the radio propagation

8/13/2019 The Radio Propagation

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At left, a wave. It is constituted of an electric (E) and a magnetic (B) fields whichamplitudes evolve together. One of the field taken in isolation cannot create this special

wave structure. At right, contrarily to a scalar field represented by dots without orientation,waves belong to the family of vectorial fields. That means that like the magnetic field of amagnet here displayed in white on this picture, outside the radiation field of an antenna,the field strength is null and you cannot pick up its emissions. Therefore users of beams

have to steer their antenna towards the country they want to work and listeners have

advantage in using conductors as long as they can to pick up the more signal they can.Below a wave (electric or magnetic) travelling in a plan. It is characterized by its frequency

or wavelength (period) and its amplitude.

Radio waves are a form of electromagnetic radiation sensible to charged particles like

free electrons. In free space they travel in straight line (in fact following geodesics) at thevelocity of light (300000 km/s) and reduce a bit in denser medium. Their intensity is

defined in volts per meter (practically in V/m), in effective or peak values like AC current

or by reference to the signal strength expressed indBan other dBW unit.

The wavelength is defined as the distance between two points of equal phase or period

taken as unit of time measurement. It is also defined as the ratio between the velocity of the

wave to the current field frequency (f). For a free space wave the wavelength is :

Radio frequencies are ranging from a few hertz, wavelengths of several thousands of kmfrom peak-to-peak for brain waves, subsonic and oscillating at less than one cycle per

second, to several thousands of gigahertz, wavelengths of a few mm from peak-to-peak for

microwaves. Above we enter in the world of light (IR, visible, UV, X-ray and gamma).

The electromagnetic spectrum from 31.2 mHz to 6.52 EHzA poster in PDF format to download designed byAnthony Tekatch (722 KB)

This spectrum is divided in octaves, the natural way to represent frequencies. An octave

represents eight diatonic degrees or a gradual frequency increasing of a 10-factor. Humans

can heard sounds (vibrations) between ~20 Hz and ~20 kHz, a range of 3 octaves. Their

wavelengths is ranging between 1500 and 15 km.
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The electromagnetic spectrum is also arbitrary divided into "bands", among them next

ones are the most important in the context of radio propagation :

Radio bands

Band Frequency Wavelength Energy Applicationys 30.3 PHz - 3 EHz 10 - 0.1 nm 125 - 12.5 keV X-ray machines, sunflare

eme ultraviolet, EUV 4.5 - 30.3 PHz 70 - 10 nm 18 - 125 eV UV, ionosphere ionization

le (red - violet) 398 - 750 THz 800 - 400 nm 1.6 - 3.1 eV Visible spectrum, light

er High Frequency, SHF 3 - 30 GHz 10 - 1 cm 13 - 132 eV Microwaves, satellite, Ham

High Frequency, UHF 300 MHz - 3 GHz 100 - 10 cm 1.1 - 13 eV Microwaves, GSM, Ham

High Frequency, VHF 30 - 300 MHz 10 - 1 m 132 neV - 1.1 eV FM Radio, Avi, Ham

Frequency, HF 3 - 30 MHz 100 - 10 m 13 - 132 neV SW Radio, Ham

um Frequency, MF 300 kHz - 3 MHz 1000 - 100 m 1.3 - 13 neV AM Radio, SW, HamFrequency, LF 30 - 300 kHz 10 - 1 km 120 peV - 1.3 neV Beacons, AM, LW Radio

Low Frequency, VLF 3 - 30 kHz 100 - 10 km 13 - 120 peV Sound, Navy, geophysics

eme Low Frequency, ELF 30 Hz - 3 kHz 10000 - 100 km 125 - 13 peV Sound, power, Navy

Each band extends over 3 octaves or so, the energy level increasing of about 10 times

between the beginning and the end of the band. Natural radiation becomes a health hazard

only from the UV light and above frequencies, even though, because all depends on the

duration of exposure to the radiation, the distance to the source, and its intensity. Refer to

the page dealing with EM radiations andhealth hazard,and well as to my pages dealing

withradioactivity(in French) for more detail.

ELF are only used by some submarines and to carry AC over power lines. Otherwhise, its

main use is of course to carry the sound of low and mid frequencies as well infrasonic

vibrations (animals). VLF are also the carrier of sound up to about 20 kHz. This band isalso used for long distance communications (few thousands km) and experimentation by

scientists and the Navy.

LF are mainly used for regional broadcasting purposes while MF are used for worldwide

broadcasting. HF are of our concern, these are formely frequencies ranging from 1.8 to 30

MHz (160-10 m bands). Know as "shortwaves", these bands are very appreciated by all

radio services and operators as they allow long distance communications, broadcasting and

trans-horizon radar operations.

VHF and UHF begin at

30 MHz (10 m) to end well

above 1 GHz and aremainly used for radio and

TV broadcasting as well as

mobile communications

over short distances (a few

hundreds km) and more

recently by cell phones.

Above these frequencies

we find centimetric and

millimetric waves, the

famous microwaves. We

know them essentiallyThe electromagnetic spectrum.
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through home devices like microwave ovens (Short or S-band), wireless LAN (compromise

or C-band), and some satellite and radar transmissions (Kurtz or K-bands).

Then close your ears and open your eyes, you enter in the near infrared and visible parts

of the spectrum ! Over it, wear your anti UV-glasses to protect you against ultraviolet

radiations. At last take your lead protection, we enter the world of X and rays.

As we see, most services work in the lower bands of the electromagnetic spectrum, the

only one frequencies able to transport information on long distances with a very simple

technology and low energy.

Each band requests special receivers and aerials according to the frequency used and the

type of waves (ground, space, ionospheric, etc). However these waves are affected by the

medium in which they propagate, its electronic density and its dielectric constant.

Alterations of radio wavesIf waves travel in straight line and at the velocity of light in free space, on Earth, the

ground, the air and the ionosphere affect wave propagation; radio waves do no more travel

from one point to another in straight line and their signals are often altered. The fading isprobably the alteration that you all know if you have listened to shortwaves or old records

from World War II.

Radio waves are mainly subject to four effects :

- Attenuation: Like the light, the intensity of the field decreases in following the

inverse-distance law : when the distance double, the signal becomes half less strong.

This is true in free space but on earth this attenuation is much stronger due to obstacles

placed between emitter and receiver and to the fact that travelling around the earth radio

waves lost their energy as they forced to bend to follow the earth curvature.

A second effect is related to ground properties. A poor ground (low conductivy and

dielectric constant) affects also performances of vertically polarized antennas due to thePseudo-Brewster Angle (PBA), a situation similar to the one we experiment when the

sun is low and its light reflects from the water's surface as glare, obscuring the

underwater view. The reflection coefficient can exceed 90% at 15 of elevation (21

MHz). Horizontally polarized antennas are differently affected and show an attenuation

factor that never exceed 42% (21 MHz).

At last, RF currents emitted by an antenna can more or less penetrate the ground at

some frequencies. For example in fresh water (lakes), RF currents penetrate up to 50mdepth (156 ft), near independent of the frequency below 30 MHz. In seawater on the

contrary, the depth is ranging between 18 and 5 cm only (7-2") between 1.8-30 MHz.

On pastoral areas (medium hills, forestation, etc) the depth is ranging between 50 and

30 cm (20-12") over the same spectrum.

- Reflection: this phenomenon, very similar to its optical counterpart, appears when a

wave enters in contact with a surface more or less thick or dense. Long wavelengths,

from 80 meters long and above don't practically "see" small obstacles like cars, trees or

buildings. Indeed these objects are proportionally too small in regards with the wave

and can't reflect its energy. The long waves pass thus across these materials without be

reflected. Due to its large surface, long waves are however reflected by the ground but

can penetrate it up to some meters depth. V/UHF waves (2m and 70 cm long) are on the

contrary very sensitive to small obstacles. Depending of their thickness metal objects

can be used as reflectors.

A second type of reflection is induced by the change of dielectric constant of themedium in wich waves travel. Like a semi-transparent glass, depending the incident


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angle, some waves will be totaly reflected back while others coming from a lower

incidence will enter the new medium without be subject to any reflection.

- Refraction: this is the main reason of the bending of waves that occurs when they pass

through a medium (air or ionosphere) having a different dielectric constant from the

medium they have just left. These differences produce variation in the velocity of

waves that tend to go further or dropping sooner that expected. Due to this effect waveschange of direction like the sun light is refracted near the horizon, displacing its

apparent position. In the air as the boudary between two areas of differing dielectric

constant changes more slowly than the refraction index of the air for example, the wave

refracts and bend gradually given the

appareance that the path is curved.

- Diffraction: Let' take for

comparison its optical counterpart.

At firt sight the shadow cast by a

small object under strong light is

very sharp. But examined closely, wecan see that the shadow borders are

not at all sharp. In fact due to its high

frequency the light bends around the

edge of the object and tends to make

the borders of it shadow lighter. That

means that some light reaches well

some places that we considered as

plunged into darkness. The same

effect applies to radio waves. A spot

located out of sight from a transmitter, say behind a hill, can receive weakly its

emissions because its signals are bending gradually by diffraction and can reach theremote receiver.

This effect has practically no influence in HF because waves arrive usually to the

receiver by many other means such as refraction or reflection in the upper

atmosphere, including sometimes ground waves if the transmitter is not too far (say

150-200 km away).

Note that if you live near the bottom of a valley, there are some chance that you had a hill

or a montain range just behind your house. If the relief is high and the landscape very close,

in HF and upper bands this direction will be simply blocked up for Yagi's. You are

"condemned" in using a high-end vertical antenna that, thanks to its omnidirectional pattern

and vertical polarization will help you in jumping over this obstacle.Interfering together, all these effects tend to replace the fine straight path followed by

radio waves by a sort of large undulating and curved beam that widen as the distance and

frequency increase and scatters in the atmosphere just like the light. This is even all benefit

for radio amateurs that can receive by these means signals under conditions as unexpected

as behind hills or thanks to atmospheric ducting or auroral events, other modes of traffic

that we will review on the next pages.

Propagation of wavesThe success or the failure of a radio transmission depends on the way that radio signals

travel around the earth. Basically there are four types of propagation :

- Ground waves: also called evanescent or surface waves, these waves propagate along

the earth surface, close to the ground, and never reach the ionosphere. Typically signals

Sometimes, effects of diffraction help to receiveradio waves in areas located in the "shadow" of

obstacles like behind a hill. Signals will be weak butreadable.


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carried by ground waves can be heard up to a distance of 160 km or more during the

daytime. They are however subject to a high attenuation throughout HF bands to reach

distances less than 15 km at 30 MHz.. Therefore these surface waves are mainly used at

low frequencies below 1.8 MHz (MW, LW and VLF) by geophysicists and the

U.S.Navy (submarines).

- Tropospheric waves: below 10 km or so of the atmosphere, where weather patternsand temperature inversions form, VHF can be refracted permitting short distances

contact (a few thousands km). This activity will be shortly discussed as well as the

atmospheric ducting, also induced by temperature inversions.

- Space waves: these waves travel directly from an antenna to another without reflection

on the ground. This phenomenon occurs when both antennas are within line of sight of

each another. The distance to the horizon is given by the next formula :

D (km) = 4.124H

where H is the heigth in meters of the antenna above ground.

This distance is longer that the line of sight because most space waves bend near theground and follow practically a curved path. In the field we must also add the effects of

the atmospheric refraction and diffraction near the earth surface that extend this

distance of about 20% in the lowest bands. On V/UHF on the contrary, diffraction is

very small and signals tend to drop off quite rapidely at a shorter distance. In this way

of propagation antennas must display a very low angle of emission in order that all the

power is radiated in direction of the horizon instead of escaping in the sky. A high gain

and horizontally polarized antenna is thus highly recommended.

At left in deep blue space wave and ground wave travelling from one antenna to another. In light bluepaths of sky waves in the upper atmosphere. Above the critical angle, these ionospheric waves

escape into space while waves emitted under a low incidence angle reflect to the ground one or moretimes (hops) to reach at the end far countries. At rightPropLab Pro,an advancedionospheric model

simulating 3D pathes of sky waves between 2 stations.

- Sky waves: They essentially concern frequencies below 30 MHz (longer than 10

meters) and V/UHF in a less extent that are able to escape into free space (that begins

over 800 km aloft). Called sky waves these waves are however stopped in their travel

by the ionospheric layers and, under low incidence angles, they are reflected to the

ground. These waves are then calledionospheric waves. They are very influenced by

the presence of electrons gas and plasma in the upper atmosphere of the Earth. Under

certain conditions these layers reflect or refract shortwaves, permitting amateurs to

reach stations located on the other side of the Earth in a succession of jumps betweenthe ground and the ionosphere, called multihops. We will develop this subject in depth
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on the next page as they are the most used by radio amateurs. In another article we will

deal aboutperturbationsaffecting sky waves propagation in the ionosphere.

- Free space waves: they are the most common but the less used ! We encounter them

working in VHF or UHF where, due to their very high frequency, at incidence angles

higher that the critical angle, shortwaves escape into space instead of be reflected by

ionospheric layers. This way of propagation is sometimes welcome to work with anham satellite in polar orbit or with ISS which signals pass through ionospheric layers.

As we told, the propagation that is our concern - amateurs and listeners - are sky waves

and mainly the ones propagating in HF bands. However, to be complete we will do a short

excursion in the higher bands as well

Next chapter

Discovery of the ionosphere

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