## 1 Introduction  
The search for new energy sources in the 21 st century is a crucial topic with essential economic and societal meaning. Energy from renewable sources has been considered an additional form of energy that has the potential to provide energy services without increasing environmental pollution (i.e., energy services with zero or near-zero emissions) [1]. Solar and wind energy are the most widely used renewable energy sources in the world today [2]. Their share in the global electricity mix exceeded 10% in 2021 [3]. Moreover, the rapid development of fifth-generation (5G) cellular networks and the Internet of Things (IoT) requires new energy sources (other than solar and wind energy) to power small autonomous IoT devices. IoT devices are battery-powered heterogeneous (in both functionality and appearance) devices that find applications in various fields, from healthcare to applications in industry and advertising [4, 5]. It is expected that the number of IoT devices will reach 29 billion in 2030 [6]. The growing proliferation of wireless IoT devices over the next few years will lead to an increase in energy consumption on the one hand and more batteries produced on the other hand, a situation that can create a significant environmental problem [7]. Hence, the question of how to provide a sustainable energy supply to IoT devices is becoming an increasingly important issue for the success of next-generation IoT networks. Energy from electromagnetic fields (EMFs) is considered a promising new energy source for ultra-low-power consumption IoT devices, such as wearable devices and IoT sensors [7, 8, 9, 10, 11, 12, 13, 14]. Radiofrequency (RF) energy harvesting, which is the conversion of energy from EMFs emitted from devices in wireless cellular networks (2G, 3G, 4G and 5G) and local networks (IEEE 802.11) into usable electrical energy, has several advantages compared to the rest of the resources [15]. First, in urban and rural areas, RF energy is present for twenty-four hours, and second, this energy is independent of environmental conditions (such as temperature and weather conditions). One key parameter when designing an energy harvesting system for low-power IoT devices is the achievement of maximum conversion efficiency [16]. The maximum energy conversion efficiency mostly depends on the incident power density on the antenna elements of the energy harvesting system, i.e., from available RF energy in the environment [17]. Moreover, available RF energy in the environment depends on the frequency, time of day and location. Consequently, it is essential to measure EMFs in practical cases in different scenarios, for example, in dense urban areas, urban areas and suburban areas, as well as in closed spaces [18]. EMF measurements have been carried out in various environments, such as dense urban areas [17; 18] and urban and suburban areas [19; 20; 21]. Only several studies have evaluated EMFs within a closed space, such as an empty vehicle cabin [22; 23]. The results reported in [23] showed that measured values for E-fields are higher in a compact-type car compared to those of a sedan-type car. Moreover, [22; 23] addressed non-realistic scenarios without any car drivers or passengers. We found only one study [24] that provided measurement results of E-fields from real mobile phones during an ongoing call (GSM mode) inside a car cabin. We could not find studies on the evaluation of EMFs from mobile phones providing LTE or 5G wireless connections in a vehicle cabin. Hence, this paper concentrates on the experimental assessment of the E-field inside a compact car for several realistic wireless communication scenarios, such as GSM, LTE, UMTS and 5G ongoing calls (voice mode), as well as GSM, LTE, UMTS and 5G wireless data transmissions (data mode), to explore the possibility of using these EMFs in energy-harvesting applications.