Antenna Propagation

Radio-Wave Propagation

• Radio waves, infrared, visible light, ultraviolet, X rays, and gamma rays are all different forms of electromagnetic radiation.

• The waves propagate as transverse electromagnetic waves (TEM) - i.e. the electric field, the magnetic field, and the direction of travel of the waves are all mutually perpendicular.

Transverse Electromagnetic Waves

x

y

z

Electric Field

Magnetic FieldDirection of Propagation

Speed & Wavelength of em Waves

• The speed of propagation and the wavelength () of an electromagnetic wave are given, respectively, by:

fvand

r

cv

where c = 3x108 m/s, r = medium’s relative permittivityor dielectric constant, and f = frequency of wave in Hz.

ReflectionRadio waves behave like light waves:• They reflect from a surface where the angle of

incidence, i = the angle of reflection, r . To minimize reflective losses, the surface should be an ideal conductor and smooth.

NormalIncidentRay

ReflectedRay

i rConductor

Refraction

• Radio waves will bend or refract when they go from one medium with refractive index, n1 to another with refractive index, n2. The angles involved are given by :

1

2

1

2

2

1

sinsin

r

r

nn

1

2n1<n2

where r = relativepermittivity of medium

Diffraction

• Diffraction is the phenomenon which results in radio waves that normally travel in a straight line to bend around an obstacle.

Direction of wave propagation

Obstacle

Refraction

If waves were not bent:

Propagation of Radio Wave• Propagation is concerned with the way that radio waves

travel between a transmitter (Tx) and a receiver (Rx) at some distant point.

• The radio frequency spectrum is divided into major bands, i.e.– VLF Very Low Frequency– LF Low Frequency– MF Medium Frequency– HF High Frequency– VHF Very High Frequency

MF

LF

VLF

Transmitter Aerial

Very Low Frequency

• 3 – 30 kHz• In this band the radio wave follows the curvature of

the earth’s surface and is known as a ground or surface wave.

• Given sufficient transmitter output power and high aerial arrays, world-wide communication is possible.

Very Low Frequency• Since there is not much bandwidth in this band of the radio

spectrum, only the very simplest signals are used, such as for radionavigation.

• VLF waves can penetrate water to a depth of roughly 10 to 40 m , depending on the frequency employed and the salinity of the water.

• VLF is used to communicate with submarines near the surface.

• VLF is also used for radio navigation beacons (alpha) and time signals (beta).

• VLF is also used in electromagnetic geophysical surveys.

Low Frequency

• 30 kHz – 300 kHz • In this band the radio wave again follows the curvature of the

earth’s surface, i.e. ground or surface wave. • However, because the frequency is now higher, the radio

wave is attenuated by the earth more quickly and so the range is reduced to approximately 1 to 2 thousand miles dependant upon transmitter output power.

• Loran C transmissions at 100 kHz, give reliable accurate ground wave coverage up to 1200 miles.

Low Frequency

• Used for– AM Broadcast service– LORAN– Weather system– Time signals

Medium Frequency

• 300 kHz – 3000 kHz • Uses ground or surface wave, but because the frequency is

now even higher, the range is reduced. • The actual range of communication now depends upon both

the transmitter output power and on the type of information being transmitted. – MF RT 2182 kHz 150 to 200 miles, – MF DSC 2187.5 kHz approximately 400 miles– Navtex 518 kHz

• The range on MF RT is less because the bandwidth is higher and therefore susceptible to attenuation.

High Frequency

• High Frequency– 3 MHz – 30 MHz– The HF band is so big that we tend to sub-divide it

into those which are used for maritime communications, i.e. 4, 6, 8, 12, 16 and 22 MHz.

Very High Frequency– 30 MHz – 300 MHz– On VHF, UHF and SHF bands, the radio waves

travel in straight lines and are known as direct or space waves, i.e. line of sight communication.

– The main consideration which determines the range obtainable is the height of both the receiving and transmitting aerials are above sea level, i.e. an increase in height gives an increase in range.

Types of Propagation• Ionospheric waves (sky waves): Main portion of the radiation that leaves the

antenna at angles above the horizon• Tropospheric waves: Radiation kept close to the earth’s surface due to bending in

the lower atmosphere (higher HF or lower VHF)• Ground waves (surface waves): Radiation directly affected by the earth’s surface - Earth-guided surface wave - Vertically polarized and absorbtion increases with freq - Travels much further over water than over land

Radio frequencies and their primary mode of propagationBand Frequency Wavelength Propagation via

ELF Extremely Low Frequency 3–300 Hz 1000-100,000 km

VLF Very Low Frequency 3–30 kHz 100–10 km Guided between the earth and the ionosphere.

LF Low Frequency 30–300 kHz 10–1 kmGuided between the earth and the D layer of the ionosphere. Surface waves.

MF Medium Frequency 300–3000 kHz 1000–100 mSurface waves. E, F layer ionospheric refraction at night, when D layer absorption weakens.

HF High Frequency (Short Wave) 3–30 MHz 100–10 m E layer ionospheric refraction. F1, F2 layer ionospheric refraction.

VHF Very High Frequency 30–300 MHz 10–1 mInfrequent E ionospheric refraction. Extremely rare F1, F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimes tropospheric ducting.

UHF Ultra High Frequency 300–3000 MHz 100–10 cm Direct wave. Sometimes tropospheric ducting.

SHF Super High Frequency 3–30 GHz 10–1 cm Direct wave.

EHF Extremely High Frequency 30–300 GHz 10–1 mm Direct wave limited by absorption.

Surface wave/Ground wave

Ground wave radio signal is made up from a number of constituents. If the antennas are in the line of sight then there will be a direct wave as well as a reflected signal.

As the names suggest the direct signal is one that travels directly between the two antenna and is not affected by the locality.

There will also be a reflected signal as the transmission will be reflected by a number of objects including the earth's surface and any hills, or large buildings. That may be present.

In addition to this there is surface wave. This tends to follow the curvature of the Earth and enables coverage to be achieved beyond the horizon.

It is the sum of all these components that is known as the ground wave.

Beyond the horizon the direct and reflected waves are blocked by the curvature of the Earth, and the signal is purely made up from the diffracted surface wave.

It is for this reason that surface wave is commonly called ground wave propagation.

Surface wave/Ground waveThe radio signal spreads out from the transmitter along the surface of the Earth.

Instead of just travelling in a straight line the radio signals tend to follow the curvature of the Earth.

This is because currents are induced in the surface of the earth and this action slows down the wave-front in this region, causing the wave-front of the radio communications signal to tilt downwards towards the Earth.

With the wave-front tilted in this direction it is able to curve around the Earth and be received well beyond the horizon.

Effect of frequencyAs the wavefront of the ground wave travels along the Earth's surface it is attenuated.

The degree of attenuation is dependent upon a variety of factors.

Frequency of the radio signal is one of the major determining factor as losses rise with increasing frequency.

As a result it makes this form of propagation impracticable above the bottom end of the HF portion of the spectrum (3 MHz).

Typically a signal at 3.0 MHz will suffer an attenuation that may be in the region of 20 to 60 dB more than one at 0.5 MHz dependent upon a variety of factors in the signal path including the distance.

In view of this it can be seen why even high power HF radio broadcast stations may only be audible for a few miles from the transmitting site via the ground wave

Effect of the groundThe surface wave is also very dependent upon the nature of the ground over which the signal travels.

Ground conductivity, terrain roughness and the dielectric constant all affect the signal attenuation.

In addition to this the ground penetration varies, becoming greater at lower frequencies, and this means that it is not just the surface conductivity that is of interest.

At the higher frequencies this is not of great importance, but at lower frequencies penetration means that ground strata down to 100 metres may have an effect.

Despite all these variables, it is found that terrain with good conductivity gives the best result. Thus soil type and the moisture content are of importance. Salty sea water is the best, and rich agricultural, or marshy land is also good.

Dry sandy terrain and city centres are by far the worst.

This means sea paths are optimum, although even these are subject to variations due to the roughness of the sea, resulting on path losses being slightly dependent upon the weather.

Effect of polarisationThe type of antenna has a major effect. Vertical polarisation is subject to considerably less attenuation than horizontally polarised signals.

In some cases the difference can amount to several tens of decibels.

It is for this reason that medium wave broadcast stations use vertical antennas, even if they have to be made physically short by adding inductive loading.

Ships making use of the MF marine bands often use inverted L antennas as these are able to radiate a significant proportion of the signal that is vertically polarised.

• Layers of the Atmosphere• The atmosphere can be split up into a variety of different

layers according to their properties. • Lowest is the troposphere that extends to a height of 10

km. • Above this at altitudes between 10 and 50 km is found

the stratosphere. This contains the ozone layer at a height of around 20 km.

• Above the stratosphere, there is the mesosphere extending from an altitude of 50 km to 80 km, and above this is the thermosphere where temperatures rise dramatically.

• There are two main layers that are of interest from a radio communications viewpoint.

• The first is the troposphere that tends to affect radio frequencies above 30 MHz.

• The second is the ionosphere. This is a region which crosses over the boundaries of the meteorological layers and extends from around 60 km up to 700 km. Here the air becomes ionised, producing ions and free electrons. The free electrons affect radio communications and radio signals at certain frequencies, typically those radio frequencies below 30 MHz, often bending them back to Earth so that they can be heard over vast distances around the world.

TroposphereThe lowest of the layers of the atmosphere is the troposphere. This extends from ground level to an altitude of 10 km. It is within this region that the effects that govern our weather occur. To give an idea of the altitudes involved it is found that low clouds occur at altitudes of up to 2 km whereas medium level clouds extend to about 4 km.

The highest clouds are found at altitudes up to 10 km whereas modern jet airliners fly above this at altitudes of up to 15 km.

Within the troposphere there is generally a steady fall in temperature with height and this has a distinct bearing on some radio propagation modes and radio communications that occur in this region. The fall in temperature continues in the troposphere until the tropopause is reached.

This is the area where the temperature gradient levels out and then the temperature starts to rise. At this point the temperature is around -50 ºC.

The refractive index of the air in the troposphere plays a dominant role in radio signal propagation and the radio communications applications that use tropospheric radiowave propagation. This depends on the temperature, pressure and humidity. When radio communications signals are affected this often occurs at altitudes up to 2 km

Description of the layers in the ionosphere

D layer: The D layer is the lowest of the layers of the ionosphere. It exists at altitudes around 60 to 90 km. It is present during the day when radiation is received from the sun. However the density of the air at this altitude means that ions and electrons recombine relatively quickly. This means that after sunset, electron levels fall and the layer effectively disappears. This layer is typically produced as the result of X-ray and cosmic ray ionisation. It is found that this layer tends to attenuate signals that pass through it.

E layer: The next layer beyond the D layer is called the E layer. This exists at an altitude of between 100 and 125 km. Instead of acting chiefly as an attenuator, this layer reflects radio signals although they still undergo some attenuation.

In view of its altitude and the density of the air, electrons and positive ions recombine relatively quickly. This occurs at a rate of about four times that of the F layers that are higher up where the air is less dense. This means that after nightfall the layer virtually disappears although there is still some residual ionisation, especially in the years around the sunspot maximum

F layer: The F layer is the most important region for long distance HF communications. During the day it splits into two separate layers. These are called the F1 and F2 layers, the F1 layer being the lower of the two. At night these two layers merge to give one layer called the F layer. The altitudes of the layers vary considerably with the time of day, season and the state of the sun. Typically in summer the F1 layer may be around 300 km with the F2 layer at about 400 km or even higher. In winter these figures may be reduced to about 300 km and 200 km. Then at night the F layer is generally around 250 to 300 km. Like the D and E layers, the level of ionisation falls at night, but in view of the much lower air density, the ions and electrons combine much more slowly and the F layer decays much less.

Accordingly it is able to support radio communications, although changes are experienced because of the lessening of the ionisation levels.

Most of the ionisation in this region of the ionosphere is caused by ultraviolet light, both in the middle of the UV spectrum and those portions with very short wavelengths.

Radiowave propagation in the troposphereFor frequencies at VHF and above different modes of propagation prevail.

Although some ionospheric modes may be experienced, the main effects are caused by changes in the troposphere.

On frequencies above 30 MHz, it is found that the troposphere has an increasing effect on radio signals and radio communications systems. The radio signals are able to travel over greater distances than would be suggested by line of sight calculations.

At times conditions change and radio signals may be detected over distances of 500 or even 1000 km and more. This is normally by a form of tropospheric enhancement, often called "tropo" for short.

At times signals may even be trapped in an elevated duct in a form of radio signal propagation known as tropospheric ducting.

This can disrupt many radio communications links (including two way radio communications links) because interference may be encountered that is not normally there.

The way in which signals travel at frequencies of VHF and above is of great importance for those looking at radio coverage of systems such as cellular telecommunications, mobile radio communications and other wireless systems as well as other users including radio hams.

Troposcatter propagation - troposcatter or tropospheric scatter is a form of radio signal propagation for radio communications links up to distances up to about 1000 km using the troposphere One useful form of radio communications technology for applications where path lengths of around 800 km are needed is known as tropospheric scatter or troposcatter.

It is a reliable form of radio communications link that can be used regardless of the prevailing tropospheric conditions.

Although reliable, when using troposcatter, the signal strengths are normally very low. Accordingly troposcatter radio communications links require high powers, high antenna gains and sensitive receivers.

Troposcatter is often used for commercial radio communications applications, normally on frequencies above 500 MHz for over the horizon links.

It is ideal for remote telemetry, or other links where low to medium rate data needs to be carried. Where viable, troposcatter provides a means of communication that is much cheaper than using satellites.

• Troposcatter basics• As the name implies, troposcatter uses the

troposphere as the region that affects the radio signals being transmitted, returning them to Earth so that they can be received by the distant receiver.

• Troposcatter relies on the fact that there are areas of slightly different dielectric constant in the atmosphere at an altitude of between 2 and 5 kilometres.

• Even dust in the atmosphere at these heights adds to the reflection of the signal.

• A transmitter launches a high power signal, most of which passes through the atmosphere into outer space.

• However a small amount is scattered when is passes through this area of the troposphere, and passes back to earth at a distant point.

• As might be expected, little of the signal is "scattered" back to Earth and as a result, path losses are very high. Additionally the angles through which signals can be reflected are normally small.

The area within which the scattering takes place is called the scatter volume, and its size is dependent upon the gain of the antennas used at either end.

In view of the fact that scattering takes place over a large volume, the received signal will have travelled over a vast number of individual paths, each with a slightly different path length.

As they all take a slightly different time to reach the receiver, this has the effect of "blurring" the overall received signal and this makes high speed data transmissions difficult.

It is also found that there are large short term variations in the signal as a result of turbulence and changes in the scatter volume. As a result commercial troposcatter propagation systems use multiple diversity systems. This is achieved by using vertical and horizontally polarised antennas as well as different scatter volumes (angle diversity) and different frequencies (frequency diversity).

Control of these systems is normally undertaken by computers. In this way troposcatter radio communications systems can run automatically giving high degrees of reliability.

Ionosphere

Ionosphere

The Blessings of Sky Wave• The medium for most all amateur radio

communication below 30 mhz• The ionosphere refracts the radio wave and returns

it to earth• The maximum usable frequency (MUF) is a function

of how highly ionized the F region is• The lowest usable frequency (LUF) is a function of

obsorbtion, signal-to-noise ratio, power and transmission mode; Correlates with movement of the sun and peaks at noon

Ground-Wave Propagation

• At frequencies up to about 2 MHz, the most important method of propagation is by ground waves which are vertically polarized. They follow the curvature of the earth to propagate far beyond the horizon. Relatively high power is required.

Direction of wave travel

IncreasingTilt

Earth

Ionospheric Propagation• HF radio waves are returned from the F-layer of the

ionosphere by a form of refraction.• The highest frequency that is returned to earth in the

vertical direction is called the critical frequency, fc.• The highest frequency that returns to earth over a

given path is called the maximum usable frequency (MUF). Because of the general instability of the ionosphere, the optimum working frequency (OWF) = 0.85 MUF, is used instead.

Sky-Wave Propagation

• From geometry (assuming flat earth):

d = 2hv tan i

• From theory (secant law):

MUF = fc sec i

i

hv

d

F-Layer

Earth

Sky-wave Propagation: Pros & Cons

• Sky-wave propagation allows communication over great distances with simple equipment and reasonable power levels : 100 W to a few kW.

• However, HF communication via the ionosphere is noisy and uncertain. It is also prone to phase shifting and frequency-selective fading. For instance, the phase shift and signal attenuation may be different for the upper and lower sidebands of the same signal. Data transmission is restricted to very low rates.

Space-Wave Propagation

• Most terrestrial communications in the VHF or higher frequency range use direct, line-of-sight, or tropospheric radio waves. The approximate maximum distance of communication is given by:

RT hhd 17where d = max. distance in km

hT = height of the TX antenna in mhR = height of the RX antenna in m

Space-Wave Propagation (cont’d)

• The radio horizon is greater than the optical horizon by about one third due to refraction of the atmosphere.

• Reflections from a relatively smooth surface, such as a body of water, could result in partial cancellation of the direct signal - a phenomenon known as fading. Also, large objects, such as buildings and hills, could cause multipath distortion from many reflections.

43

Tropospheric Ducting

Under certain conditions, VHF signals are “caught” in a duct of moist warm air, giving “propagation” over hundreds of miles.

44

Line of Sight PropagationLine of Sight PropagationWorldwideWorldwidecommunicationscommunicationsby line of sight isby line of sight isnot possible duenot possible dueto the curvatureto the curvatureof the Earthof the Earth

45

Sky wave Propagation (Skip)Sky wave Propagation (Skip)

Over the horizonOver the horizoncommunication iscommunication ispossible by sky-possible by sky-wave propagation,wave propagation,bouncing signalsbouncing signalsoff the ionosphere.off the ionosphere.

Occurs mostly at HF frequencies (less than 30 MHz).

46

VHF/UHF PropagationVHF/UHF Propagation• Generally line of sightGenerally line of sight• Can be blocked by and/or reflected off mountains Can be blocked by and/or reflected off mountains

and large buildings – even the Moon!and large buildings – even the Moon!• Temperature inversions in the troposphere can Temperature inversions in the troposphere can

cause “ducting,” and a path will open briefly for cause “ducting,” and a path will open briefly for 500 - 600 miles.500 - 600 miles.

• VHF/UHF will penetrate the Ionosphere, making VHF/UHF will penetrate the Ionosphere, making these frequencies ideal for satellite, and Earth-these frequencies ideal for satellite, and Earth-Moon-Earth (EME) operations.Moon-Earth (EME) operations.

47

VHF/UHF signalsVHF/UHF signalstravel only in straighttravel only in straightlines. We call thislines. We call this““line of sight”line of sight”propagationpropagation

Direct communications are notDirect communications are notpossible because of the mountainpossible because of the mountain

VHF/UHF PropagationVHF/UHF Propagation

48

How far can I talk with these radios?• There is never an exact

answer for any radio. Many factors come into play!– Geography– Frequency– Buildings– Distance

49

VHF & UHF• FM signals are subject to fading.

– Stronger signals trump weaker ones.• Line-of-Sight Propagation

– “Radio Horizon”• Signals are subject to multi-path distortion

– When signals hit objects, they can be refracted. Move the radio a bit and things might get better.

• Mobile stations are subject to “picket fencing.”– Rapid fluttering sound during a transmission.

How the Ionosphere is FormedHow the Ionosphere is Formed

The IonosphereThe Ionosphere

F2 Layer (Reflecting)F2 Layer (Reflecting)F1 Layer (Reflecting)F1 Layer (Reflecting)E Layer (Reflecting)E Layer (Reflecting)D Layer (Absorbing)D Layer (Absorbing)

The ionosphere is a particularly important region with regards to radio signal propagation and radio communications in general.

Its properties govern the ways in which radio communications, particularly in the HF radio communications bands take place.

The ionosphere is a region of the upper atmosphere where there are large concentrations of free ions and electrons.

While the ions give the ionosphere its name, but it is the free electrons that affect the radio waves and radio communications.

In particular the ionosphere is widely known for affecting signals on the short wave radio bands where it "reflects" signals enabling these radio communications signals to be heard over vast distances.

Radio stations have long used the properties of the ionosphere to enable them to provide worldwide radio communications coverage. Although today, satellites are widely used, HF radio communications using the ionosphere still plays a major role in providing worldwide radio coverage.

The ionosphere extends over more than one of the meteorological areas, encompassing the mesosphere and the thermosphere, it is an area that is characterised by the existence of positive ions (and more importantly for radio signals free electrons) and it is from the existence of the ions that it gains its name.

Basics•The free electrons do not appear over the whole of the atmosphere. Instead it is found that the number of free electrons starts to rise at altitudes of approximately 30 kilometres.

• However it is not until altitudes of around 60 to 90 kilometres are reached that the concentration is sufficiently high to start to have a noticeable effect on radio signals and hence on radio communications systems. It is at this level that the ionosphere can be said to start.

•The ionisation in the ionosphere is caused mainly by radiation from the Sun. In addition to this, the very high temperatures and the low pressure result in the gases in the upper reaches of the atmosphere existing mainly in a monatomic form rather than existing as molecules.

• At lower altitudes, the gases are in the normal molecular form, but as the altitude increases the monatomic forms are more in abundance, and at altitudes of around 150 kilometres, most of the gases are in a monatomic form.

•This is very important because it is found that the monatomic forms of the gases are very much easier to ionise than the molecular forms.

IonisationThe Sun emits vast quantities of radiation of all wavelengths and this travels towards the Earth, first reaching the outer areas of the atmosphere.

In creating the ionisation it is found that when radiation of sufficient intensity strikes an atom or a molecule, energy may be removed from the radiation and an electron removed, producing a free electron and a positive ion.

In the example given below, the simple example of a helium atom is give, although other gases including oxygen and nitrogen are far more common.

Ionisation of molecules by solar radiation

Ionospheric layersthe ionosphere indicates a number of distinct layers, each affecting radio communications in slightly different ways. Indeed, the early discoveries of the ionosphere indicated that a number of layers were present.

Ionisation exists over the whole of the ionosphere, its level varying with altitude

. These regions are given letter designations: D, E, and F regions.

There is also a C region below the others, but the level of ionisation is so low that it does not have any effect radio signals and radio communications, and it is rarely mentioned.

D Region•The D region is the lowest of the regions within the ionosphere that affects radio communications signals to any degree.

•It is present at altitudes between about 60 and 90 kilometres and the radiation within it is only present during the day to an extent that affects radio waves noticeably.

• It is sustained by the radiation from the Sun and levels of ionisation fall rapidly at dusk when the source of radiation is removed. It mainly has the affect of absorbing or attenuating radio communications signals particularly in the LF and MF portions of the radio spectrum, its affect reducing with frequency.

•At night it has little effect on most radio communications signals although there is still a sufficient level of ionisation for it to refract VLF signals.

E Region•The region above the D region is the E region. It exists at altitudes between about 100 and 125 kilometres.

• Instead of attenuating radio communications signals this layer chiefly refracts them, often to a degree where they are returned to earth. As such they appear to have been reflected by this layer. However this layer still acts as an attenuator to a certain degree.

•Like the D region, the level of ionisation falls relatively quickly after dark as the electrons and ions re-combine and it virtually disappears at night.

•However the residual night time ionisation in the lower part of the E region causes some attenuation of signals in the lower portions of the HF part of the radio communications spectrum.

•The ionisation in this region results from a number of types of radiation. Soft X-Rays produce much of the ionisation, although extreme ultra-violet (EUV) rays (very short wavelength ultra-violet light) also contribute.

F Region•The most important region in the ionosphere for long distance HF radio communications is the F region.

•During the daytime when radiation is being received from the Sun, it often splits into two, the lower one being the F1 region and the higher one, the F2 region. Of these the F1 region is more of an inflection point in the electron density curve (seen above) and it generally only exists in the summer.

•Typically the F1 layer is found at around an altitude of 300 kilometres with the F2 layer above it at around 400 kilometres. The combined F layer may then be centred around 250 to 300 kilometres.

•The altitude of the all the layers in the ionosphere layers varies considerably and the F layer varies the most. Being the highest of the ionospheric regions it is greatly affected by the state of the Sun as well as other factors including the time of day, the year and so forth.

•The F layer acts as a "reflector" of signals in the HF portion of the radio spectrum enabling world wide radio communications to be established. It is the main region associated with HF signal propagation.

Like the D and E layers the level of ionisation of the F region varies over the course of the day, falling at night as the radiation from the Sun disappears.

However the level of ionisation remains much higher.

The density of the gases is much lower and as a result the recombination of the ions and electrons takes place more slowly, at about a quarter of the rate that it occurs in the E region.

As a result of this it still has an affect on radio signals at night being able to return many to Earth, although it has a reduced effect in some aspects.

The F region is at the highest region in the ionosphere and as such it experiences the most solar radiation

Summary•The ionosphere is a continually changing area of the atmosphere.

• Extending from altitudes of around 60 kilometres to more than 400 kilometres it contains ions and free electrons.

•The free electrons affect the ways in which radio waves propagate in this region and they have a significant effect on HF radio communications.

•The ionosphere can be categorised into a number of regions corresponding to peaks in the electron density. These regions are named the D, E, and F regions.

•the radiation from the Sun is absorbed as it penetrates the atmosphere, different forms of radiation give rise to the ionisation in the different regions

Radio Wave Propagation

Maximum Usable Frequency• MUF stands for the Maximum Usable Frequency for communications

between two points

• The 15-meter band should offer the best chance for a successful contact if the maximum usable frequency (MUF) between the two stations is 22 MHz.

• The 20-meter band should offer the best chance for a successful contact if the maximum usable frequency (MUF) between the two stations is 16 MHz.


Critical & Maximum Usable Frequency• The frequency at which a signal sent

vertically will pass right through the ionosphere is called the critical frequency.

• The frequency at which communication just starts to fail is known as the Maximum Usable Frequency (MUF). It is generally three to five times the critical frequency, dependent upon the layer being used and the angle of incidence.


Maximum Usable Frequency (cont)

• For lowest attenuation when transmitting on HF, select a frequency just below the MUF.

• A reliable way to determine if the maximum usable frequency (MUF) is high enough to support 28-MHz propagation between your station and Western Europe is to listen for signals on a 28 MHz international beacon.

• Radio waves with frequencies below the maximum usable frequency (MUF) are usually bent back to the Earth after they are sent into the ionosphere.

Radio Wave

Maximum Usable Frequency (cont)

• The factors that affect the maximum usable frequency (MUF) are: Path distance and location Time of day and season Solar radiation and ionospheric disturbance

[All of these choices are correct]

Lowest Usable Frequency• LUF stands for the Lowest Usable Frequency for communications between

two points.• Radio waves with frequencies below the lowest usable frequency (LUF) are

usually completely absorbed by the ionosphere


Propagation “hops”• The maximum distance along the Earth's surface that is normally covered in

one hop using the F2 region is 2,500 miles.

• The maximum distance along the Earth's surface that is normally covered in one hop using the E region is 1,200 miles.


Propagation “hops” (cont)

• When the lowest usable frequency (LUF) exceeds the maximum usable frequency (MUF), no HF radio frequency will support communications over the path.

• A sky-wave signal will sound like a well-defined echo when it arrives at your receiver by both short path and long path propagation.

• Short hop sky-wave propagation on the 10-meter band is a good indicator of the possibility of sky-wave propagation on the 6-meter band.


Ionospheric layers• The D layer of the ionosphere is closest to the surface of the Earth

• The ionospheric D layer is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz

• The F2 region be expected to reach its maximum height at your location at noon during the summer

• The F2 region is mainly responsible for the longest distance radio wave propagation because it is the highest ionospheric region.

• Ionospheric Absorption will be minimum near the maximum usable frequency (MUF).


Critical angle and frequency• The term “critical angle” means the highest takeoff

angle that will return a radio wave to the Earth under specific ionospheric conditions.


HF Scatter

• Long distance communication on the 40, 60, 80 and 160-meter bands are more difficult during the day because the D layer absorbs these frequencies during daylight hours.


HF Scatter (cont)

• HF scatter signals often sound distorted because energy is scattered into the skip zone through several radio wave paths.

• A characteristic of HF scatter signals is that they have a wavering sound.

Tropo Scatter Meteor Scatter


HF Scatter (cont)

• The HF scatter signals in the skip zone are usually weak because only a small part of the signal energy is scattered into the skip zone.

• Scatter radio wave propagation allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky wave propagation.

• An indication that signals heard on the HF bands are being received via scatter propagation can be when the signal is heard on a frequency above the maximum usable frequency.


Near Vertical Incidence Sky waves• Near Vertical Incidence Sky-wave (NVIS), propagation is short distance HF

propagation using high elevation angles


Near Vertical Incidence Sky waves


Dipole placement• A horizontal dipole antenna placed between 1/8 and 1/4 wavelength

above the ground will be most effective for skip communications on 40 meters during the day.

⅛ - ¼ wavelength above ground


Dipole placement

Layers of the IonosphereLayers of the Ionosphere• D Layer, Absorbing, Disappears at nightD Layer, Absorbing, Disappears at night• E Layer, Reflecting, Disappears at nightE Layer, Reflecting, Disappears at night• F1 and F2 Layers, Reflecting, combine into a F1 and F2 Layers, Reflecting, combine into a

single F layer at night.single F layer at night.• The reflective layers are responsible for sky The reflective layers are responsible for sky

wave propagation.wave propagation.

Sporadic-E

• There are two natural phenomena that can propagate VHF signals over long distances.

• Sporadic E occurs when the E-layer reflects VHF signals!

Fading• Case of more than one propagation path (mode) exists between T and R

• Fading = the result of variation (with time) of the amplitude or relative phase, or both,of one or more of the frequency components of the signal.

• Cause: changes in the characteristics of the propagation path with time.

Antenna Propagation

Documents

Transcript of Antenna Propagation