# Low Cost Circularly Polarized Antenna for IoT Space Applications
## 1 Introduction  
In the last decade, Low Power Wide Area Network (LPWAN) technology came out as a perfect choice for enabling longer range connections at a lower cost and more efficient power consumption [1]. The number of connected objects is expected to reach 41.6 billion in 2025 [2]. Several Internet of Things (IoT) use cases must be deployed in isolated areas without any mobile terrestrial coverage or other forms of connectivity. In this context, satellite communication becomes a crucial enabler to deploy IoT connectivity all over the world [3]. On the basis of this idea, several projects are developing an IoT solution that provides a connecting link between the nodes on earth and a constellation of low earth orbit (LEO) CubeSat satellites. To deploy such a scenario, a terrestrial antenna with specifications such as a compact size, high efficiency, and low cost are needed [4]. In order to avoid polarization mismatch losses between the devices and the satellite, which are caused by their dissimilar orientations, circular polarization (CP) is preferred. Many solutions have been proposed in the literature to provide a wide-angle CP radiation pattern [5]. Several designs based on a single port and slot structure have been developed for Global Positioning (GPS) applications. In [6], a CPW-fed slot antenna was designed with a square slot shape fed by a halberd-shaped metal strip. The antenna showed a good axial ratio lower than 3 dB in the 1545-1605 MHz frequency band, but the design required a complex parametric study. The uses of the specific characteristics of the electromagnetic wave in ferrite materials have been used to generate CP using non-reciprocity phenomenon in [7]. The antenna design is based on the classical patch structure. Nonetheless, the author proposed to use the ferrite material as the substrate. Miniaturization is also naturally obtained thanks to the high permittivity and permeability of such material. However, the cost and associated loss of this material are the limitations for the low-cost application. Several solutions based on degenerate orthogonal dual-mode patch antenna are presented in [8]. These proposed solutions are extensively used for GPS applications. However, such a structure needs a high permittivity substrate such as ceramic for miniaturization, which increases the production cost. A compact folded patch was proposed in [9] with an impedance bandwidth of 2.1% and a 3-dB axial-ratio (AR) bandwidth of about 0.76%. By using the stacked configuration, several layers are placed on the ground plane of 100 mm diameter. This antenna, excited by a single port located in the patch corner is very sensitive to any fabrication errors. As previously shown, the single port structure has several limitations which could be overcome by using the multiple-port approach. Of course, the use of several ports will imply the use of an additional circuit to generate the right signal of each port (amplitude and phase). The use of a dual port square patch antenna has been used for many decades [10], but the miniaturization still requires a high permittivity ceramic substrate. In order to ensure a wide and stable radiation pattern and strong miniaturization, the axis of symmetry is required. Some compact solutions with four elements have also been studied with a wide radiation pattern [11; 12; 13; 14]. However, the proposed structures in these references use the low-loss dielectric substrates, which are expensive for industrial application. To solve this cost problem, the antenna in [15] is fabricated on FR4-Epoxy, which is a low-cost material. Nonetheless, due to the high-loss substrate, this design provides a \(-0.6\) dBic gain at 915 MHz. In this paper, we propose to study a terrestrial compact antenna for LPWAN space communication on a low-cost substrate. The use of a 3-element structure has the potential to provide better performance over a 4-element structure, as shown in [16]. Until now, a 3-element approach has not been widely used, due to the difficulty of designing a low-loss feeding network.