2 Propagation Modulation
-
Upload
salam-alecu -
Category
Documents
-
view
229 -
download
3
description
Transcript of 2 Propagation Modulation
-
Hilmi ECE GMDSS Instructor1
RADIO THEORY & PROPAGATION
The magnetic field strength in the vicinity of a conductor is
directly proportional to the magnitude of the current flowing
through the conductor. Recall the discussion of alternating
current above. A rotating generator produces current in the
form of a sine wave. That is, the magnitude of the current
varies as a function of the relative position of the rotating
conductor and the stationary magnetic field used to induce
the current. The current starts at zero, increases to a
maximum as the rotor completes one quarter of its revolution,
and falls to zero when the rotor completes one half of its
revolution. The current then approaches a negative
maximum; then it once again returns to zero.
Radio Wave Terminology
-
Hilmi ECE GMDSS Instructor2
RADIO THEORY & PROPAGATION
This cycle can be represented by a sine function. The
relationship between the current and the magnetic field
strength induced in the conductor through which the current is
flowing is shown in the diagram on the next page. Recall from
the discussion above that this field strength is proportional to
the magnitude of the current; that is, if the current is
represented by a sine wave function, then so too will be the
magnetic field strength resulting from that current. This
characteristic shape of the field strength curve has led to the
use of the term wave when referring to electromagnetic
propagation. The maximum displacement of a peak from zero
is called the amplitude. The forward side of any wave is called
the wave front. For a nondirectional antenna, each wave
proceeds out-ward as an expanding sphere (or hemisphere).
-
Hilmi ECE GMDSS Instructor3
RADIO THEORY & PROPAGATION
One cycle is a complete sequence of values, as from crest to
crest. The distance traveled by the energy during one cycle is
the wavelength, usually expressed in metric units (meters,
centimeters, etc.). The number of cycles repeated during unit
time (usually 1 second) is the frequency. This is given in hertz
(cycles per second). A kilohertz (kHz) is 1,000 cycles per
second. A megahertz (MHz) is 1,000,000 cycles per second.
Wavelength and frequency are inversely proportional. The
phase of a wave is the amount by which the cycle has
progressed from a specified origin. For most purposes it is
stated in circular measure, a complete cycle being considered
360. Generally, the origin is not important, principal interest
being the phase relative to that of some other wave.
-
Hilmi ECE GMDSS Instructor4
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor5
RADIO THEORY & PROPAGATION
Units of Measuring Hertz
We use the decimal system for simplicity and use the
following designations in measuring the frequency:
Hertz (Hz) 1 to 999 Hz
Kilo Hertz (kHz) thousand 1,000 Hz
Mega Hertz (MHz) million 1,000,000 Hz
Giga Hertz (GHz) billion 1,000,000,000 Hz
Examples: 2182.0 kHz is equal to 2,182,000 Hz
156.8 MHz is equal to 156,800 kHz or
156,800,000 Hz
-
Hilmi ECE GMDSS Instructor6
RADIO THEORY & PROPAGATION
Audio Frequencies (AF)
The human ear responds to audio frequencies between
20Hz and about 20kHz with the pitch of the note
increasing with frequency.
Above that, the human ear loses interest although other
animals such as dogs and bats can hear higher
frequencies. Human hearing becomes impaired with age
and can be damaged by working in a noisy environment,
the effect being to reduce the upper frequency limit.
No problems are experienced unless the upper-frequency
response falls to below 3kHz, which is the upper limit of
human speech.
-
Hilmi ECE GMDSS Instructor7
RADIO THEORY & PROPAGATION
Electromagnetic Spectrum
The entire range of electromagnetic radiation frequencies is
called the electromagnetic spectrum. The frequency range
suitable for radio transmission, the radio spectrum, extends
from 10 kilohertz to 300,000 megahertz. It is divided into a
number of bands, as shown in Table below.
Below the radio spectrum, but overlapping it, is the audio
frequency band, extending from 20 to 20,000 hertz. Above
the radio spectrum are heat and infrared, the visible
spectrum (light in its various colors), ultraviolet, X-rays,
gamma rays, and cosmic rays. Waves shorter than 30
centimeters are usually called microwaves.
-
Hilmi ECE GMDSS Instructor8
RADIO THEORY & PROPAGATION
Radio Frequency Spectrum
ELF Extremely Low Frequency 300 Hz 3 kHz
VLF Very Low Frequency 3 30 kHz Myriametric waves
LF Low Frequency 30 300 kHz Kilometric waves
MF Medium Frequency 300 3,000 kHz Hectometric waves
HF High Frequency 3 MHz 30 MHz Decametric waves
VHF Very High Frequency 30 300 MHz Metric waves
UHF Ultra High Frequency 300 MHz 3 GHz Decimetrik waves
SHF Super High Frequency 3 30 GHz Centimetric waves
EHF Extremely High Frequency 30 300 GHz Millimetric wavesPlease Note: We do not use all of these frequencies in the Maritime Mobile
Service, only a select portion of each
-
Hilmi ECE GMDSS Instructor9
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor10
RADIO THEORY & PROPAGATION
Radio Wave Propagation
This topic area deals with the path taken by a radio wave when
it leaves the transmitting antenna. The main factor that
determines the path taken by a radio wave is the frequency or
wavelength of the transmission.
Radio waves travel at the speed of light (300 x 106 meters per
second). The relationship between the frequency and
wavelength is expressed in the following formula:
Frequency = Speed of Light Wavelength
-
Hilmi ECE GMDSS Instructor11
RADIO THEORY & PROPAGATION
This formula shows that the longer wavelength corresponds
to lower frequencies.
Such as a shorter wavelength corresponds to higher
frequencies. We will be using this information later when we
talk about antennas.
Example: 150 MHz frequency is a 2 meter wavelength
2 MHz frequency is a 150 meter wavelength
-
Hilmi ECE GMDSS Instructor12
RADIO THEORY & PROPAGATION
Radio communication is a wonderful thing. Via radio, we send
messages at the speed of light over great distances.
Messages can travel from one side of the earth to another, or
to satellites orbiting a distant planet.
By changing the frequency of transmission, our radio can
communicate at different distances. Each frequency band has
widely varying properties with respect to its range and
behavior under different conditions. Knowing in advance what
frequency to use in communicating with a distant station is the
single most important part of learning to use your radio.
-
Hilmi ECE GMDSS Instructor13
RADIO THEORY & PROPAGATION
Approximate guide frequency bands and ranges (for 100 watt commercial services)
-
14Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
15Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
16Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor17
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor18
RADIO THEORY & PROPAGATION
The three main modes of propagation of radio waves from
transmitter (Tx) to receiver (Rx) in maritime radio
communications are:
Direct or Space Wave
Ground Waves
Sky Waves
-
Hilmi ECE GMDSS Instructor19
RADIO THEORY & PROPAGATION
-
20Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor21
RADIO THEORY & PROPAGATION
-
22Hilmi ECE GMDSS Instructor
Direct Wave Propagation
Frequencies above 30 MHz use the Direct or Space Wave
propagation. The radio waves leave the antenna in a
straight line following a line of sight path. The radio
waves do not follow the earths curvature, but will
penetrate right through the ionosphere and out into space.
For VHF communications the range depends on the
height of both the transmitting and receiving antennas.
For a ship, with an antenna mounted at the top of the
mast, communicating with another ship with a similar
antenna is approximately about 25nm.
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor23
RADIO THEORY & PROPAGATION
Communication from a coast station can produce a
greater range because the coast station antenna is at a
greater height. In the same respect, communication from
a smaller vessel, or a hand held portable, will produce a
shorter range of communication.
Remember: frequency band for VHF is 156mHz to 164
MHz, and is limited to 25 watts of output power.
-
Hilmi ECE GMDSS Instructor24
RADIO THEORY & PROPAGATION
Ground Wave Propagation
The common mode of propagation for medium frequency (MF
1605 kHz to 3800 kHz) is Ground Wave, also known as
surface wave. Radio waves leave the antenna and follow the
earths curvature. As they travel over the sea, or land, energy
is gradually lost, in technical term the wave is attenuated. The
further away from a transmitter you are, the weaker the signal
becomes, until eventually communication is lost. As ships
transmitters are typically limited to a power of 400 watts.
Maximum range for these frequencies is 300 nm. This will
depend on ships transmitter power, and antenna efficiency
and can range from 150 to 300 nm. The higher the frequency
the shorter the range of the ground wave.
-
Hilmi ECE GMDSS Instructor25
RADIO THEORY & PROPAGATION
Sky Wave Propagation
This is where radio waves are beamed up from the antenna
towards the ionosphere and are refracted back to the earth.
This gives a maximum range of about 4000 nm. Greater
ranges, giving worldwide coverage, are obtained by multi-
hop transmissions. A radio wave leaves the antenna then is
refracted by the ionosphere back to the earth, refracted
back to up to the ionosphere, then back again and so on.
-
Hilmi ECE GMDSS Instructor26
-
Hilmi ECE GMDSS Instructor27
RADIO THEORY & PROPAGATION
The ionosphere is layers of ionized gases in the upper
atmosphere, ranging from about 50 to 300 miles above the
earths surface. The density of the gas present and the
level of ultraviolet radiation from the sun determine the
degree of ionization.
Thus the density and width of the ionosphere depends on
the time of day or night, and the season, winter or summer.
During the day, there are normally four layers, known as the
D, E, F1, and F2 layers. At night the D layer disappears
and the two F layers combine into one, leaving just the E
and F layer for propagation.
-
Hilmi ECE GMDSS Instructor28
RADIO THEORY & PROPAGATION
-
29Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATIOND Layer : 20-500kHz,
E Layer : 500-2.000kHz,
F1 Layer : 1.500-30.000kHz,
F2 Layer : 3.000-30.000kHz
-
Hilmi ECE GMDSS Instructor3030
-
31Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor32
RADIO THEORY & PROPAGATION
As the radio wave enters the ionosphere it suffers both
attenuation and refraction. This is also known as weakening
and bending of the radio waves in more non-technical terms.
Both of these two effects are greater the lower the frequency
used. If refraction is sufficient the wave is returned to the earth.
Thus lower frequencies are returned from lower heights giving
shorter ranges. There is a minimum range over which a
particular frequency can communicate by sky waves, this is
known as the skip distance. The higher the frequency, the
greater the skip distance. The distance between the end of the
ground (surface) wave and the skip distance is known as dead
space, and no communications on this frequency can reach this
area.
-
33Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor34
RADIO THEORY & PROPAGATION
As a general rule, the greater the distance over which the
communication is to take place the higher the frequency you
should use. Where the signals can be heard on two bands,
the higher one should be used to minimize attenuation. One
could use tables, like the ones in the next chapter, which are a
rough guide to the ranges that different frequencies can
achieve at different times.
Although the HF band covers the frequency range from 3 to 30
MHz, only small sections have been allocated to different
services. The marine service has been allocated bands of
frequencies around 4 MHz, 6 MHz, 8 MHz, 12/13MHz, 16/17
MHz, and 22 MHz.
-
Hilmi ECE GMDSS Instructor35
RADIO THEORY & PROPAGATION
The combination of the 12/13 and the 16/17 MHz bands are
typically known as the 12 and 16 MHz bands, since this is
where the ship transmits. The maximum limit on HF output
power is 1500 watts or 1.5 kW.
-
Hilmi ECE GMDSS Instructor36
RADIO THEORY & PROPAGATION
Mode of Radio Emissions
The World Administrative Radio Conference held in 1979
assigned each emission a classification that uses a three-
character identification. These settings determine how the
modulation of the signal is to be done. If the radio were set
on the wrong mode, it would not be able to properly receive
the communication.
-
Hilmi ECE GMDSS Instructor37
On the nomenclature
RADIO THEORY & PROPAGATION
-
38Hilmi ECE GMDSS Instructor
Classification
The class of emission is a set of characteristics conforming to
below.
Emissions shall be classified and symbolized according to their
basic characteristics.
The basic characteristics (see Sub-Section IIA) are:
1) first symbol type of modulation of the main carrier;
2) second symbol nature of signal(s) modulating the main
carrier;
3) third symbol type of information to be transmitted.
RADIO THEORY & PROPAGATION
-
39Hilmi ECE GMDSS Instructor
Modulation used only for short periods and for incidental
purposes (such as, in many cases, for identification or calling)
may be ignored provided that the necessary bandwidth as
indicated is not thereby increased.
RADIO THEORY & PROPAGATION
-
40Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
41Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
42Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor43
RADIO THEORY & PROPAGATION
A few examples of the different modes of emission classification
that are used in the Maritime Mobile Service are:
A1A Continuous Wave (Morse Code)
F1B Radio telex & Navtex assignments along with DSC for VHF
F3E Frequency Modulation (Voice in VHF)
H3E Single Sideband full carrier (permitted on 2182 only)
(a.k.a. AM)
J3E Single Sideband (SSB) Suppressed Carrier (typical voice
SSB frequencies)(USB)
J2B DSC for MF/HF bands
-
Hilmi ECE GMDSS Instructor44
F3E : VHF Radio Telephone (Frequency modulation)
G3E : VHF Radio Telephone (Phase modulation-simplex chann.)
G1B : Satellite EPIRB
G2B : VHF DSC (1200 Baud)
G2B : VHF DSC EPIRB
H3E : Below of 1605kHz (and 2182kHz Watch receiver)
J3E : MF-HF/SSB (above of 1605kHz)
J2B : NBDP (Radio telex-MF/HF DSC-NAVTEX-Amplitute Mod.)
F1B : NBDP (Radio telex-MF/HF DSC-NAVTEX-freq.Mod.)
A1A : Mors (Radio-telgraphy)
F1C/F3C: FAX (weather fax)
A3E : 121,5MHz, 123,1MHz (VHF AERO)
P0N : SART
Frequently Used in modulations modes of emission
classification in GMDSS
-
Hilmi ECE GMDSS Instructor45
RADIO THEORY & PROPAGATION
Most communication on MF & HF now use single-sideband
(SSB) techniques for both speech and NBDP/telex
transmissions. In a double-sideband transmission more than
two thirds of the output power of the transmitter is contained in
the carrier, which contains no useful signal information. Also,
the upper and lower sidebands contain the same information.
By eliminating the duplicated information in the lower sideband,
along with the carrier, the transmitter efficiency is greatly
increased. In effect, the space taken up within the frequency
band is reduced and so more stations can have the ability to
transmit.
-
Hilmi ECE GMDSS Instructor46
RADIO THEORY & PROPAGATION
A narrower bandwidth for the transmitted signal means less
noise and interference is apparent at the receiver, resulting in
a relatively smaller masking effect on the wanted
transmission.
Also, the same power is used more efficiently. The net effect
is that, for the same transmitter power, the effective range of
a transmission will be greatly extended by using a narrow-
bandwidth method of modulation such as SSB.
-
Hilmi ECE GMDSS Instructor47
RADIO THEORY & PROPAGATION
Necessary bandwidth
The necessary bandwidth, determined in accordance with the
formulae and examples, shall be expressed by three numerals
and one letter.
The letter occupies the position of the decimal point and
represents the unit of bandwidth. The first character shall be
neither zero nor K, M or G.
between 0.001 and 999 Hz shall be expressed in Hz (letter H);
between 1.00 and 999 kHz shall be expressed in kHz (letter K);
between 1.00 and 999 MHz shall be expressed in MHz (letter M);
between 1.00 and 999 GHz shall be expressed in GHz (letter G).
-
Hilmi ECE GMDSS Instructor48
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor49
RADIO THEORY & PROPAGATION
Single Sideband, SSB Modulation
Single sideband modulation is a form of amplitude
modulation. As the name implies, single sideband, SSB uses
only one sideband for a given audio path to provide the final
signal.
Single sideband modulation, SSB, provides a considerably
more efficient form of communication when compared to
ordinary amplitude modulation. It is far more efficient in terms
of the radio spectrum used, and also the power used to
transmit the signal.
In view of its advantages single sideband modulation has
been widely used for many years, providing effective
communications, as well as forms being used for some
analogue television signals, and some other applications.
-
Hilmi ECE GMDSS Instructor50
RADIO THEORY & PROPAGATION
Single sideband
modulation basics single
sideband modulation can
be viewed as an
amplitude modulation
signal with elements
removed or reduced. In
order to see how single
sideband is created, it is
necessary to use an
amplitude modulated
signal as the starting
point.
-
51Hilmi ECE GMDSS Instructor
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor52
RADIO THEORY & PROPAGATION
From this it can be seen that the signal has two
sidebands, each the mirror of the other, and the carrier.
To improve the efficient of the signal, both in terms of the
power and spectrum usage, it is possible to remove the
carrier, or at least reduce it, and remove one sideband -
one is the mirror image of the other.
A single sideband signal therefore consists of a single
sideband, and often no carrier, although the various
variants of single sideband are detailed below.
-
53Hilmi ECE GMDSS Instructor
It can be seen that either the upper sideband or lower
sideband can be used. There is no advantage between
using either the upper or lower sideband. The main
criterion is to use the same sideband as used by other
users for the given frequency band and application. The
upper sideband is more commonly used for professional
applications.
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor5454
SINGLE SIDE BAND
WINDOW
FREQUENCY
LSB USB
DSB (AM) MODULATION
1.4 kHz1.4 kHz
SSB
-2.8 kHz +2.8 kHz
-
Hilmi ECE GMDSS Instructor5555
WINDOW
FREQUENCY
LSB USB
1.4 kHz1.4 kHz
-1,4 kHz +1,4 kHz2.8 kHz
Example:
2635kHz (assigned 2636.4kHz) and
2638kHz (assigned 2639.4kHz) freqeucies
-
56Hilmi ECE GMDSS Instructor
Frequency Allacotions
Regions and Areas
For the allocation of frequencies the world has been
divided into three Regions' as shown on the following
map and described in below.
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor57
For the allocations of frequencies the world has been divided into
three Regions as shown on the map
-
Frequencies vs. Channels
In order to make radio communications more user
friendly, the ITU has started assigning both simplex and
duplex frequencies to predetermined channels. In the
past all communications had to be set for both the
transmitting and receiving frequencies, including on VHF
radios.
Hilmi ECE GMDSS Instructor58
RADIO THEORY & PROPAGATION
-
The ITU agreed and started assigning channels to frequencies.
Such as 156.8 MHz is assigned to channel 16.
On the modern VHF radios, there are setting for US and
International channels. These settings are for the need of the
two different sets of channel to frequency assignments. Many
of them are identical between the US and International, but
some are different. The difference is that the US uses more
simplex frequencies for communicating with CRSs and VTS
stations, where International settings are use more duplex
operations in these areas.
Hilmi ECE GMDSS Instructor59
RADIO THEORY & PROPAGATION
-
With the introduction of GMDSS and MF/HF communications
for mariners who may not be as familiar with all the different
frequencies and the different bands of the MF/HF system, it
is the goal if the ITU to improve the system to make it
friendlier like the VHF system. The goal was to be able to
use a channel number to reference the correct frequency.
Some manufactures had units that had the capability of
programming frequently used frequencies in to memory and
assign a quick reference number.
Hilmi ECE GMDSS Instructor60
RADIO THEORY & PROPAGATION
-
When looking in the different publications, the frequency list
is typically broken into three columns. The first column is
the Ship Receive or the Shore Transmit (either way it is
expressed, it is the receive frequency you would have to
program in). The third column is the Ship Transmit or Shore
Receive.
The middle column is typically the ITU channel.
Hilmi ECE GMDSS Instructor61
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor62
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor63
RADIO THEORY & PROPAGATION
-
Hilmi ECE GMDSS Instructor64
WAVELENGTH-TO-FREQUENCY CONVERSIONS
Radio waves are often referred to by their wavelength in
meters rather than by frequency. For example, most people
have heard commercial radio stations make announcements
similar to the following: "Station WXYZ operating on 240
meters..." To tune receiving equipment that is calibrated by
frequency to such a station, you must first convert the
designated wavelength to its equivalent frequency. As
discussed earlier, a radio wave travels 300,000,000 meters a
second (speed of light); therefore, radio wave of 1 hertz would
have traveled a distance (or wavelength) of 300,000,000
meters. Obviously then, if the frequency of the wave is
increased to 2 hertz, the wavelength will be cut in half
to150,000,000 meters. This illustrates the principle that the
HIGHER THE FREQUENCY, the SHORTERTHE
WAVELENGTH.
RADIO THEORY & PROPAGATION
-
Wavelength-to-frequency
conversions of radio waves
are really quite simple
because wavelength and
frequency are reciprocals:
Either one divided into the
velocity of a radio wave
yields the other.
Remember, the formula for
wavelength is:
Hilmi ECE GMDSS Instructor65
RADIO THEORY & PROPAGATION
The wavelength in meters divided into 300,000,000 yields
the frequency of a radio wave in hertz. Likewise, the
wavelength divided into 300,000 yields
the frequency of a radio wave in kilohertz, and the wavelength
divided into 300 yields the frequency in megahertz.