EM waves can pass through some objects, and can also be reflected off of objects.
In many cases, part of the signal’s energy attempts to penetrate the object, while
the rest of the energy of the signal is reflected. (Imagine looking into a pool of
water—some of the light passes through the water and is reflected off the bottom
of the pool, allowing you to see the bottom. At the same time, if you readjust
your focus you can see a reflection of yourself.This means that some of the light
is penetrating and some of the light is reflecting.) Reflecting a signal is sometimes
referred to as bouncing a signal and/or scattering a signal.
Bouncing can degrade the performance of some systems and enhance the
performance of others. Both technology and physical conditions play a factor in
whether a specific application makes use of or is hindered by bouncing. For
example,AM broadcast radio signals can be bounced off of the upper layers of
the earth’s atmosphere. Figure 2.11 illustrates how this can extend the distance a
signal can be transmitted to well beyond the horizon. Many applications that use low frequency waves can use the layers of the
atmosphere as a passive reflector, thus enhancing their distance performance;
however, higher frequency waves do not bounce off the atmospheric layers well.
High frequency waves penetrate through the atmospheric layers and into space
without reflecting.This makes them well suited for communications with satellites.
Satellites can be used as active reflectors by receiving and then retransmitting
the signal to broadcast signals beyond the horizon; multiple satellites can be
linked together to relay a signal completely around the world.
A specific example of using bouncing to enhance propagation is a fixed wireless
telephone link this author saw in the mountains of Colorado. A house had
been built up in a canyon, well beyond where the phone lines ended, but the
residents wanted to have access to a phone.With a little monetary encouragement,
the phone company set up a fixed wireless link to the house. However,
one of the problems with choosing a wireless solution was that, obviously, the
signals could not penetrate through the mountainous walls of the canyon, and
the canyon was L-shaped with the house situated around the corner from where
the last telephone access pedestal was.The fix was creative, simple, and inexpensive,
albeit a little crude: a transceiver (a transmitter/receiver combination) and a
directional antenna were placed at the mouth of the canyon near the last telephone
access pedestal.The antenna was pointed up the canyon, aimed at a large
granite rock on the far side of the L-corner of the canyon. A similar antenna was placed at the house and pointed at the same rock face.The rock actually became
a reflector to bounce the signal around the corner of the canyon.
Not all results of bouncing are positive, however. One prevalent type of
bouncing that adversely affects most mobile communications is called multipath
scattering. Multipath scattering is where a signal reaches a receiver from multiple
paths due to part of the signal bouncing off of various objects. If these signals
arrive at the receiver out of phase, they can cancel each other. If the signals arrive
in phase but are not synchronized, you can get echo signals. Echoes are probably
most apparent on weak broadcast television signals where you see a main picture
with a fuzzy picture just off to one side of every object in the picture. Figure
2.12 illustrates how multipath scattering occurs and demonstrates how it can
cancel the signal. One technology that makes use of multipath is Code Division Multiple
Accessing (CDMA) such as Sprint’s PCS phones. CDMA uses a device called a
rake receiver to receive multiple signals and then to align them in phase so that
they all amplify each other.