# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Smart Home Appliances
#### 2.2.2 Characteristic Mode Analysis as IoT Design Tool  
The design process of reconfigurable antennas presents a complex challenge that requires the utilization of various techniques to transform a conventional antenna into a multifunctional one. The literature highlights a diversity of methodologies, each bearing unique characteristics tailored to specific design goals. Notably, there has been a growing trend in recent years towards the use of characteristic mode analysis (CMA) in antenna design [62]. CMA is valued for its effectiveness as a design tool, primarily because it provides direct insights into the radiating behavior of antennas. This facilitates a more systematic and informed approach to design, as opposed to relying solely on the engineer's experience or trial and error. CMA can be particularly effective in determining the optimal locations for excitation on the radiator. In this context, CMA emerges as a powerful technique, equipping designers with a deeper understanding of the antenna's operational mechanisms and allowing for the strategic manipulation of its properties to achieve desired reconfigurability and functionality. Various CMA-based antenna designs have been proposed for the IoT [63; 64; 65; 6, 7]. A notable example of leveraging CMA is presented in the design of an electronically reconfigurable 6-port dual-band dual-mode microstrip ring antenna (Figure 7) [7]. The design process, facilitated by CMA, identified two non-conventional higher-order modes suitable for operation within two sub-6 GHz frequency bands. The selection of these modes capitalized on the natural orthogonality of the characteristic modes. This orthogonality translated into achieved isolation exceeding 37 dB in both bands, negating the need for  
Figure 6: Simple switchable polarization cross-antenna configuration enabled by electronically controlled RF switches. complex decoupling strategies typically required to mitigate interference between multiple ports. To enable dynamic switching between the two identified operational modes, an integrated electronically reconfigurable matching network was developed, incorporating six PIN diodes and six varactor diodes. The antenna displayed a peak gain of 4.7 dB for both modes and radiation efficiency values of 44.3% and 64% at the two frequency bands, respectively. Unfortunately, the radiation pattern exhibits nulls in certain directions due to the selected characteristic mode, deviating from the desired quasi-isotropic pattern. Additionally, the selected modes differ by only about 300 MHz, which is insufficient for IoT devices that may need to switch between different Wi-Fi bands, such as 2.4 GHz and 5 GHz. Two LP states is not quite enough for a polarization-agile antenna, due to the use of CP waves in an increasing number of applications to mitigate mismatch losses. For this purpose, a linear array employing square patch slotted elements was introduced in [6]. This innovative design (Figure 8) facilitates LP, RHCP, and LHCP by exploiting two LP orthogonal modes that possess equal modal significance and a 90\({}^{\circ}\) angle difference at 2.4 GHz. A strategically designed simple feeding network incorporating PIN diodes is employed to excite the various combinations of these two modes, depending on the state of excitation chosen. Notably, a common bandwidth (with \(|S_{11}|\leq-10\) dB) that accommodates all three polarization states has been identified within the range of 2.25 to 2.55 GHz, equating to a 12.5% fractional bandwidth and 3 dB gain BW range from 2.25 to 2.7 GHz in the LP state. The axial ratio of the LHCP and RHCP states maintained values below 3 dB in their operation bands. The 1 \(\times\) 4 antenna array showed measured maximum gains of 7.2 dBi for LP and 9.78 dBic and 9.8 dBic for LHCP and RHCP states, respectively. However, such a directive radiation pattern without the ability to manipulate its beam orientation, has questionable usability in IoT appliances.