## 5 Applications of DRA
### Radio Frequency Identification (RFID)  
RFID is a revolutionary application of automatic identification and data capture technology that is contactless and does not require line of sight. It is used to identify, localize, and track an item or object provided with an RFID tag or transponder by the means of radio EM waves. It can be embedded in all kinds of consumer products and scanned several meters away, revealing information about the product and its manufacturer. Small size and low-cost RFID tags are crucially important for applications of the Internet of Small Things (IoST). There are several methods that can be used to reduce the size of RFID tags such as meandered antennas, fractal dipole antennas, and dielectric resonator antennas. In [89; 90], high-permittivity ceramic resonators are used for RFID tags, which are intended to provide size miniaturization and long-range reading distances. Moreover, ceramic materials can consistently operate under severe conditions such as extreme temperatures and chemically violent environments. In [89], a 22.9 m reading distance is obtained for the RFID tag with \(\varepsilon_{r}\) = 100. In [91], a polarization twist tag is proposed. The twist tag consists of two dielectric resonators tilted by \(\pm\)45\({}^{\circ}\) and spaced by \(\lambda/4\) for orthogonal polarizations. Figure 21 shows the operation of this system. The DR tag will reflect an equal-handed CP wave. However, another reflector is required to reflect the opposite-handed wave for a clean resonance received by the co-polarization network of the reader with less degradations due to lens reflections. In [92], a novel ultraminiature CP antenna inspired by crossed split-ring resonators (SRRs) is proposed. The proposed antenna consists of a F4 BM top substrate board, metallic screws, and a metallic ground layer. Two crossed dual-capac \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ref.** & **Geometry** & _f\({}_{r}\) (GHz)_ & **BW (\%)** & \(\varepsilon_{r}\) &
\begin{tabular}{c} **Antenna Dimension** \\ **(W \(\times\) L \(\times\) _H_)\(\lambda_{c}\)3** \\ \end{tabular} & **Gain (dBi)** & **Feed Type** & **Polarization** \\ \hline
[85] & Slot-fed & 5–6.04 & 18.8 & 7.75 & (0.72 \(\times\) 0.72 \(\times\) 1.23) & 6.69 & Slot feed & LP \\  & RDRA & & & & & Inverted & \\
[86] & Hybrid & 1.67–6.7 & 120 & 10.2 & (1.25 \(\times\) 1.11 \(\times\) 0.13) & 9.9 & T-shaped & LP \\  & RDRA & & & & & feed line & \\
[58] & RDRA & 1.83–2.85 & 43.59 & 9.2 & (0.55 \(\times\) 0.55 \(\times\) 0.32) & 7.7–8.4 &
\begin{tabular}{c} Series \\ feeding \\ \end{tabular} & CP \\  & Quarter & & & & & & \\
[87] & sectored & 2.28–2.75 & 18.7 & NM & (0.33 \(\times\) 0.31 \(\times\) 0.06) & 4.3 &
\begin{tabular}{c} Microstrip \\ feed \\ \end{tabular} & CP \\  & CDRA & & & & & & \\
[88] & Quarter & & & & & \\
[89] & sectored & 5.75–5.85 & 1.7 & 10 & (0.68 \(\times\) 0.68 \(\times\) 0.22) & 7.02 &
\begin{tabular}{c} Aperture \\ slot feed \\ \end{tabular} & CP \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison between various DRAs for RF energy harvesting. Figure 21: Operation principle of the proposed antenna [91]. DRRs), which are equivalent to two crossed magnetic dipoles, are arranged perpendicularly and excited simultaneously with an SMA connector, as shown in Figure 22a. As can be seen in Figure 22b, the measured \(-10\) dB bandwidth is from 900 to 928 MHz which is 3.1%, whereas the simulated frequency range is from 901 to 928 MHz. The proposed antenna will be a good option for RFID and low frequency applications due to its ultracompact size, low cost, high radiation efficiency, and wide CP beamwidth. A PIN diode and varactor are used in this design to achieve polarization and frequency tuning. Undoubtedly, PIN diodes and varactors need an additional bias circuit. Chipless passive tag technology has been applied to RFID and wireless sensor approaches which do not rely on microchips to form a backscatter signal. Recently, a DR has been proposed as a tag in a temperature sensor system.