Microwaves are a type of electromagnetic radiation with a frequency ranging from 300 MHz (0.3 GHz) up to 300 GHz, equivalent to a wavelength ranging from 1 m down to 1 mm (According to another common definition in RF engineering, microwaves have a frequency range from 1 GHz to 100 GHz). Their neighbours in the electromagnetic spectrum are VHF radio waves at the lower frequency border and long infrared light at the upper frequency border.
For the use in industrial applications, the permitted frequencies are limited to the so-called ISM (Industrial, Scientific and Medical) radio band to avoid interference with frequencies used for telecommunication and other purposes. Therefore microwave systems used in industrial applications, like dielectric heating and plasma generation, usually have centre frequencies of 915 MHz, 2.45 GHz and 5.8 GHz, please note that these centre frequencies might vary slightly from country to country. Because in recent years the same frequencies are shared with an increasing number of wireless telecommunication equipment like cordless phones, wireless LANs (WIFI) etc., care should be taken to avoid industrial equipment from interfering with this equipment via excessive leakage of microwave energy.
Due to the high frequencies microwaves, unlike radio waves, cannot be transmitted via coaxial cables without very high losses, therefore waveguide components are used to transmit microwave energy in high power installations, similar to the use of optical fibres for the transmission of light. Furthermore, certain microwave frequencies are strongly absorbed by water, water vapour and other materials, which on one side causes problems for their use in telecommunication applications, but on the other side is the basis for the usage in home appliances or certain industrial applications.
It is important to note that microwave radiation, against some common believe, is a non-ionising radiation, i.e. unlike X-rays or gamma rays, which can ionise atoms or molecules directly, microwaves are unable to do so because the energy of their photons is far below the required limit of approximately 10 eV. Or, to put it in a simpler context, if microwaves would be ionising, then normal visible light would be ionising too, because the energy of the photons increases with increasing frequency, and the frequency of visible light is higher than the frequency of microwaves. Perhaps this misconception is based on the fact that microwaves are also used for the generation of plasma, which is an ionised state of matter, however the underlying mechanisms are not based on direct ionisation. Therefore their is no proven negative effect on human health when getting exposed to low levels of microwave radiation, for further information please refer to our webpage microwave & health
At low frequencies and low output power, microwaves can be generated via specialised semiconductor devices, most of us use this technology daily with our mobile phones, which are operated at the GSM frequencies of 850 / 900 MHz or 1800 / 1900 MHz, or other devices like wireless LAN (WIFI), which operates at 2.45 GHz and higher frequencies. However, the efficiency of these semiconductor devices is very low and further decreases with increasing frequencies, so basically all industrial microwave applications rely on the use of specialised vacuum tubes to generate the required microwave energy.
There are a number of different types of vacuum tubes available, each of them offering special features and advantages:
Almost all industrial microwave applications belong to one of the following groups:
Please refer to the links above for more information.