Transcript Slide 1

Antennas
Once you get your license and
that first radio, you’re going
to need a good antenna to get
your signal on the air.
Antennas need not be expensive,
but you do need to take some
care in selecting the right
antenna for the job.
Release 1.0 – September 2006
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Beam Antennas
A “beam antenna” is an antenna that concentrates
signals in one direction. It is designed to focus all
of the energy produced by your transmitter in the
direction you want to work. Focusing your signal power
in one direction makes for a stronger signal in that
direction. Beams are effective, but depending on the
bands covered and type, they can be expensive.
Release 1.0 – September 2006
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Beam Antennas - Quad
The quad, Yagi, and dish are all examples of beam
antennas. A quad antenna looks something like a metal
frame for a box kite. If you look closely, you can see
the antenna wires supported by the “X” framework.
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Beam Antennas - Yagi
The yagi is a one dimensional beam
antenna consisting of several elements.
It may be mounted horizontally, as shown
here, or vertically.
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Beam Antennas - Dish
Another beam antenna is the dish or
parabolic reflector. It is often used to
receive UHF signals or TV signals beamed
from satellites, such as Dish Network ®
antennas.
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Vertical Antennas
A “vertical antenna” is an antenna that
consists of a single element mounted perpendicular
to the Earth's surface. Most mobile antennas are
verticals.
Verticals usually require some sort of
counterpoise to work their best. In a fixed
station, a vertical may either be mounted on the
ground or on a mast, and it may also have several
radials for counterpoise. These radials may be
laid out on the ground, as in the next slide, or
mounted just underneath the vertical element, as
in an elevated ground plane.
In a mobile installation, the metal body of
the car usually serves as the counterpoise.
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Typical Ground-Mounted
Vertical
This is a rough diagram of a ground-mounted
vertical. The orange radials you see may be laid along
the top of the ground or buried just beneath the
surface. Multi-band verticals are sometimes a good
compromise between price and performance for a new ham.
Release 1.0 – September 2006
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Ground Plane Antenna
Here is a ground plane antenna – another type
of vertical. It is designed to be mounted on a
mast, and it usually has three or four radials
coming from the base of the antenna.
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Vertical Antenna Design
Many vertical antennas are designed to
be equal in length to one-quarter
wavelength of the desired operating
frequency.
For a 1/4 wave vertical:
234
Length (feet) = ----------------Frequency (MHz)
You’ll need to remember this formula!
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Sample Problem from the
Question Pool
For example, suppose you want to know the
approximate length, in inches, of a quarterwavelength vertical antenna for 146 MHz.
Using the formula in the previous slide:
234
Length = ------ = 1.6 feet
146
To get inches, multiply 1.6 times 12 (since
there are 12 inches in a foot) to get 19.2 or
about 19 inches.
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5/8 Wave Verticals
Some vertical designs call for a
5/8 wave rather than a 1/4 wave.
The advantage of 5/8 wavelength over
1/4 wavelength vertical antennas is
that their radiation pattern
concentrates energy at lower angles.
(Radiation pattern describes the
shape of the radiated signal, and a
lower radiation angle usually means
traveling a greater distance –
better DX!)
Release 1.0 – September 2006
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Magnet Mount Verticals
(Mag Mounts)
As already noted, verticals are
very common in mobile installations.
One type of antenna that offers good
efficiency when operating mobile and
can be easily installed or removed
is the magnet mount vertical
antenna. They are also fairly
inexpensive and don’t require you to
drill a hole in that new vehicle!
Release 1.0 – September 2006
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Horizontal Antennas
A “horizontal antenna” is an
antenna that is a simple dipole
mounted so the elements are
parallel to the Earth's
surface. So what’s a dipole?
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Dipole Antennas
A dipole antenna is a simple
antenna designed to work best on a
single band. It consists of two
sections that are each approximately
one-quarter of the wavelength of
that band, so that the total length
is equal to about one-half
wavelength. The transmission line
from the radio is connected to this
antenna in the middle of the two
sections. It looks something like
what you’ll see on the next slide.
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Dipole Antenna
This is an example of a dipole antenna. Many hams
getting on HF for the first time often start with a
dipole. If you have the room for one, the dipole is
cheap and easy to build.
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More on Dipoles
Dipoles may be mounted either
horizontally or vertically,
depending on the intended use.
Also, they may be made from wire or
metal tubing, and are very easy for
a new ham to construct. Wire
dipoles are also fairly inexpensive
and simple to design. With an
antenna tuner, they can also be made
to work on several bands. For these
reasons, they are very popular with
new hams on the HF bands.
Release 1.0 – September 2006
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Dipole Design
Since dipoles are fairly easy to build, it is
important to know how to determine their total
length. The formula for the length of a 1/2 wave
dipole is:
468
Length (feet) = ----------------Frequency (MHz)
When designing a dipole, you should choose the
lowest frequency for the band you want to work!
You will need to remember this formula!
Release 1.0 – September 2006
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Sample Problem from the
Question Pool
What is the approximate length, in
inches, of a 6-meter 1/2 wavelength wire
dipole antenna?
Now this one can be done two ways.
First, you are given the band wavelength
already – 6 meters. This is approximate
for the band, but it will do. Half of
that wavelength is 3 meters. Since there
are about 39 inches in a meter, that gives
you an answer of 117 inches. The only
answer close to that in the question pool
question is 112 inches, so that’s the one
you go with. But there is another way...
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Using the Formula
To use the formula, you have to pick a
frequency. In the U.S., the six meter band is 50
to 54 MHz. The lowest frequency is 50 MHz, so
that’s the one you use. When you apply the
formula,
468
Length (feet) = ----- = 9.36 feet
50
When you multiply 9.36 times 12 (to get
inches), you get 112.32, which is a lot closer to
112. But remember, you use the lowest frequency
in the band as your design frequency, or at least
the lowest frequency you want to work.
Release 1.0 – September 2006
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Dipoles – Frequency Goes
Up, Length Goes Down
The physical size of half-wave
dipole antenna changes with
operating frequency. It becomes
shorter as the frequency increases.
As you might imagine, the
opposite is also true. It becomes
longer as the frequency decreases.
Release 1.0 – September 2006
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Rubber Ducky – Not for the
Bathtub
For years, the little antenna that
comes with most handheld radios has been
called a “rubber duck” or “rubber ducky.”
The main advantage of a rubber duck is its
size. However, when range is important,
the main disadvantage of the "rubber duck"
antenna supplied with most hand held radio
transceivers is that it simply does not
transmit or receive as effectively as a
full sized antenna.
If you need to increase the range of
your handheld, you’ll have to connect it
to a better antenna than the ducky!
Release 1.0 – September 2006
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Rubber Ducky – Not for the
Car, either!
You really don’t want to try
to use your "rubber duck"
antenna inside your car because
signals can be 10 to 20 times
weaker than when you are
outside of the vehicle. Much
of the RF energy leaving your
antenna will be absorbed by the
metal in your car.
Release 1.0 – September 2006
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Dummy Load – Dummies Don’t
Use Them!
You may remember that a dummy load is used when you
are testing a transmitter. It absorbs the RF energy
generated by your antenna so that the signals don’t
interfere with stations operating on the frequency. The
primary purpose of a dummy load is to avoid radiating
interfering signals when making tests.
Release 1.0 – September 2006
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Check-Up Time!
Now let’s try the questions from
this group.
You should make a note of any that
you miss for later review.
Release 1.0 – September 2006
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T9A01
What is a beam antenna?
A. An antenna built from metal Ibeams
B. An antenna that transmits and
receives equally well in all
directions
C. An antenna that concentrates
signals in one direction
D. An antenna that reverses the phase
of received signals
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T9A01 Answer - C
A beam antenna radiates best
in only one direction,
concentrating the signal power
in that direction. HINT: Think
flashlight beam or laser beam.
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T9A02
What is an antenna that
consists of a single element
mounted perpendicular to the
Earth's surface?
A.
B.
C.
D.
A
A
A
A
conical monopole
horizontal antenna
vertical antenna
traveling wave antenna
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T9A02 Answer - C
Math whizzes will immediately
recognize that perpendicular to
the earth is vertical. The
rest of us have to learn it
now!
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T9A03
What type of antenna is a
simple dipole mounted so the
elements are parallel to the
Earth's surface?
A.
B.
C.
D.
A
A
A
A
ground wave antenna
horizontal antenna
rhombic antenna
vertical antenna
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T9A03 Answer - B
The math whizzes get another
break. If the elements are
parallel to the earth's
surface, they must be
horizontal.
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T9A04
What is a disadvantage of the "rubber
duck" antenna supplied with most hand held
radio transceivers?
A. It does not transmit or receive as
effectively as a full sized antenna
B. It is much more expensive than a standard
antenna
C. If the rubber end cap is lost it will
unravel very quickly
D. It transmits a circular polarized signal
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T9A04 Answer - A
The rubber duck antenna used
on most handhelds is a
compromise antenna. It is OK
for casual operating, but there
are times when it just isn't
good enough. If you need the
range, you are better off
disconnecting the rubber duck
and connecting a full size
antenna.
Release 1.0 – September 2006
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T9A05
How does the physical size of half-wave
dipole antenna change with operating
frequency?
A. It becomes
increases
B. It must be
handle more
C. It becomes
increases
D. It becomes
decreases
Release 1.0 – September 2006
longer as the frequency
made larger because it has to
power
shorter as the frequency
shorter as the frequency
33
T9A05 Answer - C
The length of the dipole is
inversely related to frequency.
As the frequency increases, the
dipole length decreases.
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T9A06
What is the advantage of 5/8
wavelength over 1/4 wavelength
vertical antennas?
A. They are easier to match to the
feed line than other types
B. Their radiation pattern
concentrates energy at lower angles
C. They pick up less noise
D. Their radiation pattern
concentrates energy at higher angles
Release 1.0 – September 2006
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T9A06 Answer - B
The 5/8 wavelength antenna
(or simply, "5/8 wave") has the
advantage of a lower angle
radiation pattern. Radiation
at a lower angle is better,
because it sends the signal
closer to the ground, rather
than up into the sky where it
is not as likely to be heard.
Release 1.0 – September 2006
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T9A07
What is the primary purpose of a
dummy load?
A. It does not radiate interfering
signals when making tests
B. It will prevent over-modulation of
your transmitter
C. It keeps you from making mistakes
while on the air
D. It is used for close in work to
prevent overloads
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T9A07 Answer - A
A dummy load is hooked up in
the place of an antenna to
allow an operator to test or
tune a transmitter without
radiating a signal that could
interfere with other stations.
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T9A08
What type of antennas are the
quad, Yagi, and dish?
A. Antennas invented after 1985
B. Loop antennas
C. Directional or beam antennas
D. Antennas that are not
permitted for amateur radio
stations
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T9A08 Answer - C
Each of these antennas - the
quad, the yagi and the dish are directional antennas. They
are designed to radiate or
receive signals best in one
direction only.
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T9A09
What is one type of antenna that
offers good efficiency when
operating mobile and can be easily
installed or removed?
A.
B.
C.
D.
A
A
A
A
microwave antenna
quad antenna
traveling wave antenna
magnet mount vertical antenna
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T9A09 Answer - D
A magnet mount vertical
antenna (also known as a "mag
mount") uses a magnet at its
base to hold it to the metal
roof or trunk of a car. It can
easily be removed for storage
or use on another vehicle.
Release 1.0 – September 2006
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T9A10
What is a good reason not to use a "rubber
duck" antenna inside your car?
A. Signals can be 10 to 20 times weaker than
when you are outside of the vehicle
B. RF energy trapped inside the vehicle can
distort your signal
C. You might cause a fire in the vehicle
upholstery
D. The SWR might increase
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T9A10 Answer - A
Remember that a rubber duck
antenna is a compromise antenna. It
is small so that it can be portable,
but it is not as efficient as a full
size antenna. Also, when you are
inside a car using the rubber duck,
much of the signal is absorbed by
the metal that surrounds you. It is
much better to get a full size
mobile antenna outside the vehicle
where it will do the most good.
Release 1.0 – September 2006
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T9A11
What is the approximate length,
in inches, of a quarterwavelength vertical antenna for
146 MHz?
A.
B.
C.
D.
112 inches
50 inches
19 inches
12 inches
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T9A11 Answer - C
The formula for figuring the
length of a quarter wave vertical
is:
234
Length (feet) = ---------------Frequency in MHz
So...
Release 1.0 – September 2006
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T9A11 Answer - C
234
Length = --- = 1.625 feet
144
To convert that to inches,
multiply 1.625 times 12 (there are
12 inches in a foot) to give you
19.5 inches. That is very close to
the best answer of 19 inches.
Release 1.0 – September 2006
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T9A12
What is the approximate
length, in inches, of a 6-meter
1/2 wavelength wire dipole
antenna?
A.
B.
C.
D.
6 inches
50 inches
112 inches
236 inches
Release 1.0 – September 2006
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T9A12 Answer - C
Here, the wavelength is 6 meters.
A half wavelength is 3 meters.
Since there are approximately 39
inches in a meter, 3 times 39 will
give you 117, which is close to the
best answer.
There is another way that will
get you a little closer...
Release 1.0 – September 2006
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T9A12 Answer - C
The formula for determining the length of a half wave dipole in
feet is:
468
Length (feet) = ---------------Frequency in MHz
To use this formula, you have to know that the lowest frequency of
the six meter band is 50 Mhz.
Then...
468
Length = --- =
50
9.36 feet
To convert to inches, multiply 9.2 times 12 to get 112.32 inches.
Release 1.0 – September 2006
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Group T9B
Group T9B covers propagation, fading,
multipath distortion, reflections, radio
horizon, terrain blocking, wavelength
vs. penetration, and antenna
orientation.
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Propagation of Radio Waves
Radio waves at different
frequencies travel through space
very differently. How they travel
determines how far away you can
communicate. The study of
“propagation,” or how radio waves
move through space, can be a very
complicated subject, but every
amateur needs to know some of the
basics. Fortunately, the basics are
not hard.
Release 1.0 – September 2006
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The Ionosphere
There are
several layers of
charged particles
surrounding the
earth. Together,
these layers form
the “ionosphere.”
The layers of the
ionosphere are
constantly
changing. They
change as the
seasons change,
as well as during
the course of a
day.
Release 1.0 – September 2006
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The Ionosphere
The ionosphere is very important
for radio communications because it
has the ability to reflect some
radio waves back to earth. This
allows radio to be used to
communicate for long distances.
Whether or how much a radio wave is
reflected depends on the frequency
of the radio wave as well as the
condition of the ionosphere at any
given time.
Release 1.0 – September 2006
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Radio Horizon
To a certain extent, all radio
waves travel away from an antenna
along the ground. But because the
earth is curved and radio waves
travel in straight lines, sooner or
later the radio waves will be too
high above the surface to be picked
up by a radio receiver. The radio
horizon is the point where radio
signals between two points are
blocked by the curvature of the
Earth.
Release 1.0 – September 2006
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Radio Horizon and Visual
Horizon
We usually think of the horizon as the
farthest point of land or ocean where it
appears to meet the sky. Standing on a
beach and looking out on the ocean, this
may be about thirteen miles. We usually
call this “line of sight.” If we climb up
a tower (where antennas live) that line of
sight distance can be much greater. But
radio waves traveling along the ground
from that same tower can actually travel
beyond the visual line of sight. The
distance they can travel is to the radio
horizon, rather than the visual horizon.
How far that distance is depends on the
frequency of the radio wave.
Release 1.0 – September 2006
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Radio Horizon of VHF and
UHF Waves
VHF and UHF Radio signals
usually travel about a third
farther than the visual line of
sight distance between 2
stations because the Earth
seems less curved to radio
waves than to light. Another
way of saying this is that
radio waves are bent slightly
as they move along the surface.
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VHF and UHF Signals – Line of
Sight to the Radio Horizon
Only!
VHF and UHF signals are not
normally heard over long
distances because they are
usually not reflected by the
ionosphere.
Release 1.0 – September 2006
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VHF DX
While long distance VHF contacts
are fairly rare, they can occur due
to a phenomenon known as “sporadic
E.” On rare occasions, VHF waves
may be reflected off the “E” layer
of the ionosphere. So when you hear
a VHF signal from long distances,
one possible cause is sporadic E
reflection from a layer in the
ionosphere. (Note the “E” layer in
the diagram a few slides back.)
Release 1.0 – September 2006
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Receiver Overload
Sometimes you might be
listening to VHF or UHF and
hear sudden bursts of tones or
fragments of different
conversations that interfere
with these signals. If so, one
likely cause is that strong
signals are overloading the
receiver and causing undesired
signals to be heard.
Release 1.0 – September 2006
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Signal Fluctuations
Sometimes you may be talking on a
mobile or portable radio and the other
station may report a change in your signal
quality. Suppose a station reports that
your signals were strong just a moment
ago, but now they are weak or distorted.
You might want to try moving a few feet,
since random reflections may be causing
multi-path distortion. This can easily
happen near large buildings, under
bridges, or close to large metal
structures.
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UHF and VHF Signals Inside
Buildings
UHF signals often work better
inside of buildings than VHF
signals because the shorter
wavelength of UHF signals
allows them to more easily
penetrate urban areas and
buildings. Cell phones and
many cordless phones operate in
the UHF region, in part because
of this difference.
Release 1.0 – September 2006
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Vertical Polarization
Most repeater antennas are
verticals, so their signals are
vertically polarized. As a result,
if you are using your hand-held VHF
or UHF radio to reach a distant
repeater, one good thing to remember
is to keep your antenna as close to
vertical as you can. This will keep
the polarization of your signal
vertical as well, increasing your
ability to make it into the
repeater.
Release 1.0 – September 2006
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More on Polarization
Polarization of your antenna can
make a significant difference.
Signals are received much more
efficiently if both radios are
either vertical or horizontal
If
the antennas at opposite ends of a
VHF or UHF line of sight radio link
are not using the same polarization
(one vertical and the other
horizontal) signals could be as much
as 100 times weaker!
Release 1.0 – September 2006
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Obstructions on UHF and
VHF
Since VHF and UHF communications are
mostly line of sight to the radio horizon,
buildings or other large objects can
sometimes block this line of sight. If
this happens, you want to relocate if
possible. If you can’t move, another
possible way to reach a distant repeater
if buildings or obstructions are blocking
the direct line of sight path might be to
try using a directional antenna to find a
path that reflects signals to the
repeater. This way, you might be able to
use large obstructions to your advantage.
Release 1.0 – September 2006
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Picket Fencing
“Picket fencing” is a term
commonly used to describe the rapid
fluttering sound sometimes heard
from mobile stations that are moving
while transmitting. It sounds a
little like a kid dragging a stick
across a picket fence as he walks
along. This is caused by radio
waves being received with
reflections from multiple pathways
(multi-path) from the other
transmitter to you.
Release 1.0 – September 2006
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Check-Up Time!
Now let’s try the questions from
this group.
You should make a note of any that
you miss for later review.
Release 1.0 – September 2006
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T9B01
Why are VHF/UHF signals not normally
heard over long distances?
A. They are too weak to go very far
B. FCC regulations prohibit them from
going more than 50 miles
C. VHF and UHF signals are usually
not reflected by the ionosphere
D. They collide with trees and
shrubbery and fade out
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T9B01 Answer - C
The ionosphere routinely
reflects high frequency (HF)
waves so that they can travel
long distances. With the
exception of six meters, VHF
and UHF almost always travel
line of sight and are not
usually reflected by the
ionosphere.
Release 1.0 – September 2006
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T9B02
What might be happening when we hear a VHF
signal from long distances?
A. Signals are being reflected from outer
space
B. Someone is playing a recording to us
C. Signals are being reflected by lightning
storms in our area
D. A possible cause is sporadic E reflection
from a layer in the ionosphere
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T9B02 Answer - D
The E layer of the ionosphere
may sometimes reflect VHF waves
and long distance reception is
possible. This is not common,
but as the name suggests, is
sporadic.
Release 1.0 – September 2006
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T9B03
What is the most likely cause of sudden bursts of
tones or fragments of different conversations that
interfere with VHF or UHF signals?
A. The batteries in your transceiver are failing
B. Strong signals are overloading the receiver and
causing undesired signals to be heard
C. The receiver is picking up low orbit satellites
D. A nearby broadcast station is having transmitter
problems
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T9B03 Answer - B
Strong nearby signals can
overload a receiver.
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T9B04
What is the radio horizon?
A. The point where radio signals between two
points are blocked by the curvature of the
Earth
B. The distance from the ground to a
horizontally mounted antenna
C. The farthest point you can see when
standing at the base of your antenna tower
D. The shortest distance between two points
on the Earth's surface
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T9B04 Answer - A
The radio horizon is usually
farther away than the visible
horizon because radio waves can
bend slightly to follow the
curvature of the earth.
Release 1.0 – September 2006
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T9B05
What should you do if a station reports
that your signals were strong just a
moment ago, but now they are weak or
distorted?
A. Change the batteries in your radio to a
different type
B. Speak more slowly so he can understand
your better
C. Ask the other operator to adjust his
squelch control
D. Try moving a few feet, random reflections
may be causing multi-path distortion.
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T9B05 Answer - D
Many surfaces can reflect
radio waves, particularly VHF
and UHF waves. Sometimes these
reflections can mix and
interfere with the main signal,
causing your signal to be weak
or distorted. If you are near
large objects such as buildings
or bridges, the cure might be
to move away from them.
Release 1.0 – September 2006
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T9B06
Why do UHF signals often work better
inside of buildings than VHF signals?
A. VHF signals lose power faster over
distance
B. The shorter wavelength of UHF signals
allows them to more easily penetrate urban
areas and buildings
C. This is incorrect; VHF works better than
UHF inside buildings
D. UHF antennas are more efficient than VHF
antennas
Release 1.0 – September 2006
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T9B06 Answer - B
Because of their shorter
wavelength, UHF signals can
more easily penetrate
buildings. This is also true
of cell phones which usually
operate in the 800-900 MHz
range. Even though they are
relatively low powered
transceivers, they still work
inside many buildings.
Release 1.0 – September 2006
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T9B07
What is a good thing to remember
when using your hand-held VHF or UHF
radio to reach a distant repeater?
A. Speak as loudly as possible to
help your signal go farther
B. Keep your transmissions short to
conserve battery power
C. Keep the antenna as close to
vertical as you can
D. Turn off the CTCSS tone
Release 1.0 – September 2006
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T9B07 Answer - C
When trying to reach a
distant repeater, you should
try to keep your antenna
vertical. Why? Because the
antenna on the repeater is
vertical, and it can better
receive signals coming from an
antenna that is also vertical.
The fancy name for this is
vertical polarization.
Release 1.0 – September 2006
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T9B08
What can happen if the antennas at
opposite ends of a VHF or UHF line of
sight radio link are not using the same
polarization?
A. The modulation sidebands might become
inverted
B. Signals could be as much as 100 times
weaker
C. Signals have an echo effect on voices
D. Nothing significant will happen
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T9B08 Answer - B
Polarization makes a big
difference in ability to
receive. Ideally, both the
transmitting and receiving
antenna should be polarized the
same way - either both vertical
or both horizontal.
Release 1.0 – September 2006
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T9B09
What might be a way to reach a distant
repeater if buildings or obstructions are
blocking the direct line of sight path?
A. Change from vertical to horizontal
polarization
B. Try using a directional antenna to find a
path that reflects signals to the repeater
C. Ask the repeater owners to repair their
receiver
D. Transmit on the repeater output frequency
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T9B09 Answer - B
If you can use a directional
antenna, you might be able to
"bounce" the signal off of some
object such as a building that
has a clear line of sight to
the repeater.
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T9B10
What term is commonly used to
describe the rapid fluttering sound
sometimes heard from mobile stations
that are moving while transmitting?
A.
B.
C.
D.
Flip-flopping
Picket fencing
Frequency shifting
Pulsing
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T9B10 Answer - B
Think of the kid running a
stick along a picket fence and
you'll see why this is called
"picket fencing."
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T9B11
Why do VHF and UHF Radio signals usually
travel about a third farther than the
visual line of sight distance between 2
stations?
A. Radio signals move somewhat faster than
the speed of light and travel farther in
the same amount of time
B. Radio waves are not blocked by dust
particles
C. The Earth seems less curved to radio
waves than to light
D. Radio waves are blocked by dust particles
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T9B11 Answer - C
Another way of saying this is
that radio waves will bend
slightly to follow the
curvature of the earth. Either
way you look at it, VHF and UHF
waves can travel a little
farther near to the ground than
the visible horizon.
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Group T9C
Group T9C covers feedlines types, losses
vs. frequency, SWR concepts, measuring
SWR, matching and power transfer,
weather protection, and feedline failure
modes.
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Standing Wave Ratio (SWR)
In general terms, standing
wave ratio (SWR) is a measure
of how well a load is matched
to a transmitter. This load is
a combination of the antenna
and feedline.
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Impedance and Best SWR
Impedance is a quantity we measure to
determine SWR. Like resistance, it is
measured in ohms. A typical amateur radio
transmitter has an impedance of 50 ohms at
the feed point (where it connects to the
antenna). In a perfect world, the
feedline and antenna should have an
impedance of 50 ohms as well. If it does,
this is as good as it gets. We say that
the SWR is 1:1 (or “1 to 1”). A reading
on a SWR meter of 1 to 1 indicates a
perfect impedance match between the
antenna and the feed line.
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Erratic SWR Readings
Your SWR may be very good, or not
so good, but it should not change
very much when you key the
transmitter. If you see erratic
changes in SWR readings, you should
suspect a loose connection in your
antenna or feedline. When
installing and connecting any
antenna, you should always make
certain that your electrical
connections are good and tight.
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SWR Protection
Most solid state radios work at their
best with an SWR of near 1 to 1. They
will continue to work if the SWR is a
little higher than that, but if the SWR
gets too high, the radio will begin to
reduce power to avoid damage. 2 to 1 is
the SWR value where the protection
circuits in most solid-state transmitters
begin to reduce transmitter power. If you
begin to see a drop in output power, you
might want to check your SWR.
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Power Lost due to High SWR
As SWR increases, the
efficiency of your transmitted
signal decreases. This is
because some of your transmit
power is lost in the feed line.
This power lost in the feed
line is converted into heat by
losses in the line. This is
not good news to a ham.
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Forward Power and
Reflected Power
Forward Power is the power
going from the radio to the
feed line. Reflected power is
the power that is being
reflected back to the radio
because of a poor impedance
match. The presence of
reflected power indicates
higher than 1 to 1 SWR.
Because of this, you can also
use a directional wattmeter to
determine whether your
feedline and antenna are
properly matched to your
transmitter. Significant
reflected power indicates a
poor match.
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Coaxial Cable Failure
Coaxial cable is widely used as a feedline in
amateur radio. Sometimes, it fails. The most common
reason for failure of coaxial cables is moisture
contamination. If moisture gets inside of the cable, it
can corrode the wire or shielding, change the impedance
of the cable or even cause a short.
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Coaxial Cable and SWR
Coaxial cable (or
“coax”) works best
when the antenna
system has a low SWR.
It is important to
have a low SWR in an
antenna system that
uses coaxial cable
feedline to allow the
efficient transfer of
power and reduce
losses. It is more
critical to have this
match with coax than
with some other types
of feedline such as
open wire or ladder
line such as shown
here.
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Old Coax
Nothing lasts forever,
including coax. Older coaxial
cables that are exposed to
weather and sunlight for
several years can show a
dramatic increase in losses.
Coax exposed to the weather
should be replaced every few
years.
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Coax Covering
The outer sheath of most coaxial
cables black because black provides
the best protection against
ultraviolet damage.
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Impedance of Coax
The impedance of the most
commonly used coaxial cable in
typical amateur radio
installations is 50 ohms. This
represents a good match to the
typical transmitter impedance
of 50 ohms.
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Advantages of Coax
Coaxial cable is used more often
than any other feed line for amateur
radio antenna systems because it is
easy to use and requires few special
installation considerations. There
are other feed lines that have less
loss, can handle more power, or are
less expensive than coax, but
overall, coax is a good compromise
because it is easy to install and
use.
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Check-Up Time!
Now let’s try the questions from
this group.
You should make a note of any that
you miss for later review.
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T9C01
What, in general terms, is standing
wave ratio (SWR)?
A. A measure of how well a load is
matched to a transmitter
B. The ratio of high to low impedance
in a feed line
C. The transmitter efficiency ratio
D. An indication of the quality of
your station ground connection
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T9C01 Answer - A
The "load" is usually the
combination of antenna and feed
line. The radio and load
perform best when they are
closely matched. When they are
perfectly matched, the SWR is 1
to 1, or 1:1.
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T9C02
What reading on a SWR meter
indicates a perfect impedance
match between the antenna and
the feed line?
A.
B.
C.
D.
2 to 1
1 to 3
1 to 1
10 to 1
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T9C02 Answer - C
An SWR of 1 to 1 is as good
as it gets. The radio performs
at its best with a low SWR.
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T9C03
What might be indicated by erratic
changes in SWR readings?
A. The transmitter is being modulated
B. A loose connection in your antenna
or feed line
C. The transmitter is being over
modulated
D. Interference from other stations
is distorting your signal
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T9C03 Answer - B
If the SWR is not fairly
steady, you need to see whether
the antenna is getting a steady
signal from the radio. If it
isn't, the most likely cause is
a loose connection.
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T9C04
What is the SWR value where the
protection circuits in most
solid-state transmitters begin
to reduce transmitter power?
A.
B.
C.
D.
2 to 1
1 to 2
6 to 1
10 to 1
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T9C04 Answer - A
High SWR can damage modern
transceivers. However, most
transceivers are designed to
automatically reduce power output
when the SWR goes up to 2 to 1 or
higher to avoid being damaged. The
higher the SWR, the more the power
will be reduced. If you notice a
decrease in power, check your SWR.
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T9C05
What happens to the power lost in a
feed line?
A. It increases the SWR
B. It comes back into your
transmitter and could cause damage
C. It is converted into heat by
losses in the line
D. It can cause distortion of your
signal
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T9C05 Answer - C
High SWR can result in wasted
transmit power. You want all
those watts to be radiated from
your antenna as a signal, but a
high SWR will cause some of
that power to be converted to
heat. From a ham's point of
view, that's wasted energy.
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T9C06
What instrument other than a
SWR meter could you use to
determine if your feedline and
antenna are properly matched?
A.
B.
C.
D.
Voltmeter
Ohmmeter
Iambic pentameter
Directional wattmeter
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T9C06 Answer - D
A directional wattmeter
measures the power leaving your
radio, and the power returning
back to your radio. Ideally,
there should not be any power
returning to the radio. If
there is, your SWR needs to be
checked.
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T9C07
What is the most common reason
for failure of coaxial cables?
A.
B.
C.
D.
Moisture contamination
Gamma rays
End of service life
Overloading
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T9C07 Answer - A
Coaxial cable (or "coax") can
be harmed by moisture getting
inside. When you put up an
antenna, you should always use
a good sealant around
connectors to keep water out.
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T9C08
Why is it important to have a low
SWR in an antenna system that uses
coaxial cable feedline?
A. To reduce television interference
B. To allow the efficient transfer of
power and reduce losses
C. To prolong antenna life
D. To keep your signal from changing
polarization
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T9C08 Answer - B
You can use a tuner to match
the antenna to a transmitter,
but if the SWR of an antenna is
high, coax is not as efficient
for transferring power as other
types of feed line and the
losses can be fairly high when
the antenna system is not
resonant.
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T9C09
What can happen to older coaxial
cables that are exposed to weather
and sunlight for several years?
A. Nothing, weather and sunlight do
not affect coaxial cable
B. The cable can shrink and break
C. Losses can increase dramatically
D. It will short-circuit
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T9C09 Answer - C
Although coax is not likely
to shrink or break, its
efficiency will decrease after
several years of exposure to
the weather.
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T9C10
Why is the outer sheath of most
coaxial cables black in color?
A. It is the cheapest color to use
B. To see nicks and cracks in the
cable
C. Black cables have less loss
D. Black provides protection against
ultraviolet damage
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T9C10 Answer - D
Ultraviolet light in sunlight
can degrade certain types of
plastic over time. Using black
plastic as a cover for coax
helps to protect it from
harmful ultraviolet (UV) rays,
just like your shades protect
your eyes from these same UV
rays.
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T9C11
What is the impedance of the
most commonly used coaxial
cable in typical amateur radio
installations?
A.
B.
C.
D.
8 Ohms
50 Ohms
600 Ohms
12 Ohms
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T9C11 Answer - B
50 ohms is a good number to
remember. Most amateur
transceivers are designed to
use 50 ohm coax cable for
feedline.
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T9C12
Why is coaxial cable used more often than
any other feed line for amateur radio
antenna systems?
A. It is easy to use and requires few
special installation considerations
B. It has less loss than any other type of
feed line
C. It can handle more power than any other
type of feed line
D. It is less expensive than any other types
of line
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T9C12 Answer - A
There are better types of
feed line than coax for certain
purposes, but the main
advantage of coax is that it is
easy to use.
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Nine Down, One to Go!
You’re Almost There!
This concludes Study Guide # 3.
Once you are satisfied that you can answer
80% of the questions in this Sub-element, you
are ready to move on to Study Guide # 4.
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