polarizations, and (ii) it requires a sophisticated feeding network designed to excite or receive the specific polarization needed at any given time.