RADIO WAVE TRANSMISSION
Radio refers to a class of time-varying electromagnetic
fields created by varying voltages and/or currents in
certain physical sources. These sources may be "artificial,"
such as electrical power and electronic circuits, or
"natural," such as the atmosphere (lightning) and stars
(sunspots). The electromagnetic field variations radiate
outward from the source forming a pattern called a radio
wave. Thus, a radio wave is a series of electromagnetic
field variations travelling through space. Although,
technically, any varying source of voltage or current
produces a varying field near the source, here the term
"radio wave" describes field variations that propagate a
significant distance away from the source.
A sound wave has only a single "field" component (air
pressure). Variations in this component create a pattern of
air pressure changes along the direction the sound wave
travels but otherwise have no particular orientation. In
contrast, a radio wave includes both an electric field
component and a magnetic field component. The
variations in these components have the same relative
pattern along the direction the radio wave travels but they
are oriented at a 90 degree angle to each other as
illustrated in Figure 1-1. In particular, it is the orientation of
the electric field component which determines the angle of
"polarization" of the radio wave. This becomes especially
important in the design and operation of antennas.
Like sound waves, a radio wave can be described by
its frequency and its amplitude. The frequency of a radio
wave is the time rate of the field variations measured in
Hertz (Hz), where 1 Hz equals 1 cycle-per-second. The
radio spectrum, or range of frequencies, extends from a
few Hertz through the Kilohertz (KHz) and Megahertz
(MHz) ranges, to beyond the Gigahertz (GHz) range. The
suffixes KHz, MHz, and GHz refer to thousands, millions,
and billions of cycles-per-second respectively. As far as is
presently known, humans are directly sensitive to radio
waves only at frequencies in the range of a few million
GHz, which are perceived as visible light, and at those
frequencies in the range just below visible light, which are
perceived as heat (infrared radiation). The overall radio
spectrum includes both natural and artificial sources as
indicated by Figure 1-2.
The amplitude of a radio wave is the magnitude of the
field variations. It is the characteristic that determines the
"strength" of the radio wave. Specifically, it is defined to be
the amplitude of the electric field variation. It is measured
in volts per unit length and ranges from nanovolts/meter
(nV/m) to kilovolts/meter (KV/m), where nV refers to one
billionth of a volt and KV refers to one thousand volts.
The minimum level required for pickup by a typical radio
receiver is only a few tens of microvolts (uV, a millionth of a
volt) but much higher levels can be found near transmitters
and other sources. The wide range of radio wave
amplitudes that may be encountered in typical
applications requires great care in the design and use of
wireless microphone systems, particularly receivers.
Another characteristic of waves, related to frequency,
is wavelength. The wavelength is the physical distance
between the start of one cycle and the start of the next
cycle as the wave moves through space. Wavelength is
related to frequency by the speed at which the wave
travels through a given medium. This relationship is
expressed in the wave equation, which states that the
speed of the wave is always equal to the product of the
frequency times the wavelength. The wave equation
applies to any physical wave phenomenon such as radio
waves, sound waves, seismic waves, etc. (See Figure 1-3.)
5
Selection
and Operation
of Wireless Microphone Systems
CHAPTER 1
Basic Radio Principles
Figure 1-2: frequency vs. wavelength
x
y
Magnetic Field
Electric Field
Figure 1-1: radio wave
Figure 1-3: the wave equation