Page 49 - Complete Wireless Design

Wireless Essentials

48 Chapter One

1.6 Propagation
1.6.1 Introduction
An electromagnetic wave propagates through a vacuum at approximately
300,000 meters/second. This speed decreases as it passes through any type of
dielectric material—even air. The E (electric) field and the H (magnetic) field
of the electromagnetic wave are not only at 90 degree angles to each other
(orthogonal), but they also increase and decrease in amplitude together over
time, with one field regenerating the other as they travel through space. This
is referred to as TEM (transverse electric mode) propagation.
Radio wave propagation is conditional on the frequency of the RF carrier,
and is by three main modes:

Ground waves, which travel on top of and through the surface of the earth at
frequencies below 1 MHz
Surface waves—also called space or direct waves—which propagate through
the atmosphere in an almost straight line from the transmitter to the
receiver, and are the primary form of propagation for RF signals at 30 MHz
and above (surface waves reach to slightly more than line-of-sight (LOS)
distances because of atmospheric bending of the signal)
Sky waves, which are RF signals at less than 30 MHz that refract and reflect
off of the atmosphere’s ionosphere, and are the chief means of low- and high-
powered simplex long-range RF communications.

Even when direct line-of-site communications paths are used between the
transmitter and the receiver, natural signal losses will begin to decrease the
transmitted signal power to a level that comes closer and closer to the noise
floor of the receiver’s output. This will obviously decrease signal-to-noise ratio
(SNR), which increases the bit error rate (BER) of a digital system or increas-
es the noise level of an analog system.

1.6.2 Multipath
Multipath fading effects, especially problematic at microwave frequencies,
occur when a transmitted RF signal of interest bounces off a conductive
object—such as building pylons, light poles, or even the earth itself—and
reaches the receiver at a slightly different time than the direct RF signal. This
produces an out-of-phase reception condition, or phase cancellation, causing
fading of the received signal. The severity of microwave multipath effects
depends on antenna height, frequency, gain, and sidelobe suppression. The
fading effect is also a huge problem in HF communications, since it also cre-
ates an intermittent or continuous decrease in the received signal’s amplitude.
However, it is produced by the changing conditions of another reflective sur-
face that is not so close to the surface of the earth: the ionosphere. HF multi-
path can also be caused by multiple-path reception, such as when an HF

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