ARRL Amateur Radio Education & Technology Program Unit 2 Wireless Phenomena

ARRL Amateur Radio Education & Technology Program  Unit 2 Wireless Phenomena
ARRL Amateur Radio Education
& Technology Program
Unit 2 Wireless Phenomena
ELECTROMAGNETIC SPECTRUM
To understand how radio signals travel
through space, we must first understand the nature
of electromagnetic waves.
In nature there are two kinds of energy
waves: compression and transverse. Sound
waves are an example of compression waves.
They transfer their energy through things such as
air and water. Electromagnetic waves, which
include radio waves, are examples of transverse
waves. Transverse, radio waves do not need a
material to transfer their energy through. That is, they can transfer their energy through a
vacuum, like space. They do this by creating electric and magnetic fields that cycle back
and forth very fast – hundreds, thousands, millions, billions and even trillions of times
each second. The number of cycles the wave goes through each second is called its
frequency. We’ll discuss this in more detail in a moment.
Figure 2.1 shows what is called “The Electromagnetic Spectrum.” The
electromagnetic spectrum shows electromagnetic waves varying in frequency from waves
of a few hertz (or cycles) all the way through gamma rays. We use these waves to do
everything from transmitting radio and television signals to cooking our food. Yes, we
can use electromagnetic waves to cook our food. Do you have a microwave oven at
home? Notice on the electromagnetic spectrum chart, microwaves are just above TV &
FM broadcast frequencies. These electromagnetic waves can be described by their
wavelength, frequency or the amount of energy they contain. We use different units to
describe the three. Radio waves are usually described by their frequency, measured in
Hertz (Hz), thousands of Hertz (kHz) or millions of Hertz (MHz); light waves are usually
described by their wavelength measured in meters (m); and X-rays in terms of energy
measured in Electron-volts (eV).
Looking at the electromagnetic spectrum chart, what kind of electromagnetic
radiation has the shortest wavelength? Which one has the longest?
THE NATURE OF RADIO WAVES
What do all these frequencies mean to us? In Amateur Radio we work with a wide range
of frequencies starting as low as 1800 kHz to up above 300 GHz. Yes, that’s Giga Hertz.
Much of the operation, however, takes place in the lower frequencies, from 1800 kHz to
450 MHz. There is a direct relationship between the frequency of a radio wave and its
wavelength. To best understand this relationship, let’s look at the typical “ac” wave – the
kind that brings power to your house or school. Even though it travels through wires, an
“ac” (ac means alternating current) wave is a radio wave, too.
1
Figure 2.1
2
The power company uses a large machine
called an alternator to produce power at its
generating stations. The ac supplied to your home
goes through 60 complete cycles (shifting from a
positive voltage to negative and back to positive
again) each second. So, the electricity from the
power company has a frequency of 60 Hz. See Figure
2.2.
This 60-hertz ac electricity builds slowly to a
peak current or voltage in one direction, then
decreases to zero and reverses to build to a peak in
the opposite direction. If you plot these changes on a
graph, you get a gentle up-and-down curve. We call
this curve a sine wave. Figure 2.2A shows two
cycles of a sine-wave ac signal.
What you need to remember is that alternating
currents can change direction at almost any rate.
Some signals have low frequencies, like the 60-hertz
ac electricity the power company supplies to your
Figure 2.2
house. Other signals have high frequencies like, radio
signals that can alternate at more than several million
hertz. (Activity Sheet #2.1)
Wavelength is another unit that can be used to describe an ac signal. As its name
implies, wavelength refers to the distance the wave will travel through space in one single
cycle. See Figure 2.2B. All such signals travel through space at the speed of light,
300,000,000 meters per second. The Greek letter lambda (λ) is used to represent
wavelength.
Here’s another thing to remember. The faster a signal alternates, the less distance
the signal will be able to travel during one cycle. There is an equation that relates the
frequency and the wavelength of a signal to the speed of the wave. If you know either
the frequency or the wavelength, you can determine the missing one.
c=f*λ
(Equation 2.1)
Where:
c is the speed of light, 3.00 x 10 (8) meters per second
f is the frequency of the wave in meters
λ is the wavelength of the wave in meters
3
We can solve this equation for either frequency or wavelength, depending on which
quantity we want to find.
f =c/λ
(Equation 2.2)
And
λ=c/f
(Equation 2.3)
From these equations you can see that as the frequency increases the wavelength
gets shorter. Figure 2.3 shows a simple diagram that will help you remember the
frequency and wavelength relationships. You can practice using these equations on
Student Worksheet #2.2
C
f
Figure 2.3
HOW RADIO WAVES TRAVEL
Have you ever wondered how the radio signal travels from your favorite AM or
FM radio station to your radio? When you did the AM Dxing activity in unit one, did
you notice how during the day you could only hear local radio stations, but at night, you
could hear stations from far away? To understand this “phenomena” we need to
understand how radio waves travel. The study of how radio waves travel from one point
to another is called “Propagation.” Radio waves travel to their destination in four ways.
First, radio waves can travel directly from one point to another. This is called line-ofsight propagation. The second way radio waves travel is along the ground, bending
slightly to follow the curvature of the Earth for some distance. This is called groundwave propagation. Third, radio waves can be refracted or bent back to Earth by the
ionosphere. This is known as sky-wave propagation. The ionosphere is a layer of
charged particles called ions in the Earth’s outer atmosphere. These ionized gases make
long distance radio communications possible. Lastly, radio waves can be trapped in a
layer of the Earth’s atmosphere, traveling a longer distance than normal before coming
back to the Earth’s surface. This is known as tropospheric ducting. We will now take a
look at each of these four types of propagation.
4
Line-Of-Sight Propagation
Line-of-sight propagation occurs when signals travel in a straight-line form the
transmitting antenna to the receiving antenna. These signals, also know as direct waves,
are used mostly in very high frequency (VHF), ultra high frequency (UHF) and
microwave ranges. The signals you receive from your local television stations and FM
Figure 2.3
radio stations are examples of direct waves. Cable television is not considered an
example of direct waves, however, because the signal travels through a cable instead of
being transmitted through the air. Two-way radios, like police and fire departments and
Amateur Radio operators use, is another good example of
line-of-sight propagation. When transmitting on a local
repeater frequency, direct waves generally travel in a
straight line to the repeater. The repeater then retransmits
the signal in a straight line to other station. See Figure
2.3.
Tall buildings, mountains and hills, and even
airplanes affect line-of-sight propagation. These things
can get in the way of radio signals and cause disruption
of radio communications. (Activity Sheet #2.3)
Tropospheric Bending
About seven to ten miles above the Earth’s surface is the region called the
troposphere. Slight bending of radio waves occurs in this area. The troposphere can
cause radio signals to “bend” back towards the Earth, a little beyond the visible horizon,
and allow contacts between stations that are further away than would otherwise be
possible. The radio horizon is generally about 15% farther away than the true horizon.
5
This is referred to as tropospheric bending. Tropospheric bending is used in the
VHF/UHF frequency ranges. See Figure 2.4.
Figure 2.4
Tropospheric Ducting
Besides bending a radio signal back towards Earth, it
is possible for a VHF or UHF radio signal to become trapped
in the troposphere causing them to travel longer distances
than normal before coming back to the Earth’s surface. This
is referred to as tropospheric ducting. See Figure 2.5. When
a cold air mass moves in under a warm air mass, called a
temperature inversion, it can act like a tube, or duct, and
cause radio-waves to travel along the duct for many miles
before returning to Earth. (Activity Sheet #2.4)
Figure 2.5
6
Ground Wave Propagation
In ground wave propagation, radio waves travel along the Earth’s surface, even
over hills. They follow the curvature of the earth for some distance. Signals from AM
broadcast stations travel by ground wave propagation during the daytime. As you drive
away from the station the signal begins to fade until you can’t hear them anymore.
Ground waves work best at lower frequencies. Another example of ground wave
propagation is when a ham radio operator makes an HF daylight contact with a station
just a few miles away. The signal travels along the ground to the other station.
Ground wave propagation on the ham bands means relatively short-range
communications, usually 50 miles or less. But contacts of several hundred miles are
possible under the right conditions. Stations near the high frequency end of the AM
broadcast band (1600-kHz) generally carry less than a hundred miles during the day.
Stations near the low frequency end of the AM broadcast band (540-kHz) can be heard
up to a distance of 100 miles or more. Amateur Radio frequencies are higher than the
AM broadcast band, so the ground-wave range is usually shorter.
Sky-Wave Propagation (Skip)
When Marconi sent the first message across the Atlantic Ocean in 1901, he didn’t
fully understand the science of radio wave propagation. The distance was too great for
ground waves to travel. The frequencies were too low for tropospheric bending or
ducting to take place. So there had to be another reason the radio waves traveled so far.
The answer was sky-wave propagation.
We have since found that surrounding the Earth is a layer of particles called ions.
These ions have a negative charge, just like radio waves. When the negatively charged
radio wave is transmitted up near the ionosphere, they are refracted back towards the
Earth. See Figure 2.6. We are able to “bounce” radio signals off the ionosphere back to
Earth. We call the distance from the transmitting station to the receiving station the skip
distance. The area between the two stations is called the skip zone. Two-way radio
contacts of up to 2500 miles are possible with one skip off the ionosphere. Worldwide
communications using several skips (or hops) can take place if conditions are right. This
is the way long-distance (DX) radio signals travel.
Figure 2.6
7
Two factors determine sky-wave propagation:
• Frequency of the radio wave
• Level of ionization
What causes the ions? The sun, or
energy from the sun. Energy from the sun
bombards the gasses in the ionosphere
causing ions to form. We will be discussing
how this happens in just a moment.
There is one more important piece of
information you should know. The higher
the frequency of the radio wave, the less it is
bent by the ionosphere. The highest
frequency at which the ionosphere bends
radio waves back to Earth is called the
maximum usable frequency (MUF).
Now, lets look at the ionosphere and see
how it works.
Figure 2.7
Ultraviolet Radiation
from the Sun
Electron
(Negative Ion)
Electrically Neutral
Atom
(A)
Positive Ion
(B)
THE IONOSPHERE
The Earth’s upper atmosphere (25 to
200 miles above the Earth) is made up of
mainly oxygen and nitrogen, with traces of
hydrogen, helium and several other gases.
These gases are bombarded by ultraviolet
radiation (energy) from the sun. This
radiation knocks electrons out of the atoms
of the gas forming negatively charged
particles. The remaining portions of the gas
atoms form positively charged particles.
The positive and negative particles are
called ions. The process by which ions are
formed is called ionization. The area where
ionization takes place is called the
ionosphere.
The ionosphere is actually made up
of several regions of charged particles.
These regions have been given letter
designations D through F, as shown in
Figure 2.7. Why start with the letter D?
Scientists started with D just in case there
were any undiscovered lower regions. None
have been found, so there is no A, B or C
region.
8
As we mentioned, energy from the sun causes
the ionization so the sun affects the way radio waves
travel. The sun rotates through an eleven-year cycle of
increasing, then decreasing sunspot activity; this is
referred to as the sunspot cycle. During periods of
increased sunspot activity, the ionosphere can become
disturbed and disrupt radio communications worldwide.
Now lets look at the different layers (or regions)
making up the ionosphere.
The D – Region
The D region is the lowest region of the ionosphere affecting propagation. This
region is located about 35 to 60 miles above the Earth’s surface. It is very dense and
instead of refracting radio signals back to Earth, it absorbs them. So, when the D region
is most ionized, at noontime, radio communications can be affected. Ionization only lasts
while exposed to the suns rays. By sunset, the ionization has stopped and the D region
disappears.
The E – Region
The next highest region, the E-region, is located about 60 to 70 miles above the
Earth. Like the D-region, ionization lasts only while exposed to the sun’s rays. The
ionization level is lowest just before sunrise, local time. Ionization reaches its highest
level about midday and by early evening the ionization level is very low again.
Therefore, communication using the E-region is only possible during daylight hours.
The F – Region
The highest region of the ionosphere, and
most responsible for long-distance radio
communication, is the F-region. This region is very
large. It ranges from about 100 to 310 miles above
the Earth. The ionization in the F-region reaches its
highest shortly after noon local time. It then tapers
off very gradually toward sunset. At this altitude, the
ions and electrons recombine very slowly. The Fregion remains ionized during the night, reaching its
lowest just before sunrise. After sunrise, ionization
happens quickly for the first few hours, then it slows
to its noontime high.
During the day, the F-region splits into two parts, F1 and F2. The central part of
the F1 region forms about 140 miles above the Earth. For F2 region, the central region
forms at about 200 miles above the Earth. At night, these two regions recombine to form
a single F-region. The F1 region does not have much to do with long-distance
communications. Its effects are similar to those caused by the E-region. The F2 region is
responsible for almost all long-distance communication on frequencies from 1.8 to 30
9
MHz. Using the F2 region, two-way radio contacts can be made up to 2500 miles in one
skip.
The Scatter Modes
As we mentioned before, the area between where the ground wave ends and the
point where the first signals return from the ionosphere is called the skip zone. Some
radio signals that are refracted (bent) by the ionosphere are sometimes returned at a
relatively wide angle. This can cause some signals to return to Earth within the skip
zone, making communications in this area possible. This is referred to as scatter
propagation. Under ideal conditions, scatter propagation is possible over 3000 miles or
more. Scatter signals, however, are generally weak and may be distorted because the
signal may arrive at the receiver from many different directions.
AMATEUR SATELLITE OPERATIONS
Satellites have been used for many years to relay signals from one point on the
Earth to another. Government, military and businesses all have satellites circling the
Earth providing such services as television, broadcast radio, telephone, and paging
systems. Although we use these services, ham radio operators do not have direct access
to these satellites for ham communication. So hams have built and launched their own
satellites.
Since 1961, Amateur Radio operators have launched many of their own satellites.
Amateurs use these Satellites Carrying Amateur Radio (OSCAR’s) to communicate with
other amateurs around the world. Most satellites use the VHF and UHF bands because
radio signals on those bands normally go right through the ionosphere. The satellites
retransmit signals to provide greater communications range than would be possible on
those bands. Satellites use line-of-sight propagation so both operators must be in sight of
the satellite. It is helpful, when working satellites, to have an antenna you can point
directly at the satellite. To do this, you also need a computer satellite-tracking program
to locate the satellite in space. Working satellites can be a challenging and rewarding
experience in Amateur Radio.
EARTH-MOON-EARTH (EME)
COMMUNICATIONS
For a real challenge, some Amateur Radio
operators participate in a unique form of
communications involving bouncing VHF and UHF
signals off the surface of the Moon. Like satellite
communications, it requires both stations to be in
light of the Moon. There is one drawback to this
communication method, however. Due to the
distance to the Moon and back, a rather elaborate
antenna array and an enormous amount of power is
required.
10
11
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement