# 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
This antenna employs an ortho-hexagonal patch element, incorporating three groups (comprising in total six elements) of parasitic patches and varactor diodes to facilitate continuous tuning in frequency and switching polarization reconfiguration between LHCP and RHCP. Further enhancing the antenna's reconfigurability, four switchable feeding probes, each one integrated with a designed single-pole four-throw (SP4T) switch, are employed to feed each element. This setup permits feeding the array's ports with different initial phases. Consequently, by appropriately switching the feeding probes for the four elements, the radiation beam of the antenna array can be dynamically reconfigured among five distinct beam states (\(-40^{\circ}\), \(-20^{\circ}\), \(0\), \(20^{\circ}\), \(40^{\circ}\)), while presenting a beamwidth of \(22^{\circ}\). Electronically steerable parasitic array radiators (ESPARs) represent a significant category of antenna arrays that blend active and passive (parasitic) antenna elements. By switching these elements between the ON and OFF states, ESPARs achieve a reconfigurable behavior, notably in their beam-switching capabilities [107]. A key characteristic of classical ESPAR antennas is the incorporation of varactor diodes at the base of the parasitic elements [107]. This is normally achieved utilizing a variable reactive load, as depicted in Figure 18. The adapted voltage-controlled reactances of these diodes are adapted at producing varied antenna radiation patterns, enabling dynamic beam steering. As such, ESPARs are an attractive option for applications requiring directional communication capabilities  
Figure 17: Designed cubical array along with the printed slotted patch antenna (radiated element) [9]. but are constrained by budget and complexity considerations. Some notable examples of ESPAR designs include:  
1. A fan-shaped patch antenna system capable of producing four end-fire beams [104];
2. A 14-beam slot-based cylindrical cavity antenna, which exemplifies the use of ESPARs in more complex configurations, offering a higher number of beam directions for enhanced coverage and flexibility [108];
3. A 12-beam metasurface-based antenna, complemented by a reflector made of an artificial magnetic conductor reflector [107]. In recent advancements within antenna technology, efforts have been directed towards developing compact antenna arrays capable of performing beamforming in three-dimensional space. One notable contribution in this area is the development of a polarization-reconfigurable slot-ring phased array antenna designed specifically for C-band applications [109]. This antenna demonstrates operational capabilities across a frequency range from 5.42 to 7.87 GHz, achieving a significant 37% fractional bandwidth along with a gain of 7.6 dBi. The strategic placement of the microstrip line diagonally within the perimeter of a single antenna element enhances the antenna's performance and facilitates scalability. A 2 \(\times\) 2 array configuration was realized with an element spacing of 0.5\(\lambda_{0}\). This compact array is uniquely capable of supporting vertical, horizontal, or circular polarization, a flexibility achieved by feeding each antenna element port with the appropriate phase shift of the antenna elements. To excite horizontal LP, the second and fourth elements must present at 180\({}^{\circ}\) with respect to the first and third. Moreover, the array demonstrates the ability to perform beam scanning across a range of \(\pm\)30\({}^{\circ}\) with a beamwidth of 60\({}^{\circ}\). The main drawback of ESPAR antennas is their large volume requirement, which necessitates designing radomes to protect the antenna from harsh environmental conditions. Expanding upon the versatility seen in reconfigurable antennas, a novel non-uniform all-polarization 2 \(\times\) 2 array featuring \(\Gamma\)-dipoles (Figure 19) is introduced in [56], offering an innovative solution as well as advanced beamforming capabilities.