- Microwaves are a portion or “band” found at the higher frequency end of the radio spectrum, but they are commonly distinguished from radio waves because of the technologies used to access them.
- The prefix micro- in the microwave is not meant to suggest a wavelength in the micrometer range.
- Rather, it indicates that microwaves are “small” (having shorter wavelengths), compared to the radio waves used before microwave technology.
- Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF (millimeter wave) bands.
- A more common definition in radio-frequency engineering is the range between 1 and 100 GHz (wavelengths between 0.3 m and 3 mm).
Wavelengths:
- Different wavelengths of microwaves (grouped into “sub-bands”) provide different information to scientists. Medium-length (C-band) microwaves penetrate through clouds, dust, smoke, snow, and rain to reveal the Earth’s surface.
- L-band microwaves, like those used by a Global Positioning System (GPS) receiver in your car, can also penetrate the canopy cover of forests to measure the soil moisture of rainforests.
Propagation:
- Microwave transmitters and receivers are parabolic dish antennas.
- They produce microwave beams whose spreading angle is proportional to the ratio of the wavelength of the constituent waves to the diameter of the dish.
- Most communication satellites use C-, X-, and Ku-bands to send signals to a ground station.
- Microwaves are used mostly for point-to-point communications systems to convey all types of information, including voice, data, and video in both analog and digital formats, according to the Federal Communications Commission (FCC).
Various types of microwave generators and amplifiers have been developed.
- Vacuum-tube devices, the klystron, and the magnetron continue to be used on a wide scale, especially for higher-power applications.
- A Klystron (also known as a Klystron Tube or Klystron Amplifier) is a vacuum tube that is used to oscillate and amplify microwave frequency signals. It was invented by American electrical engineers Russell and Sigurd Varian.
- In a klystron, an electron beam interacts with radio waves as it passes through resonant cavities and metal boxes along the length of a tube.
- The electron beam first passes through a cavity to which the input signal is applied. The energy of the electron beam amplifies the signal, and the amplified signal is taken from a cavity at the other end of the tube.
- The gain of klystrons can be high, 60 dB (an increase in signal power by a factor of one million) or more, with output power up to tens of megawatts, but the bandwidth is narrow, usually a few percent although it can be up to 10% in some devices.
Magnetron:
- The magnetron is a high-powered vacuum tube that works as a self-excited microwave oscillator.
- Crossed electron and magnetic fields are used in the magnetron to produce the high-power output required in radar equipment.
- The cathode of a magnetron provides the electrons through which the mechanism of energy transfer is accomplished.
- The cathode is located in the center of the anode and is made up of a hollow cylinder of emissive material (mostly Barium Oxide) surrounding a heater.
- The open space between the anode block and the cathode is called the interaction space. In this space, the electric and magnetic fields interact to exert force upon the electrons.
- Microwaves travel by line-of-sight; unlike lower frequency radio waves they do not diffract around hills, follow the earth’s surface as ground waves, or reflect from the ionosphere, so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km).
- At the high end of the band, they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer.
Uses:
- Microwave technology is extensively used for point-to-point telecommunications.
- Microwaves are used in spacecraft communication.
- TV telephone communications are transmitted long distances by microwaves between ground stations and communications satellites.