For these reasons, such base-station antennas have generally not been of concern with regard to possible hazardous exposure of the public to RF radiation.
Studies at rooftop locations have indicated that high-powered paging antennas may increase the potential for exposure to workers or others with access to such sites, for example, maintenance personnel. Transmitting power levels for vehicle-mounted land-mobile antennas are generally less than those used by base-station antennas but higher than those used for handheld units.
Handheld portable radios such as walkie-talkies are low-powered devices used to transmit and receive messages over relatively short distances. Because of the low power levels used, the intermittence of these transmissions, and the fact that these radios are held away from the head, they should not expose users to RF energy in excess of safe limits. Therefore, the FCC does not require routine documentation of compliance with safety limits for push-to-talk two-way radios.
Microwave Antennas Point-to-point microwave antennas transmit and receive microwave signals across relatively short distances from a few tenths of a mile to 30 miles or more. These antennas are usually rectangular or circular in shape and are normally found mounted on a supporting tower, on rooftops, on sides of buildings, or on similar structures that provide clear and unobstructed line-of-sight paths between both ends of a transmission path or link.
These antennas have a variety of uses, such as transmitting voice and data messages and serving as links between broadcast or cable TV studios and transmitting antennas. The RF signals from these antennas travel in a directed beam from a transmitting antenna to a receiving antenna, and dispersion of microwave energy outside of the relatively narrow beam is minimal or insignificant.
In addition, these antennas transmit using very low power levels, usually on the order of a few watts or less. Measurements have shown that ground-level power densities due to microwave directional antennas are normally a thousand times or more below recommended safety limits.
Moreover, as an added margin of safety, microwave tower sites are normally inaccessible to the general public. Significant exposures from these antennas could only occur in the unlikely event that an individual was to stand directly in front of and very close to an antenna for a period of time. Satellite Systems Ground-based antennas used for satellite-earth communications typically are parabolic "dish" antennas, some as large as 10 to 30 meters in diameter, that are used to transmit uplinks or receive downlinks microwave signals to or from satellites in orbit around the earth.
The satellites receive the signals beamed up to them and, in turn, retransmit the signals back down to an earthbound receiving station.
These signals allow delivery of a variety of communications services, including long-distance telephone service. Some satellite-earth station antennas are used only to receive RF signals that is, just like a rooftop television antenna used at a residence and, since they do not transmit, RF exposure is not an issue.
Because of the longer distances involved, power levels used to transmit these signals are relatively large when compared, for example, to those used by the microwave point-to-point antennas discussed above. However, as with microwave antennas, the beams used for transmitting earth-to-satellite signals are concentrated and highly directional, similar to the beam from a flashlight.
In addition, public access would normally be restricted at station sites where exposure levels could approach or exceed safe limits. Radar Systems Radar systems detect the presence, direction, or range of aircraft, ships, or other moving objects. This is achieved by sending pulses of high-frequency electromagnetic fields EMF. Radar systems usually operate at radiofrequencies between megahertz MHz and 15 gigahertz GHz. Invented some 60 years ago, radar systems have been widely used for navigation, aviation, national defense, and weather forecasting.
People who live or routinely work around radar have expressed concerns about long-term adverse effects of these systems on health, including cancer, reproductive malfunction, cataracts, and adverse effects for children. It is important to distinguish between perceived and real dangers that radar poses and to understand the rationale behind existing international standards and protective measures used today.
The power that radar systems emit varies from a few milliwatts police traffic-control radar to many kilowatts large space tracking radars. However, a number of factors significantly reduce human exposure to RF generated by radar systems, often by a factor of at least In addition to the information provided in this document, there are other sources of information regarding RF energy and health effects.
Some States maintain nonionizing radiation programs or, at least, some expertise in this field, usually in a department of public health or environmental control. The following table lists some representative Internet websites that provide information on this topic.
The Health Physics Society neither endorses nor verifies the accuracy of any information provided at these sites. They are being provided for information only.
Radiofrequency RF Radiation Includes RF from broadcast antennas, portable radio systems, microwave antennas, satellite, and radar Kelly Classic, Certified Medical Physicist Electromagnetic radiation consists of waves of electric and magnetic energy moving together that is, radiating through space at the speed of light. However, a number of factors significantly reduce human exposure to RF generated by radar systems, often by a factor of at least Radar systems send electromagnetic waves in pulses and not continuously.
You might even hear someone talk about transmitting RF signals and not really know if they mean wirelessly or via coaxial cable. Other signals that are not RF can use coax, too. Radio frequency RF signals can be transmitted wirelessly and over an RF cable. Recall that regular AC house current around the world oscillates at either 50Hz or 60Hz. RF signals are often encased in a shielded cable e. Coaxial cable has an outer conducting shield that is typically grounded. Other channels called UHF ultra high frequency utilize an even higher frequency range of to MHz.
Note that these frequencies are those of free transmission with the user utilizing an old-fashioned roof antenna. Satellite dishes and cable transmission of TV occurs at significantly higher frequencies, and is rapidly evolving with the use of the high-definition or HD format. Microwaves are electromagnetic waves with wavelengths ranging from one meter to one millimeter frequencies between MHz and GHz.
Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently with frequencies between MHz 0. The microwave region of the electromagnetic EM spectrum is generally considered to overlap with the highest frequency shortest wavelength radio waves. As is the case for all EM waves, microwaves travel in a vacuum at the speed of light.
The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary. They are used variously between different fields of study see figure. Microwaves overlap with the high frequency portion of the radio section of the EM spectrum. The microwave portion of the radio spectrum can be subdivided into three ranges, listed below from high to low frequencies. Microwaves are the highest-frequency electromagnetic waves that can be produced by currents in macroscopic circuits and devices.
Microwaves can also be produced by atoms and molecules—e. The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation. Since it is possible to carry more information per unit time on high frequencies, microwaves are quite suitable for communications devices. Most satellite-transmitted information is carried on microwaves, as are land-based long-distance transmissions. A clear line of sight between transmitter and receiver is needed because of the short wavelengths involved.
Cosmic Microwave Background : Cosmic background radiation of the Big Bang mapped with increasing resolution. 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.
Cavity Magnetron : Cutaway view inside a cavity magnetron as used in a microwave oven. Microwaves are used by microwave ovens to heat food. Microwaves at a frequency of 2. The microwaves then induce an alternating electric field in the oven. Water and some other constituents of food have a slightly negative charge at one end and a slightly positive charge at one end called polar molecules.
The range of microwave frequencies is specially selected so that the polar molecules, in trying to maintain their orientation with the electric field, absorb these energies and increase their temperatures—a process called dielectric heating.
Radar, first developed in World War II, is a common application of microwaves. By detecting and timing microwave echoes, radar systems can determine the distance to objects as diverse as clouds and aircraft.
A Doppler shift in the radar echo can determine the speed of a car or the intensity of a rainstorm. Sophisticated radar systems can map the Earth and other planets, with a resolution limited by wavelength.
The shorter the wavelength of any probe, the smaller the detail it is possible to observe. A maser is a device similar to a laser, which amplifies light energy by stimulating photons. The maser, rather than amplifying visible light energy, amplifies the lower-frequency, longer-wavelength microwaves and radio frequency emissions. Infrared IR light is EM radiation with wavelengths longer than those of visible light from 0. Distinguish three ranges of the infrared portion of the spectrum, and describe processes of absorption and emission of infrared light by molecules.
Infrared IR light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 0. This range of wavelengths corresponds to a frequency range of approximately GHz to THz, and includes most of the thermal radiation emitted by objects near room temperature.
Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. The infrared part of the electromagnetic spectrum covers the range from roughly GHz 1 mm to THz nm. It can be divided into three parts: It can be divided into three parts:. Observations of astronomical UV sources must be done from space. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation.
Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum. The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body.
As stated above, while infrared radiation is commonly referred to as heat radiation, only objects emitting with a certain range of temperatures and emissivities will produce most of their electromagnetic emission in the infrared part of the spectrum.
However, this is the case for most objects and environments humans encounter in our daily lives. Humans, their surroundings, and the Earth itself emit most of their thermal radiation at wavelengths near 10 microns, the boundary between mid and far infrared according to the delineation above.
The range of wavelengths most relevant to thermally emitting objects on earth is often called the thermal infrared. Many astronomical objects emit detectable amounts of IR radiation at non-thermal wavelengths. Infrared radiation can be used to remotely determine the temperature of objects if the emissivity is known. This is termed thermography, mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs.
Applications of IR waves extend to heating, communication, meteorology, spectroscopy, astronomy, biological and medical science, and even the analysis of works of art.
Visible light is the portion of the electromagnetic spectrum that is visible to the human eye, ranging from roughly to nm. Visible light, as called the visible spectrum, is the portion of the electromagnetic spectrum that is visible to can be detected by the human eye. A typical human eye will respond to wavelengths from about to nm 0. In terms of frequency, this corresponds to a band in the vicinity of — THz. A light-adapted eye generally has its maximum sensitivity at around nm THz , in the green region of the optical spectrum.
The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Visible light is produced by vibrations and rotations of atoms and molecules, as well as by electronic transitions within atoms and molecules.
The receivers or detectors of light largely utilize electronic transitions. We say the atoms and molecules are excited when they absorb and relax when they emit through electronic transitions. Visible Spectrum : A small part of the electromagnetic spectrum that includes its visible components. The divisions between infrared, visible, and ultraviolet are not perfectly distinct, nor are those between the seven rainbow colors. The figure above shows this part of the spectrum, together with the colors associated with particular pure wavelengths.
Red light has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths. Blackbody radiation from the Sun peaks in the visible part of the spectrum but is more intense in the red than in the violet, making the Sun yellowish in appearance.
Colors that can be produced by visible light of a narrow band of wavelengths monochromaticlight are called pure spectral colors. Quantitatively, the regions of the visible spectrum encompassing each spectral color can be delineated roughly as:. Note that each color can come in many shades, since the spectrum is continuous.
The human eye is insensitive to electromagnetic radiation outside this range. By definition any images presented with data recorded from wavelengths other than those in the visible part of the spectrum such as IR images of humans or animals or astronomical X-ray images are necessarily in false color. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the midday sky appears blue.
The optical window is also called the visible window because it overlaps the human visible response spectrum.
This allows visible light to heat the surface. The surface of the planet then emits energy primarily in infrared wavelengths, which has much greater difficulty escaping and thus causing the planet to cool due to the opacity of the atmosphere in the infrared. Plants, like animals, have evolved to utilize and respond to parts of the electromagnetic spectrum they are embedded in.
In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product.
0コメント