Measuring electromagnetic radiation leakage from microwave ovens using spectrum analyzer and near-field probe

Microwave ovens are available at home, in the workplace, and in restaurants. Moreover, many people probably use them almost every day. Regardless of which manufacturer produces a civilian microwave oven, its typical structure is shown in Figure 1. The magnetron is a key component. It is powered by a high-voltage power supply. According to the International Electrotechnical Commission, it generates high-frequency electromagnetic waves with a frequency range of about 2.45GHz, which are irradiated to the food to be heated through a waveguide. According to the industry, at this frequency, the microwave oven causes the water molecules in the food to move at high speed and generate friction, thereby generating high temperature to achieve the purpose of heating the food.

Figure 1: Typical structure of a microwave oven

Since the magnetron inside the microwave oven will generate relatively strong electromagnetic waves when it is working, the shielding of the microwave oven is very important. Its back, bottom, top and two sides are usually wrapped with metal plates. The front is the key, with an openable sealed door with a shielding net on it. If it is tightly wrapped and well shielded, theoretically, there is no need to worry about it generating magnetic radiation to the surrounding area when using this product. The debate about the electromagnetic radiation of microwave ovens has been going on for many years. If you search online, you will see many questions and answers about this. Is it true that household microwave ovens have electromagnetic radiation? To be precise, it should be: Is it true that household microwave ovens have electromagnetic radiation leakage?

I once used a spectrum analyzer and a near-field probe to perform a leakage test on a new microwave oven sample of a well-known domestic brand. I remember that I tested the front, back, left, right, top and bottom of this microwave oven sample, especially the position of the shielding net of the front door, which was most suspected to be prone to leakage, and the joints of the entire door frame. The test results were beyond our expectations: when the microwave oven was working, no obvious leakage was detected. It seems that the shielding of this sample is well done!

In order to test the design of the front door of the microwave oven sample, the influence of the sheet metal process and the installation process on the electromagnetic radiation leakage, a small dent was deliberately made on the door frame with a hammer to cause a small mechanical deformation. When the near-field probe was placed at this position and measured again, obvious electromagnetic leakage was detected! It seems that the design and installation process of the front door of the microwave oven are very important!

So what about the microwave ovens that I use every day? With curiosity, I got a RIGOL DSA875 spectrum analyzer, as shown in Figure 2. Its measurement frequency range is 9 kHz to 7.5 GHz, plus a set of NFP-3 near-field probes, to do a qualitative test.

Figure 2: Testing for leakage using a spectrum analyzer and near-field probe

I randomly found a microwave oven sample 1. This product seems to have simple functions, so I tested it! I scanned the front and door frame of this microwave oven with a near-field probe. The results are not optimistic! It was found that the leakage around the front panel is relatively large. The measurement results are shown in Figure 3. The highest peak frequency is 2.458GHz. The signal frequency is unstable, the spectrum energy is not concentrated, and the distribution is quite wide. There are many spurious signals on both sides of the main frequency.

Figure 3: Spectral distribution of electromagnetic radiation leakage of sample 1

I randomly found a microwave oven sample 2. This product has many buttons, so I tested it! I scanned the front of the microwave oven and the surroundings of the door frame with a near-field probe, and the results were even more pessimistic! It was found that the leakage around the front panel was larger than the previous one. The measurement results are shown in Figure 4. The peak frequency is at 2.453GHz. The signal frequency is unstable, the spectrum energy is not concentrated, and the distribution is wider. There are more spurious on both sides of the main frequency. You know, WLAN also works in this frequency band. If the spectrum is spread very wide and the energy is very strong, I don’t know if it will affect the surrounding WLAN.

Figure 4: Spectral distribution of leakage of sample 2

For sample 3, the micro-test results are shown in Figure 5. The highest peak frequency is at 2.433 GHz, and the spurious distribution range is also very wide, with peak values ​​ranging from 2.17 GHz to 2.75 GHz.

Figure 5: Distribution range of the leakage spectrum of sample 3

For sample 4, the micro-test results are shown in the pink track in Figure 6. The frequency of the highest peak is at 2.47 GHz. The distribution range and amplitude of the spurious signal are significantly smaller than those of the yellow track of sample 3, and the energy is mainly concentrated on the main peak at 2.47 GHz.

Figure 6: Leakage spectrum range of sample 4

Of course, the channel power measurement function of the spectrum analyzer can also be used to integrate the power of the signal within a certain frequency band to obtain the power value of the electromagnetic radiation within a certain bandwidth, as shown in Figure 7.

Figure 7: Power measurement of electromagnetic radiation

The above is just the result of my near-field test of electromagnetic radiation leakage on several microwave oven samples out of curiosity. It is for reference only. The intensity of the leaked electromagnetic radiation is directly related to the "firepower" setting of the microwave oven. Not only did I test these samples near the microwave oven, I also tested them about 1 meter away from the front of the microwave oven. I found that some microwave ovens leaked a lot of power. In short, I came to the conclusion that when using a microwave oven, I should stay away!

The advantage of using spectrum analyzer and near-field probe to measure radiation leakage of microwave oven is that it can not only get a value, but also clearly see the distribution of spectrum of microwave signal generated by microwave oven, deviation of main frequency, etc., so as to better judge the quality of magnetron and waveguide design. It can also locate the leakage position, so as to find problems in design, sheet metal process and installation process.

The samples tested were all old products that had been used for several years, and the leakage was quite obvious. After the test, I felt a little uneasy. I wondered how the new microwave ovens on sale were doing. I wanted to buy a "tight" shielded one, but which one was tight? I went to Suning to check it out and found that the price of the microwave ovens on sale was mostly around 1,000 yuan. I specifically checked the workmanship of the doors and door frames and found that the craftsmanship and materials were generally more delicate. Have these products made great improvements in the shielding process? Will the higher the price, the better? Or should I take a spectrum analyzer and a near-field probe to the supermarket for actual testing before choosing?