# Compact Ultra-Wideband Series-Feed Microstrip Antenna Arrays for IoT Communications
## 1 Introduction  
The growing evolution of communication systems, culminating with the current deployment of the 5G system, connects the world and its people, being a powerful weapon to reduce inequalities and spread knowledge. Whether 5G or beyond 5G, new challenges are demanded of communication networks to integrate the immensity of users and connected sensors and to share information. The need to deal with this large amount of data was the decisive factor for the scientific community to look for solutions that would provide free broadband spectrum for the establishment of these networks, which are in the millimeter wave band. These networks that allow people and thousands of sensors dispersed and connected to the Internet, sharing information, interconnected, are today commonly known as Internet of Things (IoT). Therefore, it is essential that these mobile devices have the ability to handle (transmit and receive) large bandwidths of information, with antennas playing an important role in this context [1]. Antennas are a fundamental component of wireless systems, transmitting and receiving the electromagnetic waves. Microstrip antennas are very popular structures with features that fit modern communications, enabling compact, efficient, and versatile radiating elements with reduced costs. However, in its elementary format, microstrip antennas have features that do not meet the requirements of future communication networks and that must overcome, such as low gain and narrow bandwidth. New architectures need to be explored so that it is possible to improve their performance, increasing the operating bandwidth, as well as boosting the gain. Microstrip antenna arrays with broadband characteristics have been investigated to address these needs, as well as being integrated into sensors/mobile terminals. In the literature some works have been developed over the years to improve the bandwidth of an antenna array, some of the most relevant using log-periodic structures [2; 3], differentiating in the elements used. Antenna arrays designed with the log-periodic principle present the ability to have wide bandwidth, as well as high directivity and gain. In [4; 5; 6; 7; 8; 9; 10] dipole antenna arrays are identified. For operating frequencies up to 6 GHz, planar structures with several dipoles printed on a dielectric substrate are proposed [4; 5; 6], typically with one of the dipole arms printed on the top of dielectric surface and the other on the bottom. Variants of this structure, allowing greater bandwidth and operating up to 18 GHz, are proposed in [7; 8]. For high frequencies, in [9] an array of microstrip dipoles resonant at 28 GHz and with a compact structure is presented, with about 9 dBi of gain, while in [10] also with a structure printed on substrate, an array of dipoles covering frequencies between 10-40 GHz, with an average gain of 7 dBi, which features a soldered coaxial cable in the antenna structure. An array using bow-tie elements is proposed in [11] also printed on dielectric substrate, for C-band, operating between 6.12-6.58 GHz; however, the results of the radiation pattern and gain obtained were not mentioned. With a different structure, but using a log-periodic architecture, arrays of printed monopoles are also proposed in [12; 13; 14]. In [12] authors propose a compact array operating between 8.4-14.6 GHz, with a gain between 6.5 dBi and 9.5 dBi. An array with 15 monopoles, also in a log-periodic structure, is presented in [13], with multiband characteristics, and using a low-cost dielectric substrate. This array covers a frequency band between 1.58-15 GHz with a maximum gain of 9.12 dBi. In [14] the proposed monopole array has an end-fire radiation operating in a band between 6-18 GHz with a gain that varies between 2 and 8 dBi. This antenna shows, however, considerable ground plane dimensions, when compared to the size of the radiating part. Some physical modifications to the typical array of dipoles with log-periodic architecture are presented in [15; 16].