Country Home Products Wireless Microphone Systems - Manual (Page 5)

The speed of radio waves (through a
vacuum) is equal to approximately 3 x 10 8
meter/second, or about 186,000 miles/
second. This is also known as the "speed of
light," since light is just one part of the radio
spectrum. The wave equation states that the
frequency of a radio wave, multiplied by its
wavelength always equals the speed of light.
Thus, the higher the radio frequency, the
shorter the wavelength, and the lower the
frequency, the longer the wavelength. Typical
wavelengths for certain radio frequencies are
given in Figure 1-3. Wavelength also has important
consequences for the design and use of wireless
microphone systems, particularly for antennas.
Unlike sound, radio waves do not require a physical
substance (such as air) for transmission. In fact, they
"propagate" or travel most efficiently through the vacuum of
space. However, the speed of radio waves is somewhat
slower when travelling through a medium other than
vacuum. For example, visible light travels more slowly
through glass than through air. This effect accounts for the
"refraction" or bending of light by a lens. Radio waves can
also be affected by the size and composition of objects in
their path. In particular, they can be reflected by metal objects
if the size of the object is comparable to or greater than the
wavelength of the radio wave. Large surfaces can reflect
both low frequency (long wavelength) and high frequency
(short wavelength) waves, but small surfaces can reflect
only high frequency (short) radio waves. (See Figure 1-5.)
Interestingly, a reflecting metal object can be porous,
that is, it can have holes or spaces in it. As long as the
holes are much smaller than the wavelength, the metal
surface will behave as if it were solid. This means that
screens, grids, bars, or other metal arrays can reflect radio
waves whose wavelength is greater than the space
between the array elements and less than the overall array
size. If the space between elements is larger than the
wavelength, the radio waves will pass through the array.
For example, the metal grid on the glass door of a
microwave oven reflects microwaves back into the oven
but allows light waves to pass through so that the inside is
visible. This is because microwaves have a wavelength of
at least one centimeter while visible light has a wavelength
of only one-millionth of a meter. (See Figure 1-4)
Even metal objects that are somewhat smaller than the
wavelength are able to bend or "diffract" radio waves.
Generally, the size, location, and quantity of metal in the
vicinity of radio waves will have significant effect on their
behavior. Non-metallic substances (including air) do not
reflect radio waves but are not completely transparent either.
To some degree, they generally "attenuate" or cause a loss
in the strength of radio waves that pass through them. The
amount of attenuation or loss is a function of the thickness
and composition of the material and also a function of the
radio wavelength. In practice, dense materials produce
more losses than lighter materials and long radio waves (low
frequencies) can propagate greater distances through
"lossy" materials than short radio waves (high frequencies).
The human body causes significant losses to short radio
waves passing through it.
An object that is large enough to reflect radio
waves or dense enough to attenuate them can
create a "shadow" in the path of the waves which
can greatly hamper reception of radio in the area
beyond the object.
A final parallel between sound waves and
radio waves lies in the nature of the overall radio
wave pattern or "field" produced by various
sources at a given location. If reflections are
present (which is nearly always the case
indoors), the radio field will include both direct
waves (those that travel by the shortest path
from the source to the location) and indirect waves (those
that are reflected). Radio waves, like sound waves, become
weaker as they travel away from their source, at a rate
governed by the inverse-square law: at twice the distance,
the strength is decreased by a factor of four (the square of
two). The strength of radio waves that arrive at a given
location, by direct or indirect paths, is equal to the strength
of the original source(s) minus the amount of loss due to
distance (inverse square loss), loss due to material
attenuation, and loss due to reflections.
After many reflections radio waves become weaker and
essentially non-directional. They ultimately contribute to
Selection
and Operation
of W
ireless Microphone Systems
6
C
H A P T E R
1
Basic Radio Principles