# A Compact Triple Band Antenna Based on Multiple Split-Ring Resonators for Wireless Applications
## 1 Introduction
In [12], AMC metasurface antennas are optimized using a surrogate-assisted differential evolution technique, transforming uniform arrays into nonuniform geometries for enhanced wideband performance. The same is true for the technique in [13] that uses the multilayer reflective surface to enhance the broadside directivity of patch antenna. In [14], the design of a multiband monopole antenna with a metamaterial (MTM) loading technique is presented, which has a footprint of 32 \(\times\) 10 \(\times\) 1.6 mm\({}^{3}\) for vehicular communication. Although this antenna reaches the operating frequencies of 2.4 and 3.8 GHz, it showed a bandwidth of 6% and 9%, with a realized antenna of 3.2 dBi. In [15], a 60 \(\times\) 60 \(\times\) 1.6 mm\({}^{3}\) antenna with an asymmetric split-ring resonator and a tri-band slot is investigated for three-band applications. The measured impedance bandwidth of the resonant bands is 14.25%, 1.78% and 8.37%, with a peak gain of 3.1 dBi. In [16; 17; 18], antennas of almost the same size of 35 \(\times\) 35 \(\times\) 1.6 mm\({}^{3}\) on an FR4 substrate are investigated for operation in the triple [16] and dual [17; 18] frequency bands. Although these antennas are smaller, they have several disadvantages, such as a narrow frequency range in each frequency band, as the loaded elements give the antennas a narrow operating bandwidth. To overcome this limitation, the technique in [19] used parasitic strips with slots around a microstrip patch antenna (36 \(\times\) 37 \(\times\) 1.6 mm\({}^{3}\)) to improve gain and bandwidth. Although this design attained broader bandwidths of 160 MHz and 220 MHz, it achieved gains of 3.83 dBi at 3.45 GHzand 0.576 dBi at 5.9 GHz, respectively; a more compact size with a wider frequency band is required. For each of the aforementioned techniques, existing multiband antenna designs fabricated on FR4 substrates often face challenges such as larger dimensions, limited operating bandwidth and increased design complexity. The high relative dielectric constant (\(\epsilon_{\mathrm{r}}\)) of FR4 reduces the bandwidth of the antenna, potentially limiting its application range. Although FR4 is a cost-effective substrate, its performance decreases at higher frequencies. The realized gain of a standard microstrip patch antenna fabricated on an FR4 substrate and operating at 2.45 GHz typically ranges between 1.5 and 3.5 dBi, as discussed in [20]. This value depends on the antenna's dimensions, which are generally around 0.5\(\lambda\)\(\times\) 0.4\(\lambda\). The relatively low gain is primarily attributed to the high dielectric losses of FR4, which result in radiation efficiencies as low as 35-50%. In contrast, antennas fabricated on low-loss substrates such as Rogers RT5880 can achieve higher gains of 5-6 dBi due to their improved efficiency. It is important to note that these values refer to simple patch antennas without additional enhancements such as perfect electric conductor (PEC) backing or superstrate loading. To overcome these challenges, a compact triple-band antenna with multiple splitting resonators is proposed in this work, which enables significant miniaturization (81.6%) compared to the conventional patch antenna while maintaining acceptable gain and ease of fabrication. The proposed antenna is not only characterized by compact size and wide bandwidth, but also improves multiband capabilities, which makes it suitable for wireless applications especially at 5G Sub-6 GHz Mid-band (2.5/3.5/5), RFID (2.45/5.8), WiMAX (2.4/3.5/5.8 GHz), Wi-Fi 5/6/6E (2.4/5/6 GHz), WLAN (5.2/5.8 GHz). This paper is structured into five main sections. Following the Introduction, Section 2 presents the design of the split-ring resonator unit cell structure as a metamaterial and its integration into the antenna design. This section also contains the equivalent circuit analysis and the derivation of the input impedance.