# Compact Ultra-Wideband Series-Feed Microstrip Antenna Arrays for IoT Communications
## 1 Introduction
In [15] a Koch fractal structure is used in the development of the dipole array, allowing for an increase in the bandwidth and reduction of dimensions. This array was designed for the C and S bands, operating between 2.8 and 6.9 GHz, enabling a gain of up to 8 dBi. In [16] a new configuration is presented, using a helical structure in the design of the dipole arms, allowing the authors to reduce the size of the array about 40%, while still maintaining a broad band operation between 1.87-5.89 GHz with a gain ranging between 5.73 and 7.63 dBi. These logarithmic structures can also be applied to microstrip patch elements [17; 18; 19; 20]. In [17], a theoretical approach is presented for application of the transmission line model for the design of series fed arrays with log-periodic structure. The authors in [18] propose an array of five microstrip patches designed for a central frequency of 11 GHz, achieving only 11% of bandwidth despite the gain being around 12 dBi. In [19] two forms of feeding are presented, coplanar and microstrip, and two structures of arrays of five elements are compared using these methods and showing that microstrip feeding has a lower bandwidth when compared to the coplanar structure. Additionally, with five elements, an array of microstrip patches is proposed [20], operating between 3.3-4.5 GHz. This structure has the particularity of being reconfigurable, turning ON and OFF each element of the array using PIN diodes, and with that it can change between a broadband operation or tuning between four more restricted bands within this frequency range. The array gain varies between 3 and 6 dBi. Other approaches have also been presented with application to series-fed arrays in order to increase bandwidth. In [21], a 4-element waveguide slot array for band X is presented, whose bandwidth reaches 12.3%. A series array using eight dielectric resonator elements is proposed in [22] designed to operate at 7.5 GHz. This array has a bandwidth of 38.1%, and a gain of 15.7 dBi, however it incorporates a more complex and bulky structure, using reflector plane. Using slow wave transmission lines, a series-fed architecture for a 10-element array is proposed in [23], achieving 10% bandwidth. A 12-element center-fed series antenna array using a novel coaxial feeding is proposed in [24], designed to operate at 4.9 GHz, for 5G applications. A maximum gain of 19 dBi is achieved, within an impedance bandwidth of 20.5%. In [25], a patch antenna with symmetrical slits to operate at higher frequency, and then integrated into a 2 \(\times\) 16 array is presented. Simulated results show that the array operates between 14.7-17.9 GHz, corresponding to an impedance bandwidth of 19.6%. A series-fed mm-wave microstrip array is proposed in [26] for 5G applications, by introducing a series of short stubs to one edge of a microstrip line. A 4 linear has been designed, presenting an impedance bandwidth of 21.40%, with a gain of 12.80 dBi. A 4 \(\times\) 4 planar array has been also presented with a measured bandwidth of 19.15% and a gain of 18.30 dBi. A novel series-fed array working operating at 5.8 GHz has been proposed in [27] using a dual-band patch resulting from the superposition of two tapered patches with different lengths, specifically designed to solve the problems of reduced bandwidth. The designed antenna shows a fractional bandwidth of 5.92%. There are other proposed non-series configurations [28; 29] for 5G applications, which offer large bandwidths. In [28] a square patch designed for 27.5 GHz is proposed, inserted in metasurface that allows reaching 23.4% of bandwidth with a compact structure. In [29] a Fabry-Perot antenna using a half-wavelength spaced superstrate having a half-wavelength thickness, employed as a partially reflecting surface for high gain and wide impedance bandwidth is presented. Simulated and measured results show that the proposed antennas have a 27.6% bandwidth with a maximum gain of 14.1 dBi.