Radar - By Greg Sturm (all citations are in parentheses and listed at the end)

Radar, an acronym for Radio Detection and Ranging, was founded at the same time by many different scientists of various countries early in the 20th century, although the most notable were Nikola Tesla for proposing the theory behind it and Robert Watson-Watt for engineering a practical model. Interest in Radar began during World War II after rumors of a German "death ray" spread. After learning that this was false, countries realized the value in military use of radars and began funding its development.

Radar can be broken down into a few different steps and corresponding devices, the first of which would be generating an electromagnetic wave. This is accomplished with the use of a transmitter such as a magnetron. A magnetron is a cylinder with a thin filament in the center that is made to have a negative charge by a power source (the cathode). Electrons will feel an electrostatic force radially outward towards the relatively positive surface (the anode), however another force is also applied to the electrons. A magnetic force is created by a permanent magnet parallel to the filament, which creates a force perpendicular to the electrostatic force. The electrons will be pushed outward, but in a circular motion due to the magnetic force, so that the whole system resembles a wave (source 2). This follows the right-hand rule that we learned in class, where all forces act perpendicularly to each other. However, the electrons do not actually reach the surface of the magnetron. In between the anode and the cathode are multiple cavities that can vary in shape. The electrons pass through the cavities, ultimately created a capacitor at the entrance, similar to the parallel plates discussed in class, based on charges. While these electrons slide through the cavities in an oscillating manner, electromagnetic waves are created based on the natural resonant frequency of the cavities. The waves then travel through what is usually a hollow pipe called a waveguide towards the next step.

The waveguide is connected to an antenna on the other end, whose purpose is to send electromagnetic out in the targeted direction. The most basic form of antenna is just a wire made of conducting metal that is pumped energy at its center. In this case, electromagnetic waves are generated by displacing the loose electrons of the metal wire, creating a moving charge. A simple dipole antenna will disperse these waves equally in all directions. Some radars utilize phased arrays, or groups of multiple antennas that are designed to radiate waves in only a particular direction. After the waves leave the antenna, they are free to move throughout the surrounding space until they reach an object (source 1).

The waves traveling through the air will be reflected or dispersed once they enter a medium that has a dielectric or diamagnetic constant different than that of air, much like the material in between parallel plates on a capacitor. Remember that the capacitance is directly related to the material's permittivity. Buildings, planes, trees, and even the ground have a different permittivity (degree to which an electric field affects a material) than air, and so when electromagnetic waves hit them they will be affected based on the wavelength of the wave. If it is much smaller than the length of the object, it will be reflected back. If the wavelength is much larger, then the positive and negative charges of the object will be split, and it will become polarized. If the wavelength and the length of the object are comparable, then there will be resonance upon colliding (source 1). The ultimate goal of Radar is to reflect the waves back to the source, so small wavelengths are usually used. However, only a fraction of the energy of the initial wave is actually reflected back and received by the antenna, so the signal is very weak unless some device is used to amplify it. The equation for the power of the returning signal is given by: external image 6b1424601daef95eeef503895d59046b.png where Pt is the original power, Gt is the gain of the antenna, Ar is the aperture of the antenna, o is the scattering coefficient of the target, F is the pattern propagation factor, and R is the distance from the target to the radar. A simple derivation can be found here (source 2). Note that the power is proportional to the fourth power of the distance, so it decreases greatly with increasing distance.

The antenna is able to serve as the receiving device as well as the transmitting device (though not at the same time) through the use of a duplexer. The duplexer is an electrical switch that allows a signal to travel only either from the transmitter to the antenna, or from the antenna to the receiver. Typical radars alternate between these modes, sending out waves, switching the duplexer, receiving reflected waves, and then back to sending out waves (source 3). Once the waves are received, they can be interpreted to extract data about the target object that they reflected off of. The two most important pieces of information are the target's distance from the radar (or its location) and its velocity. Distance can be determined because the speed of electromagnetic waves are constant at 3 x 10^8 m/s and the time it takes for the wave to return to the receiver can be measured (although this must be divided by 2 since it is a round trip). The velocity of the target can be determined multiple ways. One historically efficient way is to write down the determined distance from the radar at different times, since velocity is defined as the change in location over time, however this pencil and paper method has been replaced by memory storage devices implanted in the radars themselves or a central control unit. A second, more complex way to measure velocity is by measuring the Doppler effect, or change in frequency of the electromagnetic wave while it is moving relative to the radar. The Doppler equation is given as: external image 6be9a7bd1b2910eccce5e1c51268ebd4.png which can be solved for V as: doppler.JPG where V is the velocity, c is the speed of the electromagnetic wave (3 x 10^8 m/s), f is the relative frequency, and f0 is the original frequency (source 3). This is the same equation used in class to find the speed of a firetruck in the classic Doppler Effect example. Some sophisticated machines called radar trackers can use this information to plot a likely course of a given object, although the data must be constantly refreshed for it to be as accurate as possible.

There are several unwanted signals that a radar can detect that must be removed or at least identified to process the right information. Signal noise is small random variation in the received signal due to the electrical components of the radar, or even thermal radiation from the Earth. It is hard to detect when measuring objects far away, since the desired signals are very weak to begin with. Clutter is the signal from unwanted objects like the ground, sea, trees, birds, or rain. Some radars can filter out constant signals from clutter using a Constant False Alarm Rate, but most use a form of polarization, confining the electromagnetic field vectors to a certain plane like a circle to remove clutter (source 1).

Radar has many uses in modern life. Police use radar guns to track the speed of passing cars, meteorologists have special radars that polarize out all signals except precipitation to predict weather, and airports control air traffic through the use of radar. One very important use of radar for the military since World War II has been detecting either enemy planes or enemy missiles. Stealth bombers have been engineered to have non-reflective paint coatings and smooth corners so that radio waves will scatter when they collide so that the planes will remain undetected (source 3). Three networks of radars have been placed in barren regions of the Earth by the U.S. for the sole purpose of detecting incoming ballistic missiles. Radar has become one of the most popular scientific breakthroughs from the 20th century for its ability to enhance our perception of our surroundings, and will continue to serve in many aspects of life.

Diagram of a Cavity Magnetron
Diagram of a Cavity Magnetron
Electron Oscillations and Capacitor Charges at the Cavities (Picture from www.radartutorial.eu)
Electron Oscillations and Capacitor Charges at the Cavities (Picture from www.radartutorial.eu)
B2 Stealth Bomber
B2 Stealth Bomber
Radar imaging of a group of ships
Radar imaging of a group of ships

Works Cited:
http://en.wikipedia.org/wiki/Radar (source 1)
http://www.radartutorial.eu/08.transmitters/tx08.en.html (source 2)
http://www.explainthatstuff.com/radar.html (source 3)

Some Questions to think about:
Why is Radar not preferred in underwater submarines, as opposed to its close counterpart, Sonar?
How could you see whether a target is friendly or not, like in a video game?
What are some other pieces of information you could find through the use of Radar?

Sonia Bansal - Could radar be used to determine the species of an animal?
-If you use waves with very small wavelengths (about a centimeter) you can detect more details of an object. Wikipedia uses the example that you can detect a loaf of bread. If you do this, you could determine the size and shape of an animal by sending a large quantity of waves in that direction. After that, you would have to hook up some sort of database of all animals in the area and have the central control system match it up by size/shape, which I assume wouldn't be too hard. The only problem is that the surface of the animal's skin (or whatever its outermost layer is) may not reflect waves in a favorable manner or at all. Radar works ideally with metallic surfaces that have corners or are at least very smooth, so while animals would be detected, the signal would be extremely weak, and it would probably not be the most efficient use of expensive radars.
Brandon Siegenfeld-How do environmental factors such as clouds and humidity effect radar?
-Back when radar was first invented for military use, they noticed little spots of interference mixed in with the desired signals. The spots were actually rain drops that reflected the waves back. This is how the meteorological radar was invented. I don't think humidity would affect the signal so much if it were uniform throughout the environment, but clouds could definitely show up on the screen. Think of the last hurricane we had, you probably saw a swirling radar image of it on the weather channel.
Robert Lopez - Can radars be used to map out entire populations or do the constantly moving people distort the signals?
-Theoretically I guess it could be done, but that would be incredibly expensive for something that a simple census could solve.
Sam Edwards - Can other types of electromagnetic waves be used to make radar-like devices?
-Other waves can be used but they either wont travel as far or won't provide a detailed enough picture of the object depending on the wavelength. It also depends on the properties of the antenna used. However even non-electromagnetic waves can be used in some cases, like in bats that use sound waves to locate objects during the night. They pretty much have an internal radar system. Its even been suggested that humans do this as well to a certain extent. There are blind people who claim to use it to learn their surroundings, its pretty amazing.
James Song- Are radars used in spacecrafts?
-There have been some radars launched into space to detect basically anything from people to planes back on earth, but as far as detecting things in space? What exactly is there to detect, its just space.
Will Chan - How do radar jamming devices work?
-Jamming devices send radio waves identical to those sent by the radar straight to its receiver. The radar will not be able to determine which signals were reflected back by objects and which were sent by the jammer. Jamming is so popular in the military because of its efficiency. You only need half the power of a normal radar since the waves travel only in one direction, and there are very few ways to counteract it.
Philip Cohn-Cort - Exactly how successful is current radar technology? Is it reliable enough for inspecting microscopic particles?
-Successful in what sense? It is widely used all around the world, and has very little inaccuracies to date. I think you could call that a success. It is not reliable enough for inspecting microscopic particles though. I don't think any form of radar is that detailed, but I don't even know why you'd want to detect individual particles.
Douglas Chin - If you ingested a radar and put a radar of the same frequency on your stomach, then would the resulting jam fry your tummy?
-Depending on the wavelength used the magnetron might spontaneously combust resulting in severe indigestion and an increase in entropy (vomit).