Microwave Theory

From plasma technology to Microwave heating and drying systems, our experience and expertise includes design and manufacture of Microwave ovens for industrial applications, with microwave power ranging from 2.5 kW to 100 kW per generator. The operating frequencies are 2450 MHz for Microwave generators with output power of 2.5 to 30 kW and 915 MHz for Microwave generators with an output power of 30 to 100 kW. Most industrial Microwave systems use magnetrons to generate a large amount of microwave energy. The system consists of the following units:

The Microwave Power Supply;
The Microwave Applicator (Oven); and
The Auxiliary Systems (Heating, Vacuum, Air Circulation, etc.)

The Microwave power unit primarily consists of a high-voltage DC power supply, the magnetron circuit, control and protective circuit, cooling system and circulator with water load. The size and shape of the applicator varies based on the type of process performed with the system. Most common types of applicators are the Batch Oven and the Continuous Conveyorized Oven. The auxiliary systems generally consist of the air circulation unit, hot air system or vacuum system, if applicable.

The following diagram shows the typical components of a Microwave Heating Unit.

The Magnetron Oscillator

The magnetron is a high vacuum electronic valve, just like the triode, consisting of a hollow copper anode incorporating a resonant Microwave structure. The anode is surrounded by a permanent magnet or electromagnet depending on the type and size of the magnetron. At the center of the magnetron is an electron-emitting cathode (filament) as shown below.

The anode, which has a set of vanes projecting inward forming slots between them, and are approximately ¼ ? (wavelength) deep and therefore are resonant at the operating Microwave frequencies. The slots are mutually coupled via the fringing field at their open ends and the whole structure forms a resonant circuit. When the filament is heated, a cloud of electrons is formed around the cathode. When the anode is supplied with high voltage DC, these electrons will travel from the cathode to the anode.

The magnetic field provided by the permanent or electromagnet installed outside the anode will provide a strong magnetic field that changes the path of these electrons. Since the field lines are parallel to the axis of the anode and perpendicular to the electron path, the electrons are forced to travel in a quasi-circular path around the cathode. By increasing the DC anode voltage or decreasing the magnetic field, some of the electrons will travel on a path closer to the anode, reaching the anode cavities where they will generate a resonant Microwave field. The Microwave power generated is extracted in one of the cavities by using an antenna that then is connected to a launcher. The launcher is connected to the circulator and subsequently to the oven by the wave-guides.

The following drawings show the principles of the power control using a magnetron with permanent magnet and with electromagnet.

Multi-mode Microwave Oven Applicators

By far the most widely used Microwave Applicator is the multi-mode type, used universally in domestic ovens and for a large number of low and high-power industrial installations. Mechanically simple, it is versatile in being able to accept a wide range of heating loads but heating uniformity is frequently a problem. In principle, the multimode applicator is a closed metal box with some means of coupling in power from a generator. The dimensions of the box should be several wavelengths long in at least two dimensions. Such a box will support a large number of resonant modes in a given frequency range. For an empty applicator, each of these modes is characterized by a sharp resonance response at a given frequency as shown below. It is important to arrange for as many of these modes as possible to lie near the operating frequency of the magnetron source which feeds the microwave energy to the oven applicator. However, when such an oven is partially filled with an absorbent workload, the Q-factor of each mode is reduced when it is empty. When the spectral density is high enough, the resonance curves of the modes will overlap in frequency to give a continuous coupling into the load. As the dielectric constant of the workload is greater than 1, the spectral density will also be increased from the empty state which gives additional overlap to the modes. This loading effect is also shown qualitatively in the drawing below where, apart from mode damping giving raise to lower Q-factors, a frequency shift of the pattern is seen to occur as well.

Field Distribution and Heating Uniformity

In a closed box, the field distribution is given by the sum of all modes excited at a given frequency, each mode giving a basic sinusoidal power variation in space along the principal coordinate axis and satisfying the field equations. There is therefore, fundamentally a spatial non-uniformity distribution of heating within a multi-mode oven. Several methods are available for improving the uniformity of heating.

Movement of the Workload

This is the most obvious and effective technique since mainly the oven walls determine the position of the standing waves within the oven and so to move the load inside the cavity so that it travels through the antinodes is clearly advantageous. For straight-line motion along one of the axes of a rectangular applicator, only the field variations along that axis are compensated by this technique and in principle the vertical and traverse variations remain. One method of compensating these two components is to design the oven and conveyor so the axis of travel is inclined to the principal axis of the oven.

In practice, the effort of moving the workload in the oven is more complicated because the characteristic impedance of the workload is substantially different from free space, giving rise to a shortening wavelength within the material and secondary reflections from the surfaces of the workload.

Mode Stirrers

A mode stirrer is a moving device introduced in the oven to perturb the field distribution continuously. Usually, it consists of a metal multi-blade fan rotated inside the oven. Even though it is a crude device, it contributes significantly to the achievement of uniform heating. There are two ways in which it operates depending on the dielectric losses of the workload. When the workload is highly absorptive, the blades (usually bent at an angle of 45 degrees) deflect the energy incident upon them from the generator feed. Since they are rotating, the path of the reflected energy is constantly changing. For low-loss high workload the blades are tuned to resonance so that they behave as rotating parasitic antenna, exciting different modes to varying extent as they rotate.