Microwaves

Group 7, 11/22/2013

Matthew Spickard – History/DefinitionAndrew Miller – Range of practical applicationDustin Morris – Detailed application and equation definition

History

• First predicted by James Maxwell in 1864 using derived equations

• Experienced and observed by Heinrich Hertz in 1888 using a Spark Gap radio transmitter

• First demonstrated usefulness by Jagdish Bose with his controlling of a bell using mm-wavelengths

Definition

• “micro” means “very small”, but does not refer to the SI unit 10^-6.

• Microwaves are actually defined from 300MHz to 300GHz, between IR and FM waves

• Emitted by electromagnetic radiation from all “warm bodies” such as people, stars, and planets, allowing us to interpret microwaves for various information

Heating and Power

• Microwave Oven 2.45 GHz• Dielectric Heating• Fusion Reactors• Weapons

Communication

• Wireless LAN Protocols• Long Distance Telephone Calls• Broadcasting

Navigation and Astronomy

• Global Navigation Satellite Systems• Radio Astronomy

Application of microwaves: Doppler Radar

• One of the most widely used applications of microwaves.

• Doppler effect is where the perceived frequency of electromagnetic (or sound) waves is altered due to the relative motion of source and observer.

• Motion of observer and/or emitter towards each other increases the frequency (“Blue-shift”).

• Motion of observer and/or emitter away from each other decreases the frequency (“Red-shift”).

• Measuring the frequency shift from electromagnetic waves reflecting off a target also measures the relative velocity of that target.

• This is how Doppler Radars find and track targets.

http://science.howstuffworks.com/science-vs-myth/everyday-myths/doppler-effect2.htmhttp://usatoday30.usatoday.com/weather/wdoppler.htm

Area sensing with Doppler • Doppler sensors can only see the radial

(front and back) component of sensed velocity, shown in magenta at right.

• As the beam rotates, circuitry stores the sensor directions associated with changing Doppler frequency values over time.

• A computer makes an image using this data to extrapolate information about its target(s). In this figure, a wind velocity map is being generated, like the one below:

http://www.srh.noaa.gov/srh/jetstream/doppler/radialvel_max.htm

Green = closing (coming closer)

Red = receding (going away)

Application: Train detection using Doppler

• Doppler sensors are sometimes used for train detection at crossings where reliability and safety are an issue.

• Certain frequencies of microwaves are less susceptible to being interfered with by rain, dust, etc. This allows all-weather target detection.

• The radar is placed parallel to the track. Only the radial motion of the train causes a signal, whereas the tangential (left and right) motion of passing traffic is invisible to the sensor.

www.oconnerengineering.com

Application: SAR imaging

• SAR stands for Synthetic Aperture Radar.

• Instead of the target moving, the observer moves.

• A Doppler radar sensor is placed at an angle to the ground, and the sensor is moved along a long path, taking readings.

• When digitally overlapped, these multiple readings simulate a much larger radar antenna taking the ‘picture,’ thus, “synthetic aperture.”

• Much higher resolution images are possible with SAR than with a stationary radar.

http://www.sandia.gov/radar/whatis.html

http://treuropa.com/technique/sar-imagery/

SAR imaging example 1: Capitol building

http://www.sandia.gov/radar/imageryku.html

Ku-band (15GHz) sensor with 1 meter resolution. Taken from a plane.

SAR imaging example 2: National Guard Vehicle Lot

Ka-band (26.5-40GHz) sensor with 4-inch resolution. Taken from a plane. http://www.sandia.gov/radar/images/ka_band_portfolio.pdf

Exercise problem 1:

Exercise problem 1 solution:

Exercise problem 2:

A certain radar module outputs a Doppler frequency of 72 Hz per mph (mile per hour). This means that if a target is moving at 1 mph, it will output 72 Hz. If the target is moving at 2 mph, it will output 72*2, or 144 Hz and so on.

a) If the measured frequency is 1,080 kHz, what is the target’s velocity?

b) If some time later the measured frequency is 432 Hz, how much did the target’s velocity change?

Exercise problem 2 solution: