# High-Gain Multi-Band Koch Fractal FSS Antenna for Sub-6 GHz Applications
###### Abstract
also developed a compact, arrowhead-shaped miniaturized antenna for the Wi-MAX and n78 bands, despite its limited bandwidth and low gain value of 2.47 dBi [37]. [PERSON] et al. have developed a compact offset edge-fed odd-symmetric double-mixed Koch-Minkowski fractal slotted antenna with a gain of 4.72 dBi, suitable for UWB applications [38]. Furthermore, [PERSON] et al. showed that the notch band may be eliminated from the passband by utilizing complementary SRR. This would produce an outer extended circular fractal antenna with a low gain value and a broad bandwidth of 1.63 to 2.91 GHz [39], [PERSON] and [PERSON] designed a CPW-fed high-gain circularly polarized L-slotted antenna at 3.5 GHz. The FSS size and gap are much larger, which increases the overall volume of the antenna. This antenna results in a 3-4 dBi enhancement in the gain with an FSS [40]. [PERSON] et al. developed a CPW-fed tree-shaped fractal elliptical antenna for dual-band applications, achieving bandwidths of 2.49-4.36 GHz and 7.96-9.92 GHz with peak gains of 2.84 and 6.15 dBi, respectively, and a gain of 9.34 dBi [41]. [PERSON] and [PERSON] have developed a dual-band FSS U-slotted patch antenna to enhance the bandwidths, gains, and reflection coefficients for 2.45 GHz and 5.8 GHz Bluetooth and WLAN applications [42]. [PERSON] et al. have demonstrated a dual-band slotted antenna in which gain enhancement is achieved using a dual-band 7 \(\times\) 7 novel FSS. The FSS is used as a superstrate that achieves dual bands of 3.48-3.65 GHz and 5.75-6.37 GHz with gains of 13 and 13.92 dBi [43]. [PERSON] and [PERSON]'s study on slotted elliptical patch FSS antennas at 5.38 GHz revealed a 2.5 dBi gain improvement from the FSS superstrate at 28.5 mm from the radiator [44]. [PERSON] et al. have developed a dual-polarized antenna for 5G wireless local networks, featuring two orthogonal bow-tie dipoles and a ground plane and wide band (4-6.5 GHz) with a ring slot connecting rectangular slots, resulting in a 40% bandwidth improvement and 5.6 dBi gain improvement [45]. [PERSON] et al. have developed a high-gain resonant cavity antenna with an FSS superstrate at 6 GHz to enhance its radiation characteristics [46]. The literature survey reveals the following research gaps. * First, traditional antenna designs struggle to achieve significant miniaturization without compromising key performance metrics such as the gain, bandwidth, and reflection coefficients. This study employs fractal geometries, specifically Koch curves, to achieve a substantial size reduction while maintaining and even enhancing the performance parameters. * Effective frequency tuning techniques are lacking in many current antenna designs, but this is crucial for flexible and adaptable 5G applications. This research presents a strong method for frequency tunability by incorporating frequency-selective surfaces (FSS), allowing the antenna to function effectively across many frequency bands. The literature [41; 43; 46; 47; 48; 49] makes it clear that the FSS size can be maintained at the same level as the antenna. However, there are instances where the FSS size is greater than the antenna size [40; 42; 50; 51]. The antenna's cost increases with the size, since a larger antenna requires more substrate. * It is still very difficult to achieve both a large bandwidth and high gain at the same time, especially in smaller antennas. An improved bandwidth and gain are obtained over the intended frequency ranges by means of the suggested design, which optimizes the fractal geometry's dimensions and its iterative processes for FSS integration. Although SRRs have been proven to improve the antenna properties, the present designs frequently fail to fully leverage their potential. This study shows how SRRs can be used to improve the gain and tunability even in small designs by including SRR loading with reduced ground planes. * The resonance frequency tuning of most antennas in the literature is not adjusted with the design frequency.