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



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).



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.



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



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.



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.



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



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



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



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.



Approximate guide frequency bands and ranges (for 100 watt commercial services)

14Hilmi ECE GMDSS Instructor




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


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.




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.



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.



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.



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.


RADIO THEORY & PROPAGATIOND Layer : 20-500kHz,

E Layer : 500-2.000kHz,

F1 Layer : 1.500-30.000kHz,

F2 Layer : 3.000-30.000kHz



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.



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.



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.



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.


On the nomenclature



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.



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.




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


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



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.



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.



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).



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.



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.



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.


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.



SINGLE SIDE BAND

WINDOW

FREQUENCY

LSB USB

DSB (AM) MODULATION

1.4 kHz1.4 kHz

SSB

-2.8 kHz +2.8 kHz


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


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.



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.



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.



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.



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.




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.


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:



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.

2 Propagation Modulation

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