Subject: [NEXTEL-1] VHF .. UHF .. 800 MHZ ... 900 MHZ ... It's all here ...

From: Marcel <[email-masked]>
Date: Mon, 16 Feb 2004 16:16:06 -0500

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.
 
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RADIO FREQUENCY BANDS

The part of the spectrum we are concerned with
are the frequency bands used for radio communications.
These are: 

VHF Lowband (25 MHz - 50 MHz) 
VHF Highband (136 MHz - 174 MHz) 
UHF Band (403 MHz - 512 MHz) 
800/900 MHz Bands 

The characteristic behavior of frequencies in different
bands in the spectrum is important when choosing a
frequency for a radio system.
 
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FREQUENCY BAND CHARACTERISTICS

The frequency band characteristics most important
to radio communications include: 

Propagation 
Noise Interference 
Range 

PROPAGATION

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.
 
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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.
 
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RANGE

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.
 
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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. 
 
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SPECTRUM MANAGEMENT

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.

http://www.myradiomall.com/arsradio/course_3.htm

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