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microwave radar



Principles of Radar

Radar involves the transmission of pulses of electromagnetic waves by means of a directional antenna; some of the pulses are reflected by objects that intercept them. The reflections are picked up by a receiver, processed electronically, and converted into visible form by means of a cathode-ray tube. The range of the object is determined by measuring the time it takes for the radar signal to reach the object and return. The object's location with respect to the radar unit is determined from the direction in which the pulse was received. In most radar units the beam of pulses is continuously rotated at a constant speed, or it is scanned (swung back and forth) over a sector, also at a constant rate. The velocity of the object is measured by applying the Doppler principle: if the object is approaching the radar unit, the frequency of the returned signal is greater than the frequency of the transmitted signal; if the object is receding from the radar unit, the returned frequency is less; and if the object is not moving relative to the radar unit, the return signal will have the same frequency as the transmitted signal.

Applications of Radar

The information secured by radar includes the position and velocity of the object with respect to the radar unit. In some advanced systems the shape of the object may also be determined. Commercial airliners are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. Planes can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot. A ground-based radar system for guiding and landing aircraft by remote control was developed in 1960.
Radar is also used to measure distances and map geographical areas (shoran) and to navigate and fix positions at sea. Meteorologists use radar to monitor precipitation; it has become the primary tool for short-term weather forecasting and is also used to watch for severe weather such as thunderstorms and tornados. Radar can be used to study the planets and the solar ionosphere and to trace solar flares and other moving particles in outer space.
Various radar tracking and surveillance systems are used for scientific study and for defense. For the defense of North America the U.S. government developed (c.1959–63) a radar network known as the Ballistic Missile Early Warning System (BMEWS), with radar installations in Thule, Greenland; Clear, Alaska; and Yorkshire, England. A radar system known as Space Detention and Tracking System (SPADATS), operated collaboratively by the Canada and the United States, is used to track earth-orbiting artificial satellites.

Development of Radar

Radar was developed (c.1935–40) independently in several countries as a military instrument for detecting aircraft and ships. One of the earliest practical radar systems was devised (1934–35) by Sir Robert Watson-Watt, a Scots physicist. Although the technology evolved rapidly during World War II, radar improved immensely following the war, the principal advances being higher power outputs, greater receiver sensitivity, and improved timing and signal-processing circuits. In 1946 radar beams from the earth were reflected back from the moon. Radar contact was established with Venus in 1958 and with the sun in 1959, thereby opening a new field of astronomy—radar astronomy.

Best Answer: A typical radio wave of say 300MHz(FM radio is cut off at 100MHz) frequency has a wave length of 1m at this frequency and lower, the wave will diffract(bend around) a typical plane even a jumbo jet.

Microwaves at 10GHz (frequency) have a wavelength of 3 cm so they bounce off even the smallest aircraft(stealth planes are an exception of course). So they are much more suited to avoid getting bombed.


Microwaves are a type of electromagnetic radiation, as are radio waves, ultraviolet radiation, X-rays and gamma-rays. Microwaves have a range of applications, including communications, radar and, perhaps best known by most people, cooking.

Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum (EM spectrum). The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays. Microwaves fall in the range of the EM spectrum between radio and infrared light.

Microwaves have frequencies ranging from about 3 billion cycles per second, or 3 gigahertz (GHz), up to about 30 trillion hertz (terahertz or THz) and wavelengths of about 30 centimeters (12 inches) to 3 millimeters (0.12 inches), although these values are not definitive. This region is further divided into a number of bands, designated as L, S, C, X and K.

Communications & radar

Microwaves are used mostly for point-to-point communications systems to convey all types of information, including voice, data and video in both analog and digital formats, according to the Federal Communications Commission (FCC). They are also used for supervisory control and data acquisition (SCADA) for remote machinery, switches, valves and signals.

Another important application of microwaves is radar. The word "radar" was originally an acronym for RAdio Detection And Ranging. However, its usage has become so common that it is now a word in and of itself. Prior to World War II, British radio engineers found that short-wavelength radio waves could be bounced off of distant objects like ships and aircraft, and the returning signal could be detected with highly sensitive directional antennas so the presence and locations of those objects could be determined.

A little-known historical fact is that an early radar installation was built atop Kahuku Point on Oahu's northernmost tip. According to the state of Hawaii's website, the station actually detected the first wave of Japanese aircraft on their way to attack Pearl Harbor when the planes were 132 miles (212 kilometers) out. However, because the system had been in operation for only two weeks, it was considered unreliable, and the warning was ignored. Over the course of the war, radar was improved and refined, and it has since become an essential element of national defense and civilian air-traffic control.

Radar has found many other uses, some of which exploit the Doppler effect. An example of the Doppler effect can be demonstrated by an approaching ambulance: As it nears, the sound of the siren seems to rise in pitch, until it wails by. Then, as it recedes into the distance, the siren seems to lower in pitch.

Robert Mayanovic, a professor of physics at Missouri State University, said that Doppler radar is used for air-traffic control and vehicular speed-limit enforcement. When an object is approaching the antenna, the returning radio waves are compressed and thus have a shorter wavelength and higher frequency. Conversely, return waves from objects moving away are elongated and have a longer wavelength and lower frequency. By measuring this frequency shift, the speed of an object toward or away from the antenna can be determined.

Common applications of this principle include simple motion detectors, radar guns for speed-limit enforcement, radar altimeters and weather radar that can track the three dimensional motion of water droplets in the atmosphere. These applications are called active sensing, because microwaves are transmitted, and the reflected signals are received and analyzed. In passive sensing, natural sources of microwaves are observed and analyzed. Many of these observations are conducted by satellites looking either back at the Earth or out into space.

Microwave heat sources

One of the most common uses of microwaves is to heat food quickly. Microwave ovens are possible because microwaves can be used to transmit thermal energy. The discovery of this phenomenon was purely accidental. In his book, "They All Laughed...: From the Light Bulbs to Lasers: The Fascinating Stories Behind the Great Inventions That Have Changed Our Lives" (HarperCollins, 1992), author Ira Flatow recounts the story of the invention of the microwave oven: "Shortly after World War II, Percy L. Spencer, an electronics genius and war hero, was touring one of his laboratories at the Raytheon Company. Spencer stopped in front of a magnetron, the power tube that drives a radar set. Suddenly he noticed that a candy bar in his pocket had begun to melt." Further investigation led him to make the first batch of microwave popcorn as well as the first exploding egg.

The first microwave ovens were quite large and expensive, but they have since become so affordable that they are common in homes worldwide. Microwave heating systems are also used in a number of industrial applications, including food, chemical and materials processing in both batch and continuous operations.
Natural microwave sources

Radio astronomers conduct observations in the microwave region, but due to attenuation by the atmosphere, most of these studies are done using high-altitude balloons or satellites. However, perhaps the most famous observation of extraterrestrial microwaves was conducted by two Bell Labs scientists working on a telecommunications system using a large ground-based horn antenna.

According to NASA's Mission: Science website, "In 1965, using long, L-band microwaves, Arno Penzias and Robert Wilson, scientists at Bell Labs, made an incredible discovery quite by accident: They detected background noise using a special low-noise antenna. The strange thing about the noise was that it was coming from every direction and did not seem to vary in intensity much at all. If this static were from something on our planet, such as radio transmissions from a nearby airport control tower, it would come only from one direction, not everywhere. The Bell Lab scientists soon realized that they had serendipitously discovered the cosmic microwave background radiation. This radiation, which fills the entire universe, is a clue to its beginning, known as the Big Bang."

Penzias and Wilson were awarded the 1978 Nobel Prize in physics for their discovery. The background radiation has since been mapped with great accuracy by satellites. These observations have revealed the minute temperature variations that eventually evolved into the galactic clusters we see today.