Microwave transmission refers to the technology of transmitting information or energy by the
use of electromagnetic waves whose wavelengths are conveniently measured in small numbers
of centimetre; these are called microwaves. This part of the radio spectrum ranges across
frequencies of roughly 1.0 gigahertz to 30 GHz. These correspond to wavelengths from 30 centimeters
down to 1.0 cm.
Uses Microwaves are widely used for point-to-point
communications because their small wavelength allows conveniently-sized antennas to direct
them in narrow beams, which can be pointed directly at the receiving antenna. This allows
nearby microwave equipment to use the same frequencies without interfering with each
other, as lower frequency radio waves do. Another advantage is that the high frequency
of microwaves gives the microwave band a very large information-carrying capacity; the microwave
band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A
disadvantage is that microwaves are limited to line of sight propagation; they cannot
pass around hills or mountains as lower frequency radio waves can.
Microwave radio transmission is commonly used in point-to-point communication systems on
the surface of the Earth, in satellite communications, and in deep space radio communications. Other
parts of the microwave radio band are used for radars, radio navigation systems, sensor
systems, and radio astronomy. The next higher part of the radio electromagnetic
spectrum, where the frequencies are above 30 GHz and below 100 GHz, are called "millimeter
waves" because their wavelengths are conveniently measured in millimeters, and their wavelengths
range from 10 mm down to 3.0 mm. Radio waves in this band are usually strongly attenuated
by the Earthly atmosphere and particles contained in it, especially during wet weather. Also,
in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by
molecular oxygen in the atmosphere. The electronic technologies needed in the millimeter wave
band are also much more difficult to utilize than those of the microwave band.
Wireless transmission of information One-way and two-way telecommunication using
communications satellite Terrestrial microwave relay links in telecommunications
networks including backbone or backhaul carriers in cellular networks linking BTS-BSC and BSC-MSC.
Wireless transmission of power Proposed systems e.g. for connecting solar
power collecting satellites to terrestrial power grids
Parabolic antenna
To direct microwaves in narrow beams for point-to-point communication links or radiolocation, a parabolic
antenna is usually used. This is an antenna that uses a parabolic reflector to direct
the microwaves. To achieve narrow beamwidths, the reflector must be much larger than the
wavelength of the radio waves. The relatively short wavelength of microwaves allows reasonably
sized dishes to exhibit the desired highly directional response for both receiving and
transmitting. Microwave radio relay
Microwave radio relay is a technology for transmitting digital and analog signals, such
as long-distance telephone calls, television programs, and computer data, between two locations
on a line of sight radio path. In microwave radio relay, microwaves are transmitted between
the two locations with directional antennas, forming a fixed radio connection between the
two points. The requirement of a line of sight limits the distance between stations to 30
or 40 miles. Beginning in the 1940s, networks of microwave
relay links, such as the AT&T Long Lines system in the U.S., carried long distance telephone
calls and television programs between cities. The first system, dubbed TD-2 and built by
AT&T, connected New York and Boston in 1947 with a series of eight radio relay stations.
These included long daisy-chained series of such links that traversed mountain ranges
and spanned continents. Much of the transcontinental traffic is now carried by cheaper optical
fibers and communication satellites, but microwave relay remains important for shorter distances.
How microwave radio relay links are formed
Because the radio waves travel in narrow beams confined to a line-of-sight path from one
antenna to the other, they don't interfere with other microwave equipment, and nearby
microwave links can use the same frequencies. Antennas used must be highly directional;
these antennas are installed in elevated locations such as large radio towers in order to be
able to transmit across long distances. Typical types of antenna used in radio relay link
installations are parabolic antennas, dielectric lens, and horn-reflector antennas, which have
a diameter of up to 4 meters. Highly directive antennas permit an economical use of the available
frequency spectrum, despite long transmission distances.
Planning considerations Because of the high frequencies used, a quasi-optical
line of sight between the stations is generally required. Additionally, in order to form the
line of sight connection between the two stations, the first Fresnel zone must be free from obstacles
so the radio waves can propagate across a nearly uninterrupted path. Obstacles in the
signal field cause unwanted attenuation, and are as a result only acceptable in exceptional
cases. High mountain peak or ridge positions are often ideal: Europe's highest radio relay
station, the Richtfunkstation Jungfraujoch, is situated atop the Jungfraujoch ridge at
an altitude of 3,705 meters above sea level.
Obstacles, the curvature of the Earth, the geography of the area and reception issues
arising from the use of nearby land are important issues to consider when planning radio links.
In the planning process, it is essential that "path profiles" are produced, which provide
information about the terrain and Fresnel zones affecting the transmission path. The
presence of a water surface, such as a lake or river, in the mid-path region also must
be taken into consideration as it can result in a near-perfect reflection, creating multipath
distortion as the two received signals swing in and out of phase. Multipath fades are usually
deep only in a small spot and a narrow frequency band, so space and/or frequency diversity
schemes would be applied to mitigate these effects.
The effects of atmospheric stratification cause the radio path to bend downward in a
typical situation so a major distance is possible as the earth equivalent curvature increases
from 6370 km to about 8500 km. Rare events of temperature, humidity and pressure profile
versus height, may produce large deviations and distortion of the propagation and affect
transmission quality. High intensity rain and snow must also be considered as an impairment
factor, especially at frequencies above 10 GHz. All previous factors, collectively known as
path loss, make it necessary to compute suitable power margins, in order to maintain the link
operative for a high percentage of time, like the standard 99.99% or 99.999% used in 'carrier
class' services of most telecommunication operators.
The longest microwave radio relay known up to date crosses the Red Sea with 360 km hop
between Jebel Erba and Jebel Dakka. The link built in 1979 by Telettra allowed to proper
transmit 300 telephone channels and 1 TV signal, in the 2 GHz frequency band.
History
In 1931 a US-French consortium demonstrated an experimental microwave relay link across
the English Channel using 10 foot dishes, one of the earliest microwave communication
systems. Telephony, telegraph and facsimile data was transmitted over the 1.7 GHz beams
40 miles between Dover, UK and Calais, France. However it could not compete with cheap undersea
cable rates, and a planned commercial system was never built.
The development of radar during World War 2 provided much of the microwave technology
which made practical microwave communication links possible, particularly the klystron
oscillator and techniques of designing parabolic antennas.
During the 1950s the AT&T Long Lines system of microwave relay links grew to carry the
majority of US long distance telephone traffic, as well as intercontinental television network
signals. The prototype was called TDX and was tested with a connection between New York
City and Murray Hill, the location of Bell Laboratories in 1946. The TDX system was set
up between New York and Boston in 1947. The TDX was improved to the TD2, which still used
klystron tubes in the transmitters, and then later to the TD3 that used solid state electronics.
The main motivation in 1946 to use microwave radio instead of cable was that a large capacity
could be installed quickly and at less cost. It was expected at that time that the annual
operating costs for microwave radio would be greater than for cable. There were two
main reasons that a large capacity had to be introduced suddenly: Pent up demand for
long distance telephone service, because of the hiatus during the war years, and the new
medium of television, which needed more bandwidth than radio.
Though not commonly known, the US Military used both portable and fixed-station microwave
communications in the European Theater during WWII. Starting in the late 1940s, this continued
to some degree into the 1960s, when many of these links were supplanted with tropospheric
scatter or satellite systems. When the NATO military arm was formed, much of this existing
equipment was transferred to communications groups. The typical communications systems
used by NATO during that time period consisted of the technologies which had been developed
for use by the telephone carrier entities in host countries. One example from the USA
is the RCA CW-20A 1–2 GHz microwave relay system which utilized flexible UHF cable rather
than the rigid waveguide required by higher frequency systems, making it ideal for tactical
applications. The typical microwave relay installation or portable van had two radio
systems connecting two LOS sites. These radios would often provide communication for 24 telephone
channels of frequency division multiplexed signal, though any channel could be designated
to carry up to 18 teletype communications instead. Similar systems from Germany and
other member nations were also in use. Similar systems were soon built in many countries,
until the 1980s when the technology lost its share of fixed operation to newer technologies
such as fiber-optic cable and communication satellites, which offer lower cost per bit.
During the Cold War, the US intelligence agencies, such as the National Security Agency (NSA),
were reportedly able to intercept Soviet microwave traffic using satellites such as Rhyolite.
Much of the beam of a microwave link passes the receiving antenna and radiates toward
the horizon, into space. By positioning a geosynchronous satellite in the path of the
beam, the microwave beam can be received.
At the turn of the century, microwave radio relay systems are being used increasingly
in portable radio applications. The technology is particularly suited to this application
because of lower operating costs, a more efficient infrastructure, and provision of direct hardware
access to the portable radio operator.
Microwave link A microwave link is a communications system
that uses a beam of radio waves in the microwave frequency range to transmit video, audio,
or data between two locations, which can be from just a few feet or meters to several
miles or kilometers apart. Microwave links are commonly used by television broadcasters
to transmit programmes across a country, for instance, or from an outside broadcast back
to a studio. Mobile units can be camera mounted, allowing
cameras the freedom to move around without trailing cables. These are often seen on the
touchlines of sports fields on Steadicam systems. Properties of microwave links
Involve line of sight communication technology Affected greatly by environmental constraints,
including rain fade Have very limited penetration capabilities
through obstacles such as hills, buildings and trees
Sensitive to high pollen count Signals can be degradedduring Solar proton
events Uses of microwave links
In communications between satellites and base stations
As backbone carriers for cellular systems In short range indoor communications
Telecommunications, in linking remote and regional telephone exchanges to larger exchanges
without the need for copper/optical fibre lines.
Troposcatter Terrestrial microwave relay links described
above are limited in distance to the visual horizon, about 40 miles. Tropospheric scatter
was a technology developed in the 1950s allow microwave communication links beyond the horizon,
to a range of several hundred kilometers. The transmitter radiates a beam of microwaves
into the sky, at a shallow angle above the horizon toward the receiver. As the beam passes
through the troposphere a small fraction of the microwave energy is scattered back toward
the ground by water vapor and dust in the air. A sensitive receiver beyond the horizon
picks up this reflected signal. Signal clarity obtained by this method depends on the weather
and other factors, and as a result a high level of technical difficulty is involved
in the creation of a reliable over horizon radio relay link. Troposcatter links are therefore
only used in special circumstances where satellites and other long distance communication channels
cannot be relied on, such as in military communications. Microwave power transmission
Microwave power transmission is the use of microwaves to transmit power through outer
space or the atmosphere without the need for wires. It is a sub-type of the more general
wireless energy transfer methods. History
Following World War II, which saw the development of high-power microwave emitters known as
cavity magnetrons, the idea of using microwaves to transmit power was researched. In 1964,
William C. Brown demonstrated a miniature helicopter equipped with a combination antenna
and rectifier device called a rectenna. The rectenna converted microwave power into electricity,
allowing the helicopter to fly. In principle, the rectenna is capable of very high conversion
efficiencies - over 90% in optimal circumstances. Most proposed MPT systems now usually include
a phased array microwave transmitter. While these have lower efficiency levels they have
the advantage of being electrically steered using no moving parts, and are easier to scale
to the necessary levels that a practical MPT system requires.
Using microwave power transmission to deliver electricity to communities without having
to build cable-based infrastructure is being studied at Grand Bassin on Reunion Island
in the Indian Ocean. Common safety concerns
The common reaction to microwave transmission is one of concern, as microwaves are generally
perceived by the public as dangerous forms of radiation - stemming from the fact that
they are used in microwave ovens. While high power microwaves can be painful and dangerous
as in the United States Military's Active Denial System, MPT systems are generally proposed
to have only low intensity at the rectenna. Though this would be extremely safe as the
power levels would be about equal to the leakage from a microwave oven, and only slightly more
than a cell phone, the relatively diffuse microwave beam necessitates a large receiving
antenna area for a significant amount of energy to be transmitted.
Research has involved exposing multiple generations of animals to microwave radiation of this
or higher intensity, and no health issues have been found.
Proposed uses
MPT is the most commonly proposed method for transferring energy to the surface of the
Earth from solar power satellites or other in-orbit power sources. MPT is occasionally
proposed for the power supply in beam-powered propulsion for orbital lift space ships. Even
though lasers are more commonly proposed, their low efficiency in light generation and
reception has led some designers to opt for microwave based systems.
Current status Wireless Power Transmission is well proven.
Experiments in the tens of kilowatts have been performed at Goldstone in California
in 1975 and more recently at Grand Bassin on Reunion Island. In 2008 a long range transmission
experiment successfully transmitted 20 watts 92 miles from a mountain on Maui to the main
island of Hawaii. See also
Wireless energy transfer Fresnel zone
Passive repeater Radio repeater
Transmitter station Path loss
British Telecom microwave network Trans-Canada Microwave
Antenna array References
Microwave Radio Transmission Design Guide, Trevor Manning, Artech House, 1999
External links RF / Microwave Design at Oxford University
http:www.kurasc.kyoto-u.ac.jpsps/history2-e.html William C. Brown's Distinguished Career
AT&T's Microwave Radio-Relay Skyway introduced in 1951
Bell System 1951 magazine ad for Microwave Radio-Relay systems.
RCA vintage magazine ad for Microwave-Radio Relay equipment used for Western Union Telegraph
Co. Digital Microwave Radio
AT&T Long Lines Microwave Towers Remembered AT&T Long Lines
Western Union Microwave Network History Trevor Manning's course 'Microwave Radio for
Next Generation Networks' at Oxford University An article about how a microwave link is planned
and how it works IEEE Global History Network Microwave Link
Networks UK Microwave Radio Case Study