Chapter 2. Wireless Methodologies
This chapter discusses some of the basic physics associated with wireless
transmission techniques and goes on to describe how the technology is
implemented in various applications. This description includes a more in-depth
look at some of the networking technologies mentioned in Chapter 1,
“Introduction” on page 1.
2.1 Radio Frequency Characteristics
The characteristics of radio waves in any frequency band determine how useful
those frequencies are for the service required. The main characteristic of
interest is how signals are changed or distorted, by absorption and reflection, by
the air and other physical media before reaching the receiver. Since a radio
signal is a particular form of energy, it is useful to consider the different forms of
energy and how they can be converted and transmitted.
2.1.1 General Aspects
In order to understand some of the properties of radio and infrared propagation
it is important that the underlying principles or the physics of electromagnetic
waves be defined. A good starting point is a discussion of energy and the ways
it can be transmitted. Energy is defined in physics as the result of multiplying
power by time and may take a number of different forms (such as sound,
electrical, kinetic, potential, chemical, heat, and nuclear energy). A fundamental
principle of physics is that of the conservation of energy - energy can be neither
gained nor lost, only converted from one form to another. When we talk about
generating a particular form of energy, say electricity, what we are really doing
is changing one form of energy (heat to electricity) in a power generating station.
Some forms of energy are mechanical in nature, for example, kinetic energy. An
object is said to have kinetic energy by virtue of its motion. A moving vehicle
has kinetic energy and this has been converted from chemical energy in the
gasoline and air used to power the engine. If the vehicle is driven to the top of a
hill, it is said to have gained potential energy. The energy is stored in the
vehicle and can be released by coasting down the other side of the hill thus
becoming kinetic energy again. If the vehicle is brought to a halt, it will lose its
kinetic energy, which will be converted to heat by raising the temperature of the
brakes. Whenever one form of energy is converted to another, the process may
not be very efficient in changing the energy into the form desired. The internal
combustion engine moving our vehicle will convert some of the chemical energy
into kinetic energy, but quite a lot will be converted to heat. This energy is not
lost, it is just not in a form which is useful in moving the vehicle. Even this
wasted heat is welcome on a cold day!
A special form of potential energy is called elastic potential energy. An example
of this is a clock spring which can store the kinetic energy used to wind it and
release it again as kinetic energy to drive the clock.
A special form of kinetic energy is heat. The temperature of an object is due to
minute vibrations of its molecules. As an object cools, it will transmit its heat
energy to its surroundings, air and other objects, by means of infrared radiation
and conduction. Eventually the object and its surroundings will all have the
same temperature and no energy will be transferred. This is known as entropy,
Copyright IBM Corp. 1994, 1995 21

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