Wireless Communications

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Wireless Communications

• Also Known as Radio
• Part II Antennas

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A radio transmitter is an electronic device which
converts sound or data into waves of electrical
current at the transmitters operating frequency.
These current waves are converted to
electromagnetic waves (radio waves) by a device
called an antenna. A radio receiver also has an
antenna, which converts electromagnetic waves to
current waves. The receiver then converts
current waves which occur at the frequency the
receiver is tuned to into their original form
sound, data, etc.
Radio Frequency Current Waves
Antenna
Radio Waves
Microphone
Transmitter
Audio Frequency Voltage Waves
Antenna
Receiver
Audio Frequency Voltage Waves
Radio Frequency Current Waves
Speaker
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Lets start by examining how the transmitters
antenna converts electrical current waves to
radio (electromagnetic) waves. Consider a
length of wire which has an electrical current
flowing through it, as shown below. The current,
which is really many tiny electrical charges (the
free electrons in the wire) moving from right to
left), creates an electrical field surrounding
the wire.
Electromagnetic Field Surrounding wire
I (current) flowing through wire
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Viewed from the end of the wire, the field would
look like this (with current flowing away from
you)
Electromagnetic Field
Wire, with current flowing away
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Reversing the direction of charge motion (that
is, reversing the current) also reverses the
direction of the magnetic field.
Electromagnetic Field
Wire, with current toward you
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If the current reverses direction periodically,
with a frequency of, say, 14.06 MHz, the
electromagnetic field surrounding the wire also
reverses itself twice per cycle at that same
frequency.
Current
Period (T)
positive
time (t)
negative
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As the current is reversing at a frequency of
14.06 MHz, the field is also traveling away from
the wire at 300,000 kilometers per second.
Suppose we start at a current wave crest, shown
below
Electromagnetic Field
Wire, with current flowing away from you
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One half-period later (35.5 nanoseconds later,
the electromagnetic field crest has moved 10.67
meters, or ½ wavelength. Also, the current
waveform is at a trough, producing an
electromagnetic wave trough just leaving the
surface of the wire.
Electromagnetic Field Trough
10.67 meters
Electromagnetic Field Crest
Wire, with current toward you
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After another half-period, the crest has moved an
additional half-wavelength from the wire, while
the trough is now one half-wavelength from the
wire. At the same time, the current has reversed
again, resulting in a second crest just leaving
the wire.
Electromagnetic Field Trough
Electromagnetic Field Crest
Wire, with current away from you
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After yet another half-period, another trough is
just leaving the wire. The previous crests and
trough have all moved ½ wavelength further.
Electromagnetic Field Crests
Electromagnetic Field Troughs
Wire, with current away from you
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In this way, the wire (actually, a simple
antenna) which has a current waveform at the
frequency of the radio signal flowing in it
creates a series of alternating crests and
troughs, all moving outward at the speed of
light. The antenna has converted a current
waveform to electromagnetic waves.
This explanation has omitted a few details for
the sake of brevity. For example, the
electromagnetic field has an electrical field
component (the E-field) as well as the magnetic
field component (the H-field) weve seen here.
The two are always oriented at right angles to
each other and to the direction of travel of the
field. If the E-field of an antenna points up and
down, the antenna is vertically polarized. If the
E-field is oriented horizontally, the antenna is
horizontally polarized.
Transmitting Antenna
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The simplest type of antenna is the half-wave
dipole, shown below. It consists of a piece of
wire ½ wavelength long, divided into two ¼
wavelength portions. Each ¼ wavelength portion
is connected to one of the conductors of a
two-conductor transmission line. The other end
of the transmission line is connected to the
transmitter. Current flows in opposite directions
in the two conductors of the transmission line,
but this results in current flowing in the same
direction in the two halves of the antenna.
l/4
l/4
Current
Current
Two-conductor Transmission line From transmitter.
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One half-cycle later, each of the currents is
reversed. Because either conductor of the
transmission carries a current which is always
equal in magnitude and opposite in direction to
the current in the other conductor, this
transmission line is said to be balanced.
Antenna Conductors
l/4
l/4
Current
Current
Two-conductor Transmission line From transmitter.
Transmission Line
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The current in the left-hand antenna conductor
flows away from the feedpoint, while the current
in the right hand conductor flows toward the
feedpoint. These two currents are always equal
and opposite, so the dipole antenna is also said
to be balanced. Driving a balanced transmission
line and antenna means the transmitters output
port should also be balanced.
Antenna Conductors
l/4
l/4
Current
Current
Two-conductor Transmission line From transmitter.
Transmission Line
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Unfortunately, most transmitters have unbalanced
output ports. One of the two conductors
(normally the outer shield conductor of a coaxial
cable) is normally connected to ground. This
means the transmission line currents are not
balanced, so proper operation of a dipole antenna
requires a balun (short for balanced to
unbalance) transformer at the feedpoint. Many
amateur radio operators use dipole antennas fed
with unbalanced transmission lines, with no
balun. This is not the best technique, but does
often give reasonably good results.
Antenna Conductors
l/4
l/4
Current
Current
Balun
Transmission Line
Outer shield to transmitter and ground
Center conductor to transmitter
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A dipole antenna may be constructed so the
antenna conductor is oriented horizontally, as
shown here, or vertically. If it is oriented
horizontally, it is said to be horizontally
polarized.
A dipole antenna is somewhat directional. The
largest portion of the energy it radiates is
concentrated in the directions approximately
perpendicular to the antenna conductor, with
somewhat less energy being radiated off the ends.

Antenna Conductors
16.5 feet
16.5 feet
Current
Current
Balun
Transmission Line
Outer shield to transmitter and ground
Center conductor to transmitter
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If a dipole is oriented vertically, it still
radiates the larger portion of the transmitters
energy perpendicular to itself. For a vertically
polarized dipole, this means that the energy is
concentrated in the horizontal plane, but is
equally distributed in all directions within that
plane. For most practical purposes, a vertical
dipole may be thought of as nondirectional,
because its radiation pattern in the two
horizontal dimensions is circular (equal in all
directions)
A truly nondirectional antenna, with a spherical
radiation pattern in three dimensions, equal in
all directions, is said to be isotropic.
Isotropic antennas exist only as theoretical
idealizations, and cannot actually be built.
Outer shield to transmitter and ground
Center conductor to transmitter
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Something interesting happens if we take the
upper half of a vertical dipole and mount it
above a ground plane of conductive material
(i.e., metal) extending at least ¼ wavelength in
all directions. The ground plane acts like a
mirror, creating a virtual quarter-wave conductor
below the ground plane. This is called a
monopole antenna, and behaves much like a
vertical dipole The monopole is an unbalanced
antenna, so no balun is needed. The coaxial
shield is simply connected to the ground plane.
Antenna Current
Real ¼ wavelength
Ground plane
Virtual ¼ wavelength
Outer shield to transmitter and ground
Center conductor to transmitter
Center conductor to transmitter
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The ground plane can actually be earth ground, if
the soil conductivity is high enough. This is
rare. The ground plane could be a conductive
sheet, but in the HF range this is usually
impractical because of the size. At VHF and
above, it is common to see a monople antenna
mounted on the roof or rear deck of a car, with
the cars body serving as the ground plane.
Real ¼ wavelength
Ground plane
Virtual ¼ wavelength
Outer shield to transmitter and ground
Center conductor to transmitter
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A screen of wire mesh can also be used as a
ground plane, but in the HF range its common to
use three or more (but some times as few as one)
¼ wavelength wires extending outward horizontally
from the base of the antenna. These are referred
to as radials, or a counterpoise.
Real ¼ wavelength
Ground plane
Counterpoise of ¼-l radials extending outward
from base and connected to coaxial shield
Virtual ¼ wavelength
Outer shield to transmitter and ground
Center conductor to transmitter
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This explains the transmitting antenna. The
receiving antenna operates on the principle of
electromagnetic induction, which says that
whenever an electrical conductor intersects a
changing electromagnetic field, a potential
difference (that is, a voltage) is induced
between the two ends of the conductor. In this
case, the changing field is the radio signal, and
the antenna is the conductor. The transmission
line carries the voltage induced in the antenna
to the receiver where it is amplified, and change
back into sound or data.