In order to transfer the power efficiently, the generator is connected to a matching network (built into the applicator) that keeps the impedance fixed at 50 \(\Omega\)[2,14,16]. The most crucial difference between both systems (FRO and 50 \(\Omega\)) resides in the higher cost of implementation and complexity of the latter type. The FRO system is commonly used industrially for drying processes (e.g., cookies, crackers), because the RF energy focuses on wet areas more than dry areas, evening out the moisture distribution throughout the process [2,17,18]. The 50 \(\Omega\) system is more suitable for pasteurisation and sterilisisation purposes because of the stability of frequency and power [2]. The majority of the research currently conducted on RF pasteurisation/sterilisation focuses on the optimisation of the heating uniformity of food products [13]. In contrast, the amount of fundamental (lab-scale) research conducted towards the bacterial inactivation mechanisms of RF heating is still limited, illustrating the possible research opportunities in this field. A notable example is the research towards the existence of non-thermal effects induced in microorganisms by RF [19]. However, commercial 50 \(\Omega\) RF setups are expensive to adapt for small lab-scale samples and therefore are less suitable for more fundamental scientific studies. Numerous RF schemes, recommendations, and lessons on how to build RF generators, amplifiers, and matching circuits can be found online, but all are difficult to implement for non-experts without specific knowledge in the field of RF behaviour and circuit design. In addition, a matching box is needed to transmit the RF energy to the load and to prevent RF reflection from the load to the power supply. In contrast, the free-running oscillator circuit, although not often used for industrial thermal inactivation applications, is suitable to implement at laboratory conditions. The free-running oscillator system is very simple in implementation, requires only a limited number of electronic parts, and can easily be modified with respect to changing research interests in the field of RFradiation [20]. The design requires preliminary work to be done on winding the coils and calibrating the system. If the frequency of the oscillations is not important for the specific application, a fluorescent or neon lamp is sufficient for the detection of the presence of RF radiation. The aim of this study was to design and construct a lab-scale low-power free-running RF oscillator operating at a frequency of 27.12 MHz (i.e., the RF frequency most commonly used for food pasteurisation) at a fixed load, i.e., for a fixed sample volume. An FRO setup design was selected over a 50 \(\Omega\) system, because the FRO type can be constructed without extensive technical knowledge, with the lower cost being an additional advantage. This is especially useful taking into account that the intended users of the setup would be part of microbiologically oriented research groups. In order to be suitable for fundamental scientific studies, the setup was specifically designed for the thermal inactivation of food-borne pathogens in fundamental lab-scale studies. Therefore, the setup was calibrated to treat two commonly used laboratory media for bacterial growth, i.e., Brain-Heart Infusion broth (BHI) and Tryptic Soy Broth (TSB). The stability of the free-running RF oscillator over the course of the inactivation treatments was investigated by means of the temperature, frequency, and power evolution with time. _Listeria monocytogenes_ (Gram-positive) and _Salmonella Typhimuriumium_ (Gram-negative) were used as model microorganisms to assess the thermal inactivation potential of the RF setup. Those bacteria were selected because of their relevance for European food safety, with _Salmonella_ being the most common causative agent of foodborne outbreaks (i.e., 30.7% out of 5098 total outbreaks in 2018), and illnesses caused by _Listeria monocytogenes_ exhibiting the highest fatality rate (i.e., 15.6% in 2018) [21].