Microwaves

Dr inż. ZDZISŁAW PÓLKOWSKI

Polkowice, 2015

University of PitestiDolnośląska Wyższa Szkoła Przedsiębiorczości i Techniki

w Polkowicach

Damian Constantin

AGENDA

What are microwaves Satellites and microwaves Microwave Frequency Band The Reflex Klystron Microwaves Properties Microwave Oven A Brief History of the Microwave Oven

Definition of microwaves

The term microwave refers to electromagnetic energy having a frequency higher than 1 gigahertz (billions of cycles per second), corresponding to wavelength shorter than 30 centimeters.Microwave signals propagate in straight lines and are affected very little by the troposphere. They are not refracted or reflected by ionized regions in the upper atmosphere. Microwave beams do not readily diffract around barriers such as hills, mountains, and large human-made structures. Some attenuation occurs when microwave energy passes through trees and frame houses. Radio-frequency (RF) energy at longer wavelengths is affected to a lesser degree by such obstacles.The microwave band is well suited for wireless transmission of signals having large bandwidth. This portion of the RF electromagnetic radiation spectrum encompasses many thousands of megahertz. Compare this with the so-called shortwave band that extends from 3 MHz to 30 MHz, and whose total available bandwidth is only 27 MHz. In communications, a large allowable bandwidth translates into high data speed. The short wavelengths allow the use of dish antennas having manageable diameters. These antennas produce high power gain in transmitting applications, and have excellent sensitivity and directional characteristics for reception of signals.

http://searchnetworking.techtarget.com/definition/microwave

http://www2.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

Below you can see the diagram of Electromagnetic Spectrum:

Satellites and microwaves

Earth satellites relaying microwave signals from the ground have increased the distance that can be covered in one hop. Microwave repeaters in a satellite in a stationary orbit 22,300 miles (35,880 kilometers) above Earth can reach one-third of Earth's surface. More than one-half of the long-distance phone calls made in the United States are routed through satellites via microwaves.

http://www.scienceclarified.com/Ma-Mu/Microwave-Communication.htmlwww.pinterest.com

Communication Tower from Berlin, Germany.

Microwave Frequency Band

http://www.google.com/patents/EP2422550A1?cl=en

The Reflex Klystron

Another tube based on velocity modulation, and used to generate microwave energy, is the REFLEX KLYSTRON. The reflex klystron contains a REFLECTOR PLATE, referred to as the REPELLER, instead of the output cavity used in other types of klystrons. The electron beam is modulated as it was in the other types of klystrons by passing it through an oscillating resonant cavity, but here the similarity ends. The feedback required to maintain oscillations within the cavity is obtained by reversing the beam and sending it back through the cavity. The electrons in the beam are velocity-modulated before the beam passes through the cavity the second time and will give up the energy required to maintain oscillations. The electron beam is turned around by a negatively charged electrode that repels the beam. This negative element is the repeller mentioned earlier. This type of klystron oscillator is called a reflex klystron because of the reflex action of the electron beam.

http://www.tpub.com/neets/book11/45d.htm

Below is the diagram of Reflex Klystron:

http://www.daenotes.com/electronics/microwave-radar/microwave-tube-devices

Microwaves Properties

Microwaves are sometimes considered to be very short radio waves (highfrequency and high-energy radio waves).

Some important properties of microwaves are:

•They are reflected by metal surfaces.•They heat materials if they can make atoms or molecules in the material vibrate. The amount of heating depends on the intensity of the microwave radiation, and the time that the material is exposed to the radiation.•They pass through glass and plastics.•They pass through the atmosphere.•They pass through the ionosphere without being reflected.•They are absorbed by water molecules, how well depends on the frequency (energy) of the microwaves.•Transmission is affected by wave effects such as reflection, refraction, diffraction and interference.

http://revisionworld.com/gcse-revision/physics/electromagnetic-radiation/radio-waves-microwaves

Microwaves Application

Microwaves have a lot of application.Below we have some application:TelecomPoint-to-point communication, Satellite, Cellular access technologiesSpaceSensing/Spectroscopy, Communication, Radio astronomyMedTechDiagnostics, imaging, and treatment applications.DefenseRadar, CommunicationSecurityCar avoidance radar, Traffic surveillance, Air traffic security “cameras”Navigation, Positioning & MeasurementGPSFoodHeating & detection of foreign bodies in food New and novel application areas are constantly being added.

http://www.microwaveroad.se/microwaves-and-application-areas.html

https://ascendingstarseed.files.wordpress.com/2013/02/electromagnetic_spectrum_full_chart.jpg

I chose this image as a example for microwaves application:

Microwave Oven

A Microwave Oven consists in: a high voltage transformer, an electron tube called magnetron, a wave guide fan and an oven chamber. The transformer passes electric energy to the magnetron and the magnetron converts this electric energy into microwave radiation.The microwaves are reflected in the oven chamber and are absorbed by food

http://www.slideshare.net/fascinating/microwaves-presentationhttp://ffden-2.phys.uaf.edu/104_spring2004.web.dir/arts_mcnulty/howmicrowaveovenswork.htm

A Brief History of the Microwave Oven

Like many of today's great inventions, the microwave oven was a by-product of another technology. It was during a radar-related research project around 1946 that Dr. Percy Spencer, a self-taught engineer with the Raytheon Corporation, noticed something very unusual. He was testing a new vacuum tube called a magnetron (we are searching for a picture of an actual 1946 magnetron), when he discovered that the candy bar in his pocket had melted. This intrigued Dr. Spencer, so he tried another experiment. This time he placed some popcorn kernels near the tube and, perhaps standing a little farther away, he watched with an inventive sparkle in his eye as the popcorn sputtered, cracked and popped all over his lab.

http://www.smecc.org/microwave_oven.htmhttp://ethw.org/Microwave_Ovens

The next morning, Scientist Spencer decided Microwave Oven Inventor Percy Spencerto put the magnetron tube near an egg. Spencer was joined by a curious colleague, and they both watched as the egg began to tremor and quake. The rapid temperature rise within the egg was causing tremendous internal pressure. Evidently the curious colleague moved in for a closer look just as the egg exploded and splattered hot yoke all over his amazed face. The face of Spencer lit up with a logical scientific conclusion: the melted candy bar, the popcorn, and now the exploding egg, were all attributable to exposure to low-density microwave energy. Thus, if an egg can be cooked that quickly, why not other foods? Experimentation began...Dr. Spencer fashioned a metal box with an opening into which he fed microwave power. The energy entering the box was unable to escape, thereby creating a higher density electromagnetic field. When food was placed in the box and microwave energy fed in, the temperature of the food rose very rapidly. Dr. Spencer had invented what was to revolutionize cooking, and form the basis of a multimillion dollar industry, the microwave oven.

http://www.smecc.org/microwave_oven.htm