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Microwave transmissionFrom Wikipedia, the free encyclopedia

The atmospheric attenuation o f microwaves in dry air with a precipitable water vapor level of 0.001 mm. The downward

spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly, such asby oxygen molecules

Microwave transmission refers to the technology of transmitting information or energy by the use of radio

waves whose wavelengths are conveniently measured in small numbers of centimeters; these are

called microwaves . This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to

30 GHz . These correspond to wavelengths from 30 centimeters down to 1.0 cm.

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

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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.

Contents

[hide ]

• 1 Properties

• 2 Uses

• 3 Parabolic (microwave) antenna

• 4 Microwave power transmission

○ 4.1 History

○ 4.2 Common safety concerns

○ 4.3 Proposed uses

○ 4.4 Current status

• 5 Microwave radio relay

○ 5.1 How microwave radio relay links

are formed

○ 5.2 Planning considerations

○ 5.3 Over-horizon microwave radio

relay

○ 5.4 Usage of microwave radio relay

systems

○ 5.5 Microwave link

5.5.1 Properties of

microwave links

5.5.2 Uses of microwave

links

• 6 See also

• 7 References

• 8 External links

[edit ]Properties

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Suitable over line-of-sight transmission links without obstacles

Provides large useful bandwidth when compared to lower frequencies (HF, VHF, UHF)

Affected by the refractive index (temperature, pressure and humidity) of the atmosphere, rain (see rain

fade ), snow and hail, sand storms, clouds, mist and fog, strongly depending on the frequency.[edit ]Uses

Wireless transmission of information

One-way (e.g. television broadcasting ) and two-way telecommunication using communications satellite

Terrestrial microwave radio broadcasting relay links in telecommunications networks including e.g.

backbone or backhaul carriers in cellular networks linking BTS -BSC and BSC- MSC .

A parabolic antenna for Erdfunkstelle Raisting , based in Raisting , Bavaria , Germany .

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Military microwave set in Switzerland

Wireless transmission of power

Proposed systems e.g. for connecting solar power collecting satellites to terrestrial power grids

[edit ]Parabolic (microwave) antenna

Main article: Parabolic antenna

To direct microwaves in narrow beams for point-to-point communication links or radiolocation ( radar ),

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.

[edit ]Microwave power transmissionMicrowave power transmission (MPT ) 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.

[edit ]History

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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. [1] 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 .

Microwave spying

During the Cold War , the US intelligence agencies, such as NSA , were reportedly able to intercept Soviet

microwave messages using satellites such as Rhyolite .[2] Microwave also used in mobile communication.

[edit ]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 alarge rectenna 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 .[3]

[edit ]Proposed uses

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Main article: Solar power satellite

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.

[edit ]Current status

Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of kilowatts have

been performed at Goldstone in California in 1975 [4][5][6] and more recently (1997) at Grand Bassin on Reunion

Island .[7] In 2008 a long range transmission experiment successfully transmitted 20 watts 92 miles (148 km)

from a mountain on Maui to the main island of Hawaii .[8]

[edit ]Microwave radio relay

Heinrich-Hertz- Turm in Germany

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

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sight radio path. In microwave radio relay, radio waves are transmitted between the two locations

with directional antennas , forming a fixed radio connection between the two points. Long daisy-chained series

of such links form transcontinental telephone and/or television communication systems.

[edit ]How microwave radio relay links are formed

Relay towers on Frazier Mountain ,Southern California

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 (High gain ); 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 , shell antennas and horn radiators, 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.

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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 (a 4/3

equivalent radius effect). 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.

Portable microwave rig for Electronic news gathering (ENG) for television news

[edit ]Over-horizon microwave radio relay

In over-horizon, or tropospheric scatter, microwave radio relay, unlike a standard microwave radio relay link,

the sending and receiving antennas do not use a line of sight transmission path. Instead, the stray signal

transmission, known as "tropo - scatter" or simply "scatter," from the sent signal is picked up by the receiving

station. 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. Over horizon

radio relay links are therefore only used where standard radio relay links are unsuitable (for example, in

providing a microwave link to an island).

[edit ]Usage of microwave radio relay systems

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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.

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 optical radio relay links, both of which offer larger

data capacities at lower cost per bit. Communication satellites , which are also microwave radio relays, better

retained their market share, especially for television.

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.

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[edit ]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 toseveral 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.

[edit ]Properties of microwave links

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Involve line of sight (LOS) 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 [citation needed ]

Signals can be degraded [citation needed ]during Solar proton events [9]

[edit ]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 (main) exchanges

without the need for copper/optical fibre lines.

[edit ]See also

energy portal

Wireless energy transfer

Fresnel zone

Passive repeater

Radio repeater

Transmitter station

Path loss

British Telecom microwave network

Trans-Canada Microwave

Antenna array (electromagnetic)

[edit ]References

1. ^ EXPERIMENTAL AIRBORNE MICROWAVE SUPPORTED PLATFORM Descriptive Note : Final rept. Jun

64-Apr 65

2. ^ James Bamford, The Shadow Factory , Doubleday, 2008, p 176

3. ^ Environmental Effects - the SPS Microwave Beam

4. ^ NASA Video, date/author unknown

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5. ^ Wireless Power Transmission for Solar Power Satellite (SPS) (Second Draft by N. Shinohara), Space

Solar Power Workshop, Georgia Institute of Technology

6. ^ Brown., W. C. (September 1984). "The History of Power Transmission by Radio Waves" . Microwave

Theory and Techniques, IEEE Transactions on (Volume: 32, Issue: 9 On page(s): 1230- 1242 + ISSN:

0018-9480).

7. ^ POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND 48th International

Astronautical Congress, Turin, Italy, 6–10 October 1997 - IAF-97-R.4.08 J. D. Lan Sun Luk, A. Celeste, P.

Romanacce, L. Chane Kuang Sang, J. C. Gatina - University of La Réunion - Faculty of Science and

Technology.

8. ^ http://www.wired.com/wiredscience/2008/09/visionary-beams/

9. ^ Analyzing Microwave Spectra Collected by the Solar Radio Burst Locator

Microwave Radio Transmission Design Guide, Trevor Manning, Artech House, 1999