## 1 Introduction  
Microwave field-assisted techniques are widely used in agricultural production technologies and in the food industry [1; 2; 3; 4]. In these processes, the uniformity of the microwave field distribution over the volume of the product under processing is normally ensured by the application of in-line processing, conveying units [5]. In such processing units, a product is evenly distributed over a transporter to form a layer whose thickness is selected so that the microwave field penetrates to its depth without considerable attenuation. In such processing units, the productivity level of several tons per hour is hardly attainable, and it normally does not exceed 250 kg/h to 350 kg/h. At the same time, microwave field-assisted grain drying, immediately in the course of harvesting, requires the application of grain-drying equipment with a performance rate from 5 t/h to 30 t/h [6; 7]. As a rule, convective-type grain dryers are used for this purpose [8; 9]. The major part of such grain dryers is designed to process grain in a dense layer, in which case the grain moves downward and is blown through with a drying agent. The drying agent (hot air) is delivered directly into the grain layer via air channels (ducts). The grain is heated by the air, and the moisture evaporated from it is transported with the drying agent outside the processing plant. In grain dryers using microwave effects produced on grain, a wet material is heated owing to the interaction with the microwave field. That is why this process requires less energy for drying. Therefore, grain dryers based on microwave field-assisted techniques are preferable in terms of energy-saving criteria. Two methods of microwave field application in shaft-type grain dryers are known. In the first case [10], microwave effects are employed in the recirculation process, thus intensifying the moisture exchange between the wet and dried grain. In the second case [11; 12], the grain layer is exposed to the convective-microwave effect. The microwave field supports moisture extraction from single seeds into the inter-grain space, while heated air brings moisture out of the grain layer. The authors deal with convective-microwave processing units of modular design, one of which is shown in Figure 1, as an example. Inside the module, waveguides with slot radiators are installed, designed to deliver an electromagnetic microwave field into the grain. The number of waveguides in a module and the number of modules in a unit have to be defined with respect to the required grain processing performance, the location of the unit, and its type (mobile or stationary). For example, the productivity of the installation can be increased in two ways: One is by widening the modules. In this case, the volume of grain processed in one module increases, along with the number of emitting waveguides in one module. This option is applicable when placing microwave convective grain dryers in areas where there are no restrictions on the occupied area. In this scenario, the height of the installations will be lower, reducing capital expenditures on loading pits. The second is by increasing the number of modules placed on top of each other. This option is suitable for mobile installations, where there are limitations on the dimensions of transported installations. Grain moves downwards, in the form of a natural flow, being exposed to the microwave field. It is important to ensure the uniformity of grain exposure to the microwave field while it is within the active zones, between the waveguides with slot radiators. The fulfillment of this condition defines the operational efficiency of the unit (i.e., grain treatment uniformity and energy consumption). The authors' approach of placing emitting waveguides directly in the grain layer allows for regulating the intensity of the microwave field in the grain layer and its uniformity. This is achieved by adjusting the distance between the emitting waveguides in the array, which can vary depending on the moisture content of the processed grain.