# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Smart Cities
#### 2.3.2 State-of-the-Art Compact Antennas  
In [9], a simplified unique single-fed four-faced cubical array with slotted inset-fed patch antenna designed for operation at 2.45 GHz is introduced. This antenna, structured as a cube and referred to as a \(4\times 2\times 1\) antenna array (Figure 17), incorporates four RF switches, located at each arm of the four faces. These switches play a critical role in selective pattern reconfigurability choosing from six different azimuth states. A significant advantage of this design is its ability to switch beam while maintaining stability in the reflection coefficient. This offers full \(360^{\circ}\) azimuth coverage by six beams with average \(HPBW=50^{\circ}\) in azimuth. On the other hand, this specific design was originally intended for CubeSat applications. However, it can also be utilized in IoT applications as a gateway in a local area network to receive signals from all directions. In contrast, another study [105] presents a low-profile, compact \(16\times 16\) antenna array that offers reconfigurability in frequency, radiation pattern, and polarization but at the expense of a complicated system. This advanced antenna design features a square patch element loaded with four capacitors and four switchable feeding ports, all controlled by an FPGA. The innovative aspect of this design lies in its frequency adjustability, which ranges from 1.35 to 2.19 GHz, achieved by selecting capacitance values between 0.31 and 3.14 pF. Additionally, by setting specific coding patterns for the digital radiating elements, the antenna can produce various radiation beams embodying three distinct LP states (\(0,45,90^{\circ}\)). This level of versatility and control, however, necessitates the use of 64 PIN diodes and 64 tunable capacitors, underscoring the complexity and advanced nature of the antenna's design. However, the 12 different radiation pattern states present peculiar behavior such as the creation of multiple front lobes and randomness in beam steering on the azimuth and elevation planes. A dual-polarized frequency-reconfigurable patch antenna array designed for millimeter-wave (mmWave) frequencies was investigated in [106]. The use of varactor diodes, strategically positioned at the four corners of the square patch antenna, allows for dynamic adjustment of the antenna's operational frequency. This design enables the antenna array to be tuned across a broad frequency range from 23.2 to 30.2 GHz, at millimeter range,simply by varying the capacitance of the varactor diodes. A key feature of the proposed antenna array is its exceptional isolation capabilities--achieving over 20 dB isolation between orthogonally polarized antenna elements and more than 15 dB isolation between antenna elements with the same polarization. This high level of isolation is critical for reducing interference and improving the quality of signal transmission and reception in densely populated frequency bands. Additionally, the antenna array exhibits impressive gain performance, with peak gains ranging from 8.9 to 10.5 dBi across its tunable frequency range. The beamforming capability of this \(1\times 8\) uniform linear array further enhances its utility in advanced communication systems. By feeding the different ports with appropriate phase differences, the array can continuously steer its main beam across a wide azimuth angular range, from \(-60^{\circ}\) to \(60^{\circ}\), with a mean \(HPBW=30^{\circ}\). However, this property was estimated using the antenna element radiation pattern with the use of the analytical array factor without any numerical validation and prototype implementation of its feeding network. In contrast with the previously referred to antennas, a \(1\times 4\) CP patch array antenna was introduced in [103] for the higher bands of 5G.