Chapter 7
Antennas
Antennas
Jim Siemons, AF6PU
1
Brief Review of a few
Chapter 6
Exam Questions
What segment of the 20-meter
band is most often used for
digital transmissions?
●
14.275 - 14.350 MHz
●
14.150 - 14.225 MHz
14.070 - 14.100 MHz
●
14.070 - 14.100 MHz
●
14.000 - 14.050 MHz
G2E04
2017 MDARC/SATERN General Class License Course
Page 6-1
2
Brief Review of a few
Chapter 6
Exam Questions
How are the two separate
frequencies of a Frequency Shift
Keyed (FSK) signal identified?
●
On and Off
●
Dot and Dash
●
Mark and Space
●
High and Low
Mark and Space
G8C11
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Page 6-5
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Brief Review of a few
Chapter 6
Exam Questions
What does the number 31
represent in "PSK31"?
• The number of characters that can
be represented by PSK31
The
approximate
• The
version of thetransmitted
PSK protocol
symbol rate
• The year in which PSK31 was
invented
• The approximate transmitted
symbol rate
G8C09 Page 6-7
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Brief Review of a few
Chapter 6
Exam Questions
What is the approximate
bandwidth of a PACTOR3 signal at
maximum data rate?
• 2300 Hz
2300 Hz
• 1800 Hz
• 500 Hz
• 31.5 Hz
G8B05
Page 6-9
Refer to Table 6.2, Page 6-10
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Chapter 7
7.1 Antenna Basics
 Elements are the conducting portions
of an antenna that radiates or receives a
signal.
 Polarization refers to the orientation of
the electric field radiated by the antenna.
 Feed Point Impedance is the ratio of RF
voltage to current at an antenna’s feed
point.
 Radiation Pattern is a graph of signal
strength in every direction or at every
vertical angle.
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Chapter 7
7.1 Antenna Basics
 An Azimuthal pattern shows signal
strength in horizontal directions.
 An Elevation pattern shows signal
strength in vertical directions.
 Lobes are regions in the radiation
pattern where the antenna is radiating a
signal.
 Nulls are the points at which radiation
is at a minimum between lobes.
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Chapter 7
7.1 Antenna Basics
 An Isotropic Antenna radiates equally
in every possible direction.
 An Omnidirectional Antenna radiates a
signal of equal strength in every
horizontal direction.
 A Directional Antenna radiates
preferentially in one or more directions.
 Gain is the concentration of a signal
transmitted or received from a specific
direction.
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
A basic Dipole antenna consists of
two symmetrical linear halves
(Radiators) and a Feed Line.
Basic Dipole
Antenna
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
This drawing of an All Band HF Dipole
Antenna may look complicated, but like the
basic dipole, it contains two radiators and a
feed line.
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Radiation Pattern in the plane
of a dipole located in space
Based on Figure
7.1, Page 7-2
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
 The half-wave dipole has its maximum current
in the middle and maximum voltage at each
end
 The wire length for a ½ wave HF dipole
antenna is computed as follows:
Length in feet =
492
frequency in MHz
For a 20 meter antenna (14.250 MHz) the
antenna would be:
L = 492 = 34.53 feet
14.250
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Typical Dipole Antenna Configurations
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
For best performance, a dipole should be
mounted at least a half wavelength above
the ground.
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Ground Planes (Verticals)
Good choice when you do not have
room for a dipole or beam.
 Vertical polarization.
 Used extensively for mobile
operations (whip).
 Omni-directional pattern.
 ¼ wavelength long (some ½
wavelengths are available).
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Ground Planes (Verticals)
For a vertical to work effectively, it needs artificial
ground wires since the ground acts as the other half
of the antenna.
These wires are called radials. Lots of radials are
sometimes needed.
Radials should be placed on the surface of the
ground or buried a few inches below the ground.
Length of λ/4 ground plane is:
λ (Lambda) is
commonly used
for wavelength)
246
Length ft 
f MHz
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Example of a Ground Plane Antenna
Based on
Figure 7.3,
Page 7-4
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Random Wires
 This antenna is what the name
suggests.
 Random Wires are multiband antennas.
 The Random Wire antenna is connected
directly to the output of the transmitter.
 The radiation pattern is often
unpredictable.
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Example of a Long Wire Antenna
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Effects of Height above Ground
 Feed point impedance is affected
because the electrical image is
electrically reversed.
 The radiation pattern is affected by the
antenna’s height above ground because
of the antenna’s radiated energy from the
ground.
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
The elevation of the dipole above the
ground affects its electrical ground
image and causes it to flatten out.
Based on
Figure 7.6,
Page 7-6
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Different radiation patterns at
different heights
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Chapter 7
7.2 Dipoles, Ground-planes and
Random Wires
Effects of Polarization
Polarization affects the amount of signal that
is lost from the resistance of the ground.


Radio waves reflecting from the ground have
lower losses when the polarization of the wave
is parallel to the ground.

Ground mounted vertical antennas are able to
generate stronger signals at low angles of
radiation.

They are preferred for DX contacts at lower HF
bands.
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Chapter 7
7.3 Yagi Antennas
o Multi-element (at least two).
o Can be for one band (monobander) or
multiple bands (tribander).
o Produces gain over a dipole in a specific
direction.
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Chapter 7
7.3 Yagi Antennas
o Whether a driven array or a parasitic
array, it's radiation pattern is determined
by constructive and destructive
interference.
o When two waves interfere with each
other, they can reinforce each other if they
are in phase and can cancel each other if
they are out of phase.
o Radiated fields from two different
antennas may add and/or subtract at
different angles around the antennas so
that lobes and nulls are formed.
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Chapter 7
7.3 Yagi Antennas
For two antennas 1 wavelength (λ) apart (seen on-end)
and fed identical, in-phase signals, the radiated signals
add and cancel at different angles around the antenna.
This creates the lobes and nulls of the radiation pattern
seen at right.
Based on
Figure 7.7,
Page 7-8
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Chapter 7
7.3 Yagi Antennas
Yagi Gain Antenna
Radiation pattern is directional
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Chapter 7
7.3 Yagi Antennas
A simple illustration of how a Yagi
Antenna works:
1) The original signal from the Driven
Element (DE) travels to the reflector
where it causes current to flow, reradiating a signal.
2) Re-radiated signals are 180° out of
phase with the original signal, so that reradiated and (DE) Signals cancel in the
direction of the reflector (to the back of
the antenna).
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Chapter 7
7.3 Yagi Antennas
3) To the front of the antenna, the extra
travel time for the radiated signal
from the reflector causes it to
reinforce the (DE) Signal.
4) A director element, placed in front of
the (DE) increases forward gain.
5) Additional reflectors make little
difference in either gain or front to
back ratio.
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Chapter 7
7.3 Yagi Antennas
The Yagi is a
parasitic
antenna
array with a
single driven
element and
at least one
parasitic
element.
Based on Figure
7.8, Page 7-9
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Chapter 7
7.3 Yagi Antennas
The primary variables for Yagi antennas are
the length and number of each element and
their placement along the boom of the antenna.
These variables affect Gain, SWR, and front-toback ratio in differing ways.
 More directors increase gain.
 A longer boom with a fixed number of directors
increases gain up to a maximum length beyond
which gain is reduced.
Larger diameter elements reduce SWR variation
with frequency (increases SWR bandwidth).
Placement and tuning of elements affects gain
and feed point impedance (and SWR).
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Chapter 7
7.3 Yagi Antennas
 The process of modifying a design for a certain
level of performance is called optimizing.
 Most Yagi designs that have desirable
radiation patterns may also have impedance
problems.
 The most common technique to change the
feed point impedance to 50 Ω is the gamma
match.
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Chapter 7
7.3 Yagi Antennas
The transmission
line transforms the
low impedance of
the feed point to a
higher value using
either an adjustable
capacitor or a short
piece of insulated
wire inside of the
hollow gamma rod.
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Chapter 7
7.3 Yagi Antennas
 Other techniques to change the feed
point impedance include
–Beta Match
–The Omega Match
–Impedance transformers
–Transmission line stubs
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Chapter 7
7.4 Loop Antennas
Loop Antennas can be circular,
square, or various other shapes.
Horizontally oriented loop antennas result
in most of their signal going straight up
making them good antennas for local and
regional contacts.
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Chapter 7
7.4 Loop Antennas
Vertical Loop Antenna
Polarization of a vertical loop antenna
depends on the location of the feed point.
Based on Figure
7-11, Page 7-13
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Chapter 7
7.5 Specialized Antennas
 Near Vertical Incidence Sky-wave
(NVIS)
 Stacked Antenna
 Log Periodic
 Beverage Antenna
 Multiband
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Chapter 7
7.5 Specialized Antennas
Near Vertical Incidence Sky-wave
(NVIS)
A military NVIS antenna is the AS-2259 Antenna, which
consists of two V-shaped dipoles: the four dipole wires
also serve as guy rope for the antenna mast.
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Chapter 7
7.5 Specialized Antennas
Stacked Antenna
BWG = Beam Waveguide antenna
Stacked set of 2 meter 13 element BWG Antennas
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Chapter 7
7.5 Specialized Antennas
Log Periodic Antenna
Log Periodic Antenna, 250–2400 MHz
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Chapter 7
7.5 Specialized Antennas
Beverage Antenna
KW2P Coax Beverage Antenna
with End Feed
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Chapter 7
7.5 Specialized Antennas
Multi-Band Mobile Antenna
Diamond HV7A Multi-Band Mobile Antenna
Magnetic Mount
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Chapter 7
7.6 Feed Lines
 All feed lines have two conductors.
 Feed lines have different characteristic
impedances (Z0) that characterize how
electromagnetic energy is carried by the
feed line.
 The common characteristic impedance for
coaxial lines is 50 Ω in radio applications
and 75 Ω in video applications.
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Chapter 7
7.6 Feed Lines
Common Feed Lines
2017 MDARC/SATERN General Class License Course
Based on Figure
7.16, Page 7-17
44
Chapter 7
7.6 Feed Lines
 Forward Power, Reflected Power and
Standing Wave Ratio (SWR).
o Power traveling toward the antenna is
known as forward power.
o Power reflected by the antenna is known
as reflected power.
o The Standing Wave Ratio (SWR) is the
peak voltage in the standing wave
compared to the minimum voltage in the
standing wave.
o A perfectly matched antenna and feed line
have an SWR of 1:1 a perfect match.
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Chapter 7
7.6 Feed Lines
 Impedance Matching
o Matching feed line and load (antenna)
impedances eliminates standing waves from
reflected power and maximizes power
delivered to the load.
o A device to minimize SWR at the transmitter
connection to the feed line is called an
impedance matcher or an antenna tuner.
o The most common circuit configuration is the T
network.
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Chapter 7
7.6 Feed Lines
 Feed Line Loss
o All feed lines dissipate a little of the energy
they carry as heat -- this is a attenuation or
loss.
o Feed line losses are measured in dB/100 ft.
o Increasing the SWR in a feed line also
increases the total loss in the line.
o The higher the feed line loss, the lower the
measured SWR will be at the input to the line.
o The higher the frequency, the greater the feed
line loss.
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Chapter 7
7.6 Feed Lines
 Feed Line Characteristics
Based on Table
7.1 Page 7-19
28.4 MHz is in the 10 meter band and 144 MHz is in the 2
meter band. 2017 MDARC/SATERN General Class License Course
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Next Class
Session
Assignment
In preparation for the next class
session, do the following…….
Study Chapter 8
“Propagation”
Study the Question Pool questions
found in the ”blue boxes.”
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Please follow the instructions from the
Elmers for this room set up.