Why do Microwave Ovens Operate at 2.45 GHz?

Image result for microwave oven popcorn

I get this question all the time when we discuss dielectric loss in my classes. The common answer I get back is that this is the frequency for the resonance of a water molecule. It would make it the ideal frequency to run the oven so we get the most absorption.

But not always is the obvious answer the correct answer.

After all, is it a coincidence that 2.45 GHz is also the same frequency as 802.11ab wifi, the same as Bluetooth, and the Nyquist for PCIe gen 2, operating at 5 Gbps?

The use of 2.45 GHz is because this is in the center of the unlicensed ISM band of the FCC.  One of the frequency bands of the Industrial, Scientific and Medical (ISM) bands is from 2.4 GHz to 2.5 GHz. This band is reserved for non telecommunications applications and does not require an FCC certification of compliance.

If you use this band for communications, buyer beware. Your protocol must be robust to interference and the FCC will not help patrol emissions in this frequency range.

If microwave ovens use this band so they don’t have to be certified by the FCC, how is it related to heating water?

The frequency of 2.45 GHz is a wavelength in air of 122 mm or 12.2 cm. When we look at the absorption spectrum of liquid water, the frequency is usually reported as the wavelength, instead.

This low frequency is well below the energy of vibration bands, and is more related to the loose, rather “fluid” lattice of the water molecules, smeared out due to fluctuating hydrogen bonding.  Here is a typical absorption curve for liquid water, from 0 degC to 100 degC. There is a broad peak in the dissipation factor, around 3 cm which increases in frequency as temperature goes up.

Note that 12.2 cm, the wavelength of the microwave oven, is the black line. At 0 decC, the absorption of liquid water, the blue curve that is a measure of the dissipation factor, is actually sitting on the tail of the peak. It’s a very broad peak. As the water heats up, the absorption peak actually moves to higher frequency, shorter wavelength and the absorption for water actually gets worse.

The broad absorption by water, in the frequency range from 1 GHz to 100 GHz, is an important issue. This says that as an interconnect polymer absorbs water from the humidity in the air, we would expect the losses in the material to increase. This is why the humidity sensitivity to the dielectric loss of a laminate material is an important metric.

But why a microwave oven works at 2.45 GHz is more about the FCC than about the absorption resonance of water.

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