|#3 - FREQUENCY AND SPECTRUM|
THE ELECTROMAGNETIC SPECTRUM
|The electromagnetic spectrum is the total range of frequencies of electromagnetic radiation. It extends from the lowest audio waves (15 Hz) to the highest light waves (900,000 GHz). To make it more manageable, the electromagnetic spectrum is divided into bands.|
RADIO FREQUENCY BANDS
|The part of the spectrum we are concerned with
are the frequency bands used for radio communications. These are:
The characteristic behavior of frequencies in different bands in the spectrum is important when choosing a frequency for a radio system.
FREQUENCY BAND CHARACTERISTICS
The frequency band characteristics most important
to radio communications include:
The atmosphere that surrounds the earth acts
to attenuate and refract radio signals just as it does light. Just how
much it is affected depends on the frequency. As a general rule, the
lower the frequency, the less the attenuation, or loss of signal.
Lower frequency radio waves travel better through fog or dust than higher frequency waves. Low frequency AM broadcast radio signals will travel far beyond the horizon and can be reflected back to earth for reception at great distances.
Higher frequency television or FM commercial broadcast stations are absorbed by the earth's atmosphere and are therefore limited to line of sight transmission.
Below 300 KiloHertz, the characteristics are just the opposite. Here, radio waves follow the curvature of the earth for great distances. This type of propagation is called a ground wave. Radio communications over distances up to several thousand miles are possible by making use of low frequency ground waves.
Above 300 KHz to about 30 MHz, the ionosphere will sometimes reflect and/or refract the radio signals. when returned to earth, they are received hundreds or even thousands of miles away. This is called skip and these radio signals are called sky waves.
The frequencies from 30 MHz to about 900 MHtz are the most suitable for two-way radio transmission. Generally, this range of radio frequencies is characterized by line-of-sight propagation.
NOISE AND INTERFERENCE
|Electromagnetic noise interference comes from machines and engines. VHF lowband signals are very susceptible to noise interference because noise occurs in the lower frequency ranges. VHF high band has minimal susceptibility to noise interference. UHF and 800/900 MHz signals have virtually no susceptibility to noise interference. From this we can see that low band is not a good choice in high noise areas.|
In rural areas, VHF lowband signals have the
best range because they tend to follow the curvature of the earth. VHF
highband signals have good range characteristics. The UHF and 800/900
MHz bands have only fair range characteristics because signals in this
range can be attenuated by foliage and other terrain found in rural
areas. From this we can see that the VHF bands are the best choices for
use in rural areas.
In suburban areas, VHF lowband signals have the good range characteristics. VHF highband signals have excellent range characteristics. The UHF band has good range characteristics, and the 800/900 MHz bands have only fair range characteristics.
In urban areas, VHF lowband signals have poor range because they cannot penetrate buildings. Also, urban areas tend to have high levels of noise interference which affects lowband signals. VHF highband signals have good range characteristics and the UHF band and 800/900 MHz bands have excellent range characteristics because their signals can bounce off of, and penetrate buildings. From this you can see that lowband is not a good choice in urban areas.
|SELECTING THE RIGHT BAND|
There are many compromises in selecting a
frequency band. To aid you there are two general rules that apply:
As the frequency increases, range decreases but so does the ambient electrical noise.
Reflections from buildings increase with frequency.
The result is that lowband VHF is generally more suited for large rural areas. Highband VHF is generally more suited for suburban and mixed urban and rural areas. The UHF and 800/900 MHz bands are general most suited for urban usage.
Because frequency spectrum is an infinite
resource, and the number of users in many areas is high, many radio
channels are becoming crowded. Channel loading is a term used to
describe the number of users assigned to the same frequency. Channel
loading is so heavy in some areas that additional users are no longer
allowed on particular channels, or frequencies. The use of channels is
authorized and licensed by government agencies in most countries.
In the United States, the FCC, or Federal Communications Commission is in charge of regulating communications by radio (both commercial broadcast and two-way), television, wire, satellite, and cable.
International regulations fall under the jurisdiction of the ITU, or International Telecommunications Union.
In all cases, a license to operate radio equipment is required and must be applied for with the appropriate governing body. The license is granted to operate on a particular frequency, or set of frequencies, with specific eligibility rules that must be met.
Spectrum management and conservation is a global issue. Not only are government agencies using regulations to help conserve and manage our spectrum, but they are also encouraging the adoption of new technologies—technologies that have the potential to make our use of the spectrum more efficient.
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