by Seyar Azizi 12O
This report is about microwaves and I will be writing about some of their properties and uses. I will also be writing about their history and their social and health aspects along with the benefits and risks.
Microwaves are part of the electromagnetic spectrum, on the side with the longer wave lengths and smaller frequencies. It has wave lengths which are shorter than radio waves and longer than infra-red. Microwaves have frequencies from 0.3 GHz to 3Ghz with wavelengths of 1mm to 1m. Microwaves, and in fact all waves in the electromagnetic spectrum, are created by oscillating electric fields and magnetic fields. Due to not requiring a medium they can travel through a vacuum at a speed of 3 x E8 m/s.
High-power microwave sources use specialized vacuum tubes to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron (used in microwave ovens), klystron, traveling-wave tube (TWT), and gyrotron. These devices work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream of electrons.
Cutaway view inside a cavity magnetron as used in a microwave oven
Low-power microwave sources use solid-state devices such as the field-effect transistor (at least at lower frequencies), tunnel diodes, Gunn diodes, and IMPATT diodes. Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A maser is a solid state device which amplifies microwaves using similar principles to the laser, which amplifies higher frequency light waves.
All warm objects emit low level microwave black-body radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of objects or terrain. The sun and other astronomical radio sources such as Cassiopeia A emit low level microwave radiation which carries information about their makeup, which is studied by radio astronomers using receivers called radio telescopes. The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology’s Big Bang theory of the origin of the Universe.
Microwaves can travel through some materials such as plastics but not metals. Some of their wavelengths are able to pass through the atmosphere which makes them very useful for satellites. Like all transverse waves they can be reflected, refracted, diffracted and polarised.
Uses of different frequencies/wavelengths of microwaves
L band 1 to 2 GHz 15 cm to 30 cm military telemetry, GPS, mobile phones (GSM), amateur radio
S band 2 to 4 GHz 7.5 cm to 15 cm weather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
C band 4 to 8 GHz 3.75 cm to 7.5 cm long-distance radio telecommunications
X band 8 to 12 GHz 25 mm to 37.5 mm satellite communications, radar, terrestrial broadband, space communications, amateur radio
Ku band 12 to 18 GHz 16.7 mm to 25 mm satellite communications
K band 18 to 26.5 GHz 11.3 mm to 16.7 mm radar, satellite communications, astronomical observations, automotive radar
Ka band 26.5 to 40 GHz 5.0 mm to 11.3 mm satellite communications
Q band 33 to 50 GHz 6.0 mm to 9.0 mm satellite communications, terrestrial microwave communications, radio astronomy, automotive radar
U band 40 to 60 GHz 5.0 mm to 7.5 mm
V band 50 to 75 GHz 4.0 mm to 6.0 mm millimetre wave radar research and other kinds of scientific research
W band 75 to 110 GHz 2.7 mm to 4.0 mm satellite communications, millimetre-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
F band 90 to 140 GHz 2.1 mm to 3.3 mm SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
D band 110 to 170 GHz 1.8 mm to 2.7 mm EHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimetre wave scanner
James Clerk Maxwell in 1864 and Heinrich Hertz 1888 were the first people to discover and analyse radio waves. In 1894, Indian radio pioneer Jagdish Chandra Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves.
Jagadish Chandra Bose, CSI, CIE, FRS (30 November 1858 – 23 November 1937) was a Bengali polymath, physicist, biologist, botanist, archaeologist, as well as an early writer of science fiction. He pioneered the investigation of radio and microwave optics.
Perhaps the first, documented, formal use of the term microwave occurred in 1931:
“When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon.” Telegraph & Telephone Journal XVII. 179/1
In 1943, the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there, worked as a radar, and could be used to measure distance, as well as to study the moon.
Zoltán Lajos Bay (July 24, 1900 in Gyulavári – October 4, 1992 in Washington, D.C.) was a Hungarian physicist, professor, and engineer who developed microwave technology, including tungsten lamps. He was the second person to observe radar echoes from the Moon. From 1930, he worked at the University of Szeged as a professor of theoretical physics.
Benefits and uses
Microwaves are mainly used to transmit data. They do not require a lot of space because each transmitter and receiver station is powerful enough not to need massive towers. Their high operating frequencies allow them to store large amounts of information. This makes them very cost effective. Physical obstacles such as water and high mountains do not interfere with them very much
They can send information across long distances, which make them useful for mobile phones signals and TV stations to send the video and sound to the receivers. Microwaves have good range and amplification is cheap since not many repeaters are needed. Microwave radio communication is more reliable and requires less maintenance than other types of radio station.
They are more easily focused into narrower beams than radio waves, allowing frequency reuse; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to transmitted frequency. Microwaves are used in spacecraft communication, and much of the world’s data, TV, and telephone communications are transmitted long distances by microwaves between ground stations and communications satellites as most microwave frequencies can pass through the atmosphere. Microwaves are also employed in microwave ovens and in radar technology.
Radar is an object-detection system that uses radio waves to determine the range, altitude, direction, or speed of objects.
The most obvious use for microwaves is the microwave oven. This is a kitchen appliance that heats and cooks food by bombarding it with electromagnetic radiation in the microwave spectrum causing polarized molecules in the food to rotate and build up thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm of a dense (high water content) food item; food is more evenly heated throughout (except in thick, dense objects) than generally occurs in other cooking techniques.
In 1945 the specific heating effect of a high-power microwave beam was accidentally discovered by Percy Spencer (19 July 1894 – 8 September 1970), an American self-taught engineer from Howland, Maine. Employed by Raytheon at the time he noticed that microwaves from an active radar set he was working on started to melt a candy bar he had in his pocket.
He then created what we might call the first true microwave oven by attaching a high density electromagnetic field generator to an enclosed metal box.
Microwaves are also used in spectroscopy. The technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II).
Disadvantages and risks
As the microwave signals move so quickly and carry a lot of information more complicated circuitry is needed to receive the signals.
They are propagated in a straight line and can pass through many media but can only be transmitted in one direction at a time. So in order to cover a large circular area you need to broadcast each signal multiple times over different directions.
Microwaves do not contain sufficient energy to chemically change substances by ionization, and so is an example of non-ionizing radiation. The word “radiation” refers to energy radiating from a source and not to radioactivity. It has not been shown conclusively that microwaves (or other non-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect. This is separate from the risks associated with very high-intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.
Exposure to microwave radiation can produce cataracts because the microwave heating denatures proteins in the crystalline lens of the eye. The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burns that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.
It has not been shown conclusively that microwaves (or other nonionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some but not all studies suggest that long-term exposure may have a carcinogenic effect. This had led to concerns over the prolonged use of mobile phones by children. The Stewart Report was commissioned to look into this in 2000. It recommend that a precautionary approach to the use of mobile phone technologies be adopted until much more detailed and scientifically robust information on any health effects becomes available. Since then a further report was made in 2005 in which Sir Stewart expressed concern that no clear indication of safety had been confirmed and that the precautionary principle should still be adhered to.
Edexcel AS Physics book by miles Hudson and Patrick Fullick ISBN: 978140589638