A very brief introduction to microwaves, by Thomas D. Williams, Ph.D., radio amateur WA1MBA.

Preface:

When microwave ovens became popular in the 1980's, the culture began to call those ovens "microwaves", just as in the 1960's people began to call the transistor radio a "transistor". These are just vernacular handles that allow people to carry on conversation without the need for extra words. If you want to learn about microwave ovens click here. Also, if you are interested in appliances that changed history follow this link to an excellent page on important appliances developed as a student project by Abby in Delaware.

In this brief introduction, we discuss neither microwave ovens nor the use of microwaves to induce heat in objects, but are instead concerned with the use of microwaves to communicate.

Definition
By "microwaves" we mean the range of radio frequencies between about 1 GHz (one gigahertz, or one billion oscillations per second) and about 300 GHz. For comparision, television transmissions normally occupy frequencies below the microwave region, from about 50 Mhz to 600 MHz (one Megahertz is one million oscillations per second, one GHz is 1,000 MHz). Cellular telephones operate in two bands, one from about 800 to 900 MHz and another around 1.8 to 1.95 GHz, again just below this definition of microwave frequencies.

Although there is no formal definition of the frequency range for "microwaves", some text books will define all frequencies above 300 MHz as microwaves.

The term "microwaves" seems to have first appeared in writing in a 1932 paper by Nello Carrara in the first issue of Alta Frequenza. The Italian word is microonde. The term gained acceptance during the second world war to describe wavelengths less than about 30 cm. These waves were much shorter than those normally used for communications (at that time), but were being used in RADAR.

A 30 centimeter wavelength is equivalent to 1 GHz (to convert from frequency to wavelength, just divide the speed of light 300,000,000 meters per second by the frequency in cycles per second to get meters of wavelength).

What are Radio Waves?
Electromagnetic radiation is a wave that combines electric and magnetic fields, moving out from its source as an expanding sphere and having waves as the feilds alternate in value. Its formal name is Transverse Electro Magnetic wave, or TEM. This kind of radiation has different utility as its wavelength changes.

Waves of a very long wavelength, such as thousands of meters, tend to travel along the surface of the earth and even penetrate into the water. These are useful for communication with submarines, and for broadcasting time signals. Broadcast radio, short-wave radio, television, cellular telephones, walky-talkies, 2-way police radios, satellite television, and other such communication/broadcast systems all use electromagnetic radiation, or "Radio Frequency Waves". Each communication service uses a part of the spectrum that is suitable for its needs.

Light, infra-red heat, ultra-violet (black light), and even X-rays and Gamma-rays are all forms of electromagnetic waves. All of these last forms are thousands of times shorter wavelengths than the shortest wavelengths of microwaves. They also behave in ways where thier particle-like nature becomes apparent. The discovery of the dual nature of light waves has led to significant discoveries in physics (see below: What are photons?).

Early Microwaves
Surprisingly enough, some of the first electromagnetic experiments conducted by Heinrich Hertz in 1886 and also by Marconi used frequencies near the microwave region - some around 500 MHz and some even in the multiple GHz (Gigahertz) region. By the way, the current unit frequency "Hz" is the "Hertz" named after Heinrich. GHz is pronounced "Giga Hertz". There were some very interesting experiments conducted in 1895 with frequencies/wavelengths that are even today considered a challenge. For a reference to this work see http://www.tuc.nrao.edu/~demerson/bose/bose.html

Why use microwaves?
Communication using electromagnetic radiation (except for light) began early in this century, and most early practical systems used very long wavelengths (low frequencies) which traveled great distances. Eventually, electronics were developed, including the vacuum tube (or "valve") which allowed controlled frequencies and modulation. This led to the use of higher frequencies, many channels, and commercial and industrial radio. During the 1930's and 1940's various experimenters discovered that higher frequencies could bring other advantages to communications. Some of these experimenters were government agencies and the military - some were universities, and some were private individuals.

Among these discoveries were that microwaves are easier to control (than longer wavelengths) because small antennas could direct the waves very well. One advantage of such control is that the energy could be easily confined to a tight beam (expressed as narrow beamwidth). This beam could be focused on another antenna dozens of miles away, making it very difficult for someone to intercept the conversation. Another characteristic is that because of their high frequency, greater amounts of information could be put on them (expressed as increased modulation bandwidth). Both of these advantages (narow beamwidth and modulation bandwidth) make microwaves very useful for RADAR as well as communications.

Eventually, these qualities led to the use of microwaves by the telephone companies. They placed towers every 30 to 60 miles each with antennas, receivers and transmitters. These would relay hundreds or even thousands of voice conversations across the country. The ability to modulate with a wide bandwidth permitted so many conversations on just one signal, and the reduction in beamwidth made this reasonably secure. In the 1950s experiments were conducted that showed the potential to connect the two coasts of the US via these microwave circuits to produce television programming on a continental basis, and true television networks were born.

Amateur radio interests in microwaves have mostly been for the challenge of working with such esoteric frequencies that require specialized techniques in design, fabrication and testing. Furthermore, in order to reach beyond LOS (line-of-sight) amateurs have spent countless hours carefully measuring propagation phenomena. Amateurs have carried on conversations using 10GHz well over 1,000 miles, and have bounced signals at that frequency off the moon. For more information about amateur radio uses of microwaves set your browser to www.wa1mba.org, contact a local VHF/Microwave Amateur radio club, or contact the ARRL.

What are photons, and how does this relate to microwaves? Someone recently asked: does the word "photon" only apply to light? And if so, what word should be used when referring to microwave energy?

A photon is a quantum of electromagnetic energy. Physicists think of electromagentic energy as having a "dual nature", in that some experiments reveal its nature as a particle which we call a photon and other experiments reveal its nature as a wave.

When it comes to lower frequencies (longer wavelengths), such as microwaves, VHF, and the like, it becomes much less convenient to think of energy in the form of photons, but there is no specific reason to decide that only one nature exists at these longer wavlengths. Sometimes photons are reffered to when describing an RF interaction with matter. The author does not know of any other word to describe the particulate nature of a propogating RF energy field except "photon". When the interaction with matter converts the energy into a mechanical form, we sometimes refer to the energy packets as "phonons". This is not a propogating Electro-Magnetic (EM) field, but rather a sound wave, and at the most minute level, even mechanical energy is quantized.

In most antenna, transmission line, waveguide, and quasi-optic formulations, the EM field is described according to its wave-like nature. When dealing with the interaction bewteen a microwave field and a molecule of Oxygen (for instance), in order to understand just why there are specific resonant freqencies of the molecule, a quantized nature re-appears, and the notion of the field expressed as photons can make sense.

The interactions between matter and EM fields have clearly different properties when comparing the interaction that causes a change in mechanical vibration with the interaction that causes a change in electron orbital state. The first occurs in the microwave and millimeter wave range - such as the serious absorption of 22 GHz signals by water vapor in the atmosphere. Here the interaction causes vibration and heat. To cause changes in electron orbital states, infrared, visible and UV range wavelengths are involved - such as is evidenced by floresence and lasers. In these cases much more than conversion to heat occurs. We call the second group of wavelengths "light" and the word "photon" is derived from Greek for light.

How are the higher frequencies designated?
What is meant by "Millimeter waves"?

People often misuse the term "millimeter waves" to mean any microwaves of higher frequencies than those normally used.

Here are some frequency bands, exact frequencies and approximate wavelengths.

Band Starts at Ends at
VHF 30 MHz 300 MHz
- 10 meters 1 meter
UHF 300 MHz 3 GHz
- 1 meter 10 cm< /TR>
SHF 3 GHz 30 GHz
- 10 cm 1 cm
EHF 30 GHz 300 GHz
- 1 cm 1 mm
Sub Millimeter 300 GHz Infinity
- 1 mm zero

EHF is considered "Millimeter Waves" because the range of wavelengths is between 1 mm and 1 cm. Above 300 GHz is considered "Sub-millimeter waves" because the wavelength is below one millimeter. Frequencies above 1 THz (1000 GHz) are called "Terahertz frequencies" or "Very long wave infrared"

Various designators and names have been associated with the wavelengths between 300 GHz and 300 THz. They usually are called Infrared in one form or another, but as you can see, this range covers three orders of magnitude - or one thousand fold.

300 THz - or 1 um (one micrometer, or 1000 nano-meters, or 10,000 angstroms) is still infrared, because visual sensitivity begins around 480 THz or about 625 nm which is a very very deep red color.


Amateur Radio Bands above 30 MHz

VHF ham bands = 50 MHz, 144 MHz, 222 MHz
UHF ham bands = 432 MHz, 903 MHz, 1296 MHz, 2304 MHz
VHF ham bands =3456 MHz (also called 3400 MHz), 5760 MHz (also called 5800), 10368 MHz, 24192 MHz
EHF ham bands = 47 GHz, 80 GHz, 122 GHz, 133 GHz, 240 GHz

Sub millimeter wave ham bands = One band defined as all frequencies above 300 GHz. Therefore, a contact at 411 GHz is in the same ham band as a red laser and the same as a blue laser and the same as an X-ray contact.



This page is last updated 4/16/11

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