# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Smart Home Appliances
#### 2.2.3 Alternative Methods for Agile Antennas
This is achieved through the introduction of side walls on the upper element and conductor posts. These components not only maintain the necessary distance between the two antenna layers but also serve as an electrical shield for the feeding cable to the upper antenna. This effectively mitigates potential interference and preserves the integrity of the signal transmission. The tuning in this antenna design can only be accomplished mechanically, requiring an electronically controlled rotor system. The large size and complexity of this system make it impractical for IoT applications, where compactness and simplicity are often critical requirements. In the realm of advanced antenna design, the pursuit of multi-polarization capabilities has led to the exploration of innovative configurations that extend far beyond conventional solutions. Such approaches involve the use of high-order-mode cavity-fed patch antenna arrays (Figure 14) [73], which leverage the electric field distribution of high-order modes to achieve diverse polarization states. Similarly, high-order cavity-based resonant circular slot arrays [74] and orthogonally placed bowtie antennas with double printed rings [75] may be employed. Figure 12: Configuration of inverted-V antenna array fed by a dual Butler matrix, enabling beam or polarization switching [69]. A particularly interesting design feature is exhibited by a 16-switchable linear polarization sectioned circular stacked antenna, which operates within a reconfigurable frequency band ranging from 1.84 GHz to 2.64 GHz [76]. A schematic of this circular antenna is presented in Figure 15, along with its biasing network. This antenna employs an odd-even strategy to manage polarization states, with diode control facilitated by a field-programmable gate array (FPGA) coupled with the biasing network. This sophisticated control mechanism allows for significant reductions in linear polarization (LP) mismatching loss and equips the antenna with robust anti-interference capabilities, making it well suited for operation in modern, electromagnetically complex environments. The reconfiguration behavior of this antenna is achieved through the strategic distribution of PIN diodes across the slots on both patch surfaces. By activating different groups and numbers of sector patches, the effective current paths within the antenna are altered, thereby adjusting its operating frequency band. Figure 14: Multi-polarized high-order-mode cavity-fed antenna 2 \(\times\) 2 sub-array [73]. Figure 13: Mechanically tuned quad-polarized stacked patch antenna [72]. A quite intriguing and smart realization of an all-polarization reconfigurable antenna (Figure 16) was introduced in [77]. The patch antenna is capable of altering its polarization states among arbitrary LPs, LHCP, and RHCP. The core of this design is a truncated square patch antenna featuring two isolated H-shaped aperture-coupling feeds, tailored for operation at 2.45 GHz. This configuration enables the antenna to operate in two orthogonal CP modes with minimal cross-talk. The mechanism for achieving polarization reconfigurability in this antenna involves the selective excitation of one of two feeding microstrip lines. Port 1 generates LHCP and port 2 RHCP; this means with the appropriate use of variable attenuators someone can generate only one. Furthermore, by simultaneously exciting both feeding microstrip lines with equal amplitudes--but with differing initial phases (phase shifters)--the antenna can produce different LP modes. Remarkably, the orientation of the LP mode's polarization plane can be steered to any desired azimuth direction, offering unprecedented control over the antenna's polarization state. Simulated and measured performance metrics for this antenna demonstrate its efficacy, with a consistent peak gain of approximately 6.8 dBi at 2.45 GHz. Moreover, the antenna's impedance-matched bandwidth, which spans from 2.25 to 2.60 GHz, fully accommodates all polarization states without compromise.