Microwaves

This part of the Radio Spectrum has wavelengths such short that they are easily absorbed by water and this is the core principle that they are used in microwave ovens. When we keep our food in microwave, the energy of the microwaves is converted into heat and makes the water molecules vibrate faster.

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

Why Microwaves are considered better than Radio waves for safer communication?

Microwaves are easier to control (than longer wavelengths) because small antennas could detect the waves very well. One advantage of such control is that the energy could be easily confined to a tight beam (narrow beam width). 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 (increased modulation bandwidth). Both of these advantages (narrow beam width 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 beam width made this reasonably secure.

Note: Please note that above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna (like the satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other.

Use of Microwaves

Please note that microwave transmission is weather and frequency dependent. Microwave communication was widely used for long distance telephone communication, cellular telephones, television distribution and other uses that a severe shortage of spectrum has developed.

• Microwave is relatively inexpensive in comparison to fiber optics system. For example, putting up two simple towers and antennas on each one may be cheaper than burying 50 km of fiber through a congested area or up lower a mountain.
• Microwave systems permit data transmission rates of about 16 Giga (1 giga = 109) bits per second. At such high frequencies, microwave systems can carry 250,000 voice channels at the same time.
• This is the core principle that they are mostly used to link big metropolitan cities where have heavy telephone traffic between them.

But, there is a big limitation. Since microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way. Consequently, repeaters are needed periodically. The higher the towers are, the further apart they can be. The distance between repeaters goes up very roughly with the square root of the tower height. For 100 meter high lowers, repeaters can be spaced 80 km apart.