Researchers Tame 2 GHz Microwave Oven Magnetron Noise

If your station uses 2 GHz ENG frequencies A08 or A09, in the 2450 to 2483.5 MHz range, you are probably aware of the potential for microwave oven interference. Researchers at the University of Michigan have found a way to dramatically reduce the noise (sidebands and other spurious signals) from the magnetrons used in microwave ovens to the many devices operating around 2.4 GHz, including cordless phones, IEEE 802.11b wireless LANs, Bluetooth wireless devices and video extenders in addition to licensed broadcast auxiliary systems.

The principal researchers are professors Ronald M. Gilgenbach, Yue Ying Lau and Bogdan Neculaes. The solution is surprisingly simple - install four permanent magnets around the existing upper magnet to generate an azimuthally varying axial magnetic field. Placement is not critical. Installing the magnets reduced the noise near the carrier by about 30 dB and eliminated or substantially reduced microwave sidebands. For the study, the four magnets had a strength of about 3.5 kGauss on the face and were arranged around the existing magnet to cause the total axial magnetic field outside the magnetron cavities to vary azimuthally by about 50 percent.

Installing the magnets reduces power output by about 10 percent for fresh magnetrons and 20 percent for old magnetrons. Reducing the number of added magnets increases power out but also increases noise, compared with four magnets.

More technical information, including drawings showing the location of the added magnets and spectrum analyzer photos showing the results of adding the magnets can be found in the Applied Physics Letters paper Low-noise microwave magnetrons by azimuthally varying axial magnetic field. The paper notes that "noise generation mechanisms in crossed-field devices are not presently understood and predictive computational calculations do not exist."

While it would be nice to see microwave ovens that cause less interference to consumer and broadcast equipment, it is hard to imagine manufacturers spending the extra money to modify their magnetron designs. Magnetrons, however, are used in other applications, including radar. Improving the signal-to-noise ratio in these systems and making them more sensitive could be worth the extra cost. The University of Michigan study was supported by DUST (S&T) under the Innovative Microwave Vacuum Electronics MURI Program managed by the Air Force Office of Scientific Research.

The Kilowatt Magnetron page has a photo of the equipment setup used to conduct the experiments. The ABC News story Muting Meddling Microwaves - Scientists Create Microwave Oven that's Wireless-Friendly looks at the study from a less technical viewpoint.