# 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  
Microfluidically polarization-tunable probe-fed patch antennas represent a fascinating advancement in antenna technology, as detailed in [67]. The proposed design is presented in Figure 11. This functionality is achieved through the alternative insertion or removal of metallic ink within 3D-printed channels made of polylactic acid (PLA), demonstrating the potential of integrating microfluidic systems with an antenna. For the second antenna discussed in [67], both polarization states exhibit stable reflection coefficients, with a fractional bandwidth of 9.6% (ranging from 4.48 to 4.93 GHz) and a 3 dB axial ratio bandwidth (ARBW) of 5.2% (spanning 4.48 to 4.72 GHz). The design incorporates slots etched into the top copper layer of the patch antenna, with fluidic channels filled with EGaIn (eutectic gallium-indium) liquid metal ink serving as switching elements. These elements can create open or shortcircuit electrical connections, enabling the antenna's polarization reconfigurability. Additionally, two slots are etched on a diagonally fed patch antenna to excite two modes, \(TM_{10}\) and \(TM_{01}\), simultaneously, contributing to the antenna's ability to support multiple polarizations effectively. This tuning mechanism is quite intriguing, but it is challenging to incorporate into a fully integrated system due to the complex process required to pump fluid in and out of the cavity. Quad-polarized antennas represent a significant advancement in antenna technology, offering support for four distinct polarizations [68; 69; 70; 71]. To approximate an all-polarization-capable antenna, it is essential to accommodate both right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP), as well as two linear polarizations, typically horizontal polarization (HP) and vertical polarization (VP). This capability ensures that any arbitrarily polarized wave can be detected with minimal efficiency degradation. A noteworthy antenna design is proposed in [69], featuring a 1 \(\times\) 4 crossed inverted-V antenna array combined with a dual-Butler-matrix-based feed network (Figure 12). This setup facilitates both polarization diversity and beam-steering capabilities. The crossed inverted-V array incorporates a planar director to boost antenna gain. The outputs from  
Figure 11: Double CP antenna manufactured by 3D printing and tuned through metallic ink [67]. Figure 10: Triple-polarized metasurface antenna [66]. the dual-Butler-matrix network are connected to each pair of the gain-enhanced inverted-V antenna elements. Additionally, the dual Butler matrix is integrated with a polarization mode selection switch matrix. This polarization selector, comprising a simple switch matrix, allows the RF signal to be directed into a single or both Butler matrices with a relative phase difference. This arrangement enables the generation of quad-polarization based on the switching operations performed. Excluding the contributions of the dual Butler matrix, the measured gain averages 12.8 dBi across all polarizations. However, the beamforming capabilities of the antenna are limited due to the use of a 4 \(\times\) 4 Butler matrix, which only covers the azimuth range between \(-30^{\circ}\) and \(+30^{\circ}\). Additionally, the occupied volume of the entire structure poses a challenge, especially due to its height when integrated into an IoT device. There are cases where antennas with four linear polarizations (LPs) are referred to as quad-polarized [72]. In the design proposed by [PERSON] et al. (Figure 13), a stacked structure is employed that passively receives four distinct linear polarizations without relying on diodes for tuning. This design consists of a three-stacked element configuration. The bottom patch is dedicated to orthogonal-polarized elements, while the upper patch is rotated at a \(45^{\circ}\) angle to facilitate slant-polarized elements. A significant feature of this design is the reduction in mutual coupling between the two antenna layers.