CST – Computer Simulation Technology

Magnetron and microwave oven design to solve Wi-Fi interference issues

Magnetrons are widely used as RF power sources because of their high energy conversion efficiency (~75%) at a low price. After the magnetron was invented as radar technology during World War II, mass production and automated manufacturing techniques have established the magnetron in the application area of microwave ovens at home.

Radiated noise from microwave ovens may interfere with several communication systems such as Wi-Fi and Bluetooth operating at 2.45 GHz as these communication systems occupy the same region of the microwave spectrum (Figure 1). The significant potential for interference from noisy magnetrons forces manufacturers and researchers to look for a solution to prevent EMI issues by reducing heater power, using non homogeneous magnetic fields or by modifying the shape of the pole pieces. In this article a solution via shifting the magnetron frequency (see Figure 1) is shown....

Figure 1: Table of radio frequency allocation

The structure of the cooker magnetron includes a heated filament cathode, multiple resonant cavities with a pair of permanent ceramic ring magnets to force electron beams into helical orbits, and an output antenna as shown in Figure 2.

Figure 2: Structure of the cooker magnetron for a microwave oven

Analyzing the Interaction Region in CST PARTICLE STUDIO

CST PARTICLE STUDIO (CST PS) can calculate both efficiency and hot frequency of a cooker magnetron. To be more precise, it can predict how much RF output power a cooker magnetron can generate and at which frequency considering a beam loading effect.

Figure 3 shows the time evolution of the electron spokes in the xy-plane of the interaction space obtained by the PIC solver of CST PS. Five spokes are formed azimuthally around the cathode according to the π-mode in a 10 vane resonator. Here, the π-mode is the operating mode.

Figure 3: Beam formation in the magnetron interaction region

The generated RF voltage from the interaction between the electron beam and the π-mode field of the resonator is shown in Figure 4. The voltage is evaluated at the top of the interaction space. From this, an output power of 980 W -using the information of the anode current- and a real operating frequency (2.481 GHz) of the magnetron can be predicted. The difference (39 MHz) between CST MICROWAVE STUDIO® (Cold simulation) and CST PS (Hot simulation) results from a beam loading effect. The RF power is extracted by means of an antenna, which is connected to the anode. The predicted RF power, 980 W, matches the design value of 1000 W quite well.

Figure 4: Voltage signal and corresponding spectrum in the interaction region

Static Calculations in CST EM STUDIO

In order to force the particles to gyrate around the cathode, two ceramic permanent magnets are applied. Both the resulting magnetic field distribution and the absolute field values along an axis (r=2 mm) are shown in Figure 5. Because of the B-field, emitted electrons from the cathode experience a force perpendicular to their instantaneous direction of motion. The B-field maximum from a simulation agrees to the design value of 0.19 Tesla.

Figure 5: Magnetostatic field distribution and absolute field value along a line of the structure

Furthermore, the electric potential distribution (isolines) resulting from potentials applied to the metal bodies are shown in Figure 6. This electrostatic field is responsible for the acceleration of the particles towards the anode. In combination with the magnetostatic field it defines the drift velocity of the particles which has to be synchronous to the operating mode.

Figure 6: Isolines of the electrostatic potential distribution

Temperature and Mechanical Analysis in CST MPHYSICS STUDIO

Since an efficiency of a magnetron is not 100%, cooling is extremely important. In fact, 30 to 50% of energy are converted into heat. The weakest parts are the ceramic magnets because they will lose their magnetism if heated above the Curie temperature. Figure 7 shows the temperature distribution for an efficiency of 42%. The cooling air flow of 1.0 m3/min is considered by the heat transfer coefficient. Also the deformation by the thermal expansion is shown in Figure 7.

Figure 7: Temperature distribution (left) and resulting deformation (right) of the magnetron

RF considerations

Figure 8 shows the resonant mode (π-mode) of a cooker magnetron, where the phase shift of the electric field between neighboring vanes is 180°. The intended use of a microwave oven is to heat food evenly. Therefore, the electric field inside the oven has to be analyzed. In addition, the radiation through the gap between door and microwave oven should be minimal. CST MWS gives the resulting farfield plot and E-field pattern as shown in Figure 9. To ease the simulation, compact models are available for most standard geometries like vents, slots, seams and panels to design a door part.

Figure 8: Electric field of the resonant mode

Figure 9: Farfield (left) and electric field of the microwave oven

Comparison to Measurements

A simulation can be a great way to reduce costs, if the simulation is able to replicate measurements. High accuracy and short simulation times can efficiently reduce time to market. CST STUDIO SUITE® shows good results compared to measurements (see Figure 10). The temperature difference in the yoke is due to the contact condition between yoke and heat sink, which is different in measurement and simulation.

Figure 10: Comparison between measurement and simulation

Figure 11: Contact between yoke and heat sink

The article shows a possible way to solve EMI issue by shifting the frequency of the microwave oven magnetron from 2.45 GHz to 2.48 GHz. Furthermore, it demonstrates the benefit of an integrated design environment. The whole range of solver technologies necessary for such a simulation is available within one GUI: a static solver for magnet and electrode design, transient and eigenmode solver for the resonator analysis, PIC solver for the simulation of charged particle dynamics, thermal solver and mechanical stress analysis for heat sinks. Additional features such as GPU and MPI computing are available for speed up.

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