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Most industrial microwaves operated in one of two unlicensed, government allocated radio bands called the Industrial, Scientific, and Medical (ISM) bands.  The lower “L” band ranges from 902 to 928 MHz.  In Europe and some other regions 896 MHz is used to avoid interference to the 900 MHz GSM cellular phone band.  The upper “S” band ranges from 2400 to 2500 MHz.  Consumer microwave ovens also use this band, as do many other unlicensed wireless devices such as WIFI.

Most high power industrial microwaves (30 or more kilowatts of power) use the 900 MHz band because it is possible to efficiently generate high power levels at the lowest cost per watt.   Because of its relatively longer wavelength is also penetrates about 3 times more deeply into materials (typically about 10”) compared to S band.

S band systems tend to be physically much smaller and have lower power levels (about 1 to 10 kilowatts per generator).   As mentioned previously, the penetration depth is less, about 3” for food products. Waveguide used to carry the energy from the generator to the applicator is much smaller and less expensive compared to L band.  S-band Magnetron units of exactly 1 kilowatt are relatively inexpensive because the tubes are manufactured in astronomical volumes for consumer microwave ovens.  At any power level above 1 kilowatt the magnetron tubes become much more expensive.