Chapter 6 Signals in the Soil: Underground Antennas 6.1 Introduction Antenna is a major design component of Internet of Underground Things (IOUT) communication system. This chapter focuses on developing a theoretical model for understanding the impact of soil on antenna by conducting experiments in different soil types (silty clay loam, sandy, and silt loam soil) and indoor testbed. Figure 6.1 shows the organizational structure of the chapter. The purpose of the model is to predict UG antenna resonance for designing efficient communication system for IOUT. Based on the model a wideband planar antenna is designed considering soil dispersion and soil–air interface reflection effect which improves the communication range five times from the antennas designed only for the wavelength change in soil [62, 71]. IOUT is being used for implementing many applications [1, 2, 16, 44, 58, 60, 69]. In all these applications, major challenge is to establish a reliable communication. To that end, an underground-specific antenna design challenge is necessary to address. Three major factors that can impact the performance of a buried antenna are: (1) effect of high soil permittivity changes the wavelength of EM waves, (2) variations in soil moisture with time affecting the permittivity of the soil, and (3) difference in how EM waves propagate during aboveground (AG) and underground (UG) communications. For the third challenge above, it to be noted that lateral waves [26] are dominant component in EM [8, 47, 54, 58] during UG2UG communication and suffers lowest attenuation as compared to other, direct and reflected, components. Therefore, antennas used for over-the-air (OTA) communication will not be suitable for UG communication because of impedance mismatch. The chapter also focuses on developing an impedance model to study the effect of changing wavelength in underground communication. Furthermore, it is discussed how soil–air interface and soil properties effect the return loss of dipole antenna. © Springer Nature Switzerland AG 2020 A. Salam, U. Raza, Signals in the Soil, https://doi.org/10.1007/978-3-030-50861-6_6 189