PROPAGATION OF RADIO WAVES
An undisturbed radio wave in space will travel in straight line and at a constant speed. However, the earth’s surface and atmosphere is a mix of uneven mass of solids and liquids, surrounded by a mixture of gases with varying density and even electrical charge (ions). All of these factors affect the ‘propagation’ of the waves to a greater or lesser effect.
General Properties of Radio Waves. The general properties of radio waves are:
- In a given medium, radio waves travel at a constant speed.
- When passing from one medium to another of different refractive index, the velocity of the waves changes. The waves are also deflected towards the medium of higher refractive index, that is, they change their direction.
- Radio waves are reflected by objects commensurate with their wave lengths.
- Uninfluenced, radio waves travel in a straight line.
The whole electromagnetic spectrum includes radiation in the form of light, X-rays and gamma rays, but radio waves comprise only the bottom end of the complete spectrum. Voice frequencies fall immediately below this radio spectrum, but sound waves are actually pressure waves and are propagated differently from electromagnetic waves.
The Surface of Earth
The earth is approximately a sphere. This means that the horizon curves away with distance from the transmission point, and if the radio waves travelled only in straight lines, the reception ranges would be limited to ‘optical’ distance only. This distance is given by the formula:
D = 1.05 √H
Where D is the range in nm and H is the height of transmitter in feet AMSL.
However Radio waves do curve along the surface of earth and also in the atmosphere, therefore the above formula is just a theoretical one.
Earth is formed by a mixture of many things like, rocks, water, sand, minerals etc. These materials affect the propagation of radio waves by reflecting them from their surface or by absorbing their energy. The absorption of energy will depend upon the conductivity of the surface over which the radio wave is travelling. On the surface of the earth this happens mostly to the lower frequency band of Radio waves. They lose energy in inducing current to the surface of earth and in turn slow down. this process is called Attenuation of radio waves. As mentioned above this surface attenuation will depend not only on frequency but also on conductivity of the surface. The conductivity of the earth’s surface itself varies, sea water provides a medium of high conductivity whereas the conductivity of the land surface depends on its composition. It is fairly high where the soil is rich in minerals & very poor in the sands of a desert or the polar ice caps. Similarly, dry sand produces greater attenuation and speed reduction than wet loam, and sea water produces less attenuation than either of them.
Radio waves (ElectroMagnetic Waves) when transmitted will travel in all directions equally ( Omni Directional Antenna), some of it will travel along the surface of the earth. These waves travelling along the surface are called ‘surface waves’ or ‘ground waves’. If there are no obstruction, radio waves will travel in straight line as per its property, however, under appropriate conditions they tend to follow the earth’s surface giving us increased ranges.
Diffraction and Attenuation.
Two factors result in the curving of radio waves along the surface of earth. They are
Radio waves tend to be reflected by objects larger than about half their wavelength. At higher frequencies, most obstacles will cause reflection, or absorption because of their small wavelengths, and therefore there will be shadows behind these obstructions, but at lower frequencies the waves will curve around a small obstacle, even a hill. This curving of radio waves around the corners is called ‘diffraction’. The amount of diffraction is inversely proportional to the frequency. Radio waves can also be redirected by scattering between molecules in the atmosphere, and reflected from neighbouring solid objects. At small wavelengths an upstanding obstacle stops wave front, causing a shadow behind it. It is because of this effect that low frequency broadcasts give good field strength behind a range of hills but there is no reception when going under a railway bridge.
This bending downward is further assisted (the other factor) by the fact that as a part of the waveform comes in contact with the surface it includes currents in it, thereby losing some of its energy and slowing down. This is called surface attenuation. This slowing down of the bottom gives the waveform a forward and downward tilt encouraging it to follow the earth’s curvature
Thus, bending due to diffraction and tilting due to attenuation (imperfect conductivity of the surface) cause the waves to curve with the surface. Waves continue until they are finally attenuated, that is, become undetectable. Attenuation, in its turn, depends on the following factors:
- Type of Surface. Different surfaces have different conductivities. For a given transmission power a radio wave will travel a longer distance over the sea than over dry soil.
- Frequency in Use. The higher the frequency, the greater the attenuation
- Polarisation of Radio Waves. Vertically polarised waves are normally used with minimum attenuation.
To summarise the ground ranges expected from frequencies in various frequency bands.
- VLF Attenuation is least, maximum bending is due to diffraction. Given sufficient power, ranges of several thousand nm may be obtained.
- LF Attenuation is less and the signals will bend with the earth’s surface and ranges to a distance of 1500 nm may be expected.
- MF Attenuation is now increasing, signals still bend with the surface and the ranges are approximately 300 to 500 nm, maximum is 1000 nm over the sea.
- HF Severe attenuation, bending is least. The maximum range obtainable due to surface waves is around 70 to 100 nm.
- VHF and Above. The signals do not bend and the radio waves travel in a straight line giving line-of-sight ranges.
Disadvantages of Low Frequencies
Although low frequencies produce very long ranges, there are considerable drawbacks which prohibit their use. These drawbacks are:
(a) Low Efficiency Aerials. Ideally the length of the transmitter and receiver aerials should each be equal to the wavelength. An aerial approximately half the size of the wavelength is also considered to be suitable for satisfactory operation. Any further reduction in the aerial size would result in a loss of efficiency. The largest aerials are found in the lowest frequency band i.e. VLF.
(b) Static. Static is severe at lower frequencies and additional power must be supplied to combat its effect. The effect of static decreases as the frequency is increased. VHF is considered to be practically free from static.
(c) Installation and Power. The cost of initial installation is high and subsequent power requirement to maintain the desired range, giving satisfactory reception, is very large. It should be noted that the range of a surface wave varies as the square of its power
The range of a signal (surface waves) therefore is inversely proportional to its frequency, or directly proportional to its wavelength, as well as being directly proportional to the power at the transmitter. Surface waves are the primary means of propagation in the MF band, virtually the sole means of propagation in the LF band and lower frequencies, and the means of transmitting HF signals to receivers outside the range of direct waves but too close to receive sky waves.
- ABOVE THE TROPOPAUSE LIES THE STRATOSPHERE, AND ABOVE THAT A REGION CALLED THE IONOSPHERE. HERE RADIATION FROM THE SUN HAS A CONSIDERABLE EFFECT ON THE MOLECULES OF A THIN ATMOSPHERE, AND ELECTRONS ARE SET FREE FROM THEIR ATOMS. THE FREE ELECTRONS PROVIDE SEVERAL ELECTRICALLY CHARGED LAYERS IN THIS IONOSPHERE, BUT THEIR EXISTENCE DEPENDS ON EXCITATION FROM THE SUN’S RAYS. THE NUMBER OF FREE ELECTRONS, AND THEIR DISTRIBUTION, DEPEND ON THE ANGLE AT WHICH THE SUN’S RAYS MEET THE IONOSPHERE, AS WELL AS THE INTENSITY OF THE RAYS THEMSELVES.
- THE LAYERS WERE DISCOVERED BY THEIR EFFECT ON RADIO WAVES,THE DENSITY OF FREE ELECTRONS CHANGES, IT CHANGES THE ‘REFRACTIVE INDEX’ OF THE AIR. ELECTROMAGNETIC WAVES PASSING THROUGH THE LAYERS IN THE IONOSPHERE AT AN ANGLE ARE REFRACTED, OR BENT, AWAY FROM AREAS OF HIGHER ELECTRON DENSITY, WHICH HAPPEN TO BE IN THE HIGHER PART OF THE IONOSPHERE.
- THE AMOUNT OF REFRACTION DEPENDS ON THREE FACTORS VIZ. THE FREQUENCY OF THE WAVES, THE CHANGE IN ELECTRON DENSITY, AND THE ANGLE AT WHICH THE WAVES HIT THE LAYER. THE WAVES ARE ALSO ATTENUATED, BY AN AMOUNT DEPENDING ON THE ELECTRON DENSITY AND THE FREQUENCY.
- THE D LAYER IS GENERALLY REGARDED AS BEING BETWEEN 50 AND 100 KM ABOVE THE SURFACE OF THE EARTH, WITH AN AVERAGE ALTITUDE OF 75 KM.
- THE E LAYER EXISTS BETWEEN 100 AND 150 KM, WITH AN AVERAGE ALTITUDE OF 125 KM.
- THE F LAYER SPREADS BETWEEN 150 AND 350 KM, WITH AN AVERAGE ALTITUDE OF 225 KM.
- DURING THE DAY F LAYER APPEARS TO SPLIT INTO TWO LAYERS, THE LOWER ONE BEING CALLED F1 LAYER AND THE UPPER LAYER AS F2.
- THE D LAYER, WHERE AIR DENSITY IS HIGH, AND ELECTRON DENSITY IS COMPARATIVELY LOW, TENDS TO ABSORB RADIO WAVES RATHER THAN REFRACT THEM.
- THE E LAYER, WITH GREATER ELECTRON DENSITY OF UP TO 105 / CM3 AND LESS AIR DENSITY, PRODUCES SOME REFRACTION OF WAVES IN THE HF BAND, AND THE F LAYERS WITH EVEN LOWER AIR DENSITY AND HIGHER ELECTRON DENSITY (UP TO 106 / CM3 ) DO MOST OF THE REFRACTING.
- WAVES REFRACTED AT LOW LEVELS WILL BE REFRACTED FURTHER AT HIGHER LEVELS, PROVIDED THEY ARE NOT ABSORBED BEFORE THEN. THE REFRACTION OF ELECTROMAGNETIC WAVES IN THE IONOSPHERE CAN BE SUFFICIENT TO BEND A SIGNAL SENT SKYWARD DOWN TOWARDS THE EARTH AGAIN.
- WE USE THIS FACILITY IN HF COMMUNICATION, BUT IT CAN CAUSE PROBLEMS WHEN USING MF NAVIGATION AIDS.
- THE ANGLE AT WHICH A RADIO SIGNAL STRIKES A LAYER IS A MAJOR FACTOR IN DECIDING WHETHER A SIGNAL WILL RETURN TO THE SURFACE OF THE EARTH OR NOT.
- IF IT STRIKES THE LAYER AT A SMALL ANGLE TO THE PERPENDICULAR, IT WILL NOT BE REFRACTED SUFFICIENTLY TO RETURN.
- AS THE ANGLE TO THE PERPENDICULAR PROGRESSIVELY INCREASES, THE SIGNAL WILL BEND PROGRESSIVELY MORE, UNTIL AT A CRITICAL ANGLE, THE SIGNAL WILL REFRACT ENOUGH TO RETURN TO THE EARTH.
- THIS CRITICAL ANGLE IS MEASURED FROM THE PERPENDICULAR AT THE TRANSMITTER (A LINE NORMAL TO THE EARTH’S SURFACE).
- THE CRITICAL ANGLE DEPENDS ON THE IONOSPHERIC CONDITIONS AT THE TIME.
- IT ALSO DEPENDS ON THE FREQUENCY OF THE SIGNAL, A LOWER FREQUENCY WILL BEND MORE, AND THEREFORE HAVE A LOWER CRITICAL ANGLE.
- A FREQUENCY OF MORE THAN 30 MHZ (VHF BAND IS FROM MHZ TO 300 MHZ) WILL NOT USUALLY RETURN TO EARTH.
- SKIP DISTANCE IS THE DISTANCE FROM THE POINT OF TRANSMISSION TO THE POINT WHERE THE FIRST SKY WAVE IS RECEIVED FOR A GIVEN FREQUENCY.
- DEAD SPACE IS THE DISTANCE BETWEEN THE LIMIT OF GROUND WAVE AND THE POINT WHERE THE FIRST SKY WAVE IS RECEIVED FOR A GIVEN FREQUENCY. IN THIS SPACE NO RECEPTION IS AVAILABLE FROM THAT FREQUENCY IN USE.
- IF A SKY WAVE HAS ENOUGH POWER THAN IT WILL STRIKE THE SURFACE OF EARTH AND RETURN BACK TO THE IONOSPHERE AND THEN REFRACTED BACK TO THE SURFACE, THIS IS CALLED MULTI HOP
Line Of Sight
- THE IONOSPHERE
- ITS AN ELECTRICALLY CONDUCTING SPHERE
- D LAYER : 50 – 100 KM, AVG 75 KM
- E LAYER : 100 – 150 KM, AVG 125 KM
- F LAYER : 150 – 350 KM, AVG 225 KM
- DENSITY OF IONOSPHERE
- D LEAST — F MAXIMUM
- DIURNAL ACTIVITY : DAY — DENSITY INCREASES AND THE REFLECTING HT MOVES DOWN
- SEASONAL ACTIVITY : MAX WHEN EARTH IS CLOSEST TO SUN. CAUSES SPORADIC ACTIVITY, RESULTING IN “SPORADIC-E” RECEPTION IN VHF BAND (~150 MHz ).
11 YEAR SUN-SPOT CYCLE : ENHANCED UV & X-RADIATION, IN ABSORPTION, VHF SIGNALS MAY RETURN
- ATTENUATION IN ATMOSPHERE DEPENDS ON FOLLOWING FACTORS
- i ) DENSITY OF LAYERS :
- GREATER DENSITY — GREATER ATTENUATION ( MAX AT MID-NIGHT )
- ii) FREQ IN USE
- LOWER FREQ — GREATER ATTENUATION ( HIGHER FREQ IN HF BY DAY )
- iii) PENETRATION DEPTH
- HIGHER THE FREQ — GREATER THE PENETRATION–GREATER ATTENUATION (VHF AND ABOVE NO SKY WAVES)
- c ) CONDITION FOR TOTAL INTERNAL REFLECTION
- i ) CRITICAL ANGLE
- ii) FREQUENCY IN USE
- * UPTO 500 K Hz — ‘D’ LAYER
- * 500 K Hz — 2 MHz — ‘E’ LAYER
- * 2 M Hz — 30 MHz — ‘F’ LAYER
- * ABOVE 30 M Hz ( VHF & ABOVE ) — FREE SPACE
- d) RANGES AVAILABLE
- i ) TRANSMISSION POWER – GREATER THE TRANSMISSION POWER GREATER THE RANGE
- ii) DEPTH OF PENETRATION- THE DEEPER THE SIGNAL PENETRATES THE GREATER THE RANGE
- iii) CRITICAL ANGLE — MAX RANGE BY WAVE LEAVING TANGENTIAL TO EARTH (GREATER THE CRITICAL ANGLE, GREATER IS THE RANGE)
- NOTE : RANGES AT NIGHT ARE GREATER THAN DAY (FOR A GIVEN FREQUENCY) BECAUSE OF IONIZATION LAYER HT WHICH INCREASES BY NIGHT
- FOR A GIVEN FREQ, SKIP DIST VARIES WITH TIME OF THE DAY ( AND ALSO SEASONS), MAINLY BECAUSE OF IONISATION LAYER HEIGHT AND DENSITY.
DUCT – PROPAGATION OR SUPER REFRACTION
NORMALLY IN ATMOSPHERE, REFRACTIVE INDEX REDUCES WITH HT .
- DURING CONDITIONS OF INVERSION, TEMPERATURE IN ATMOSPHERE INCREASES WITH HEIGHT (THUS REFRACTIVE INDEX INCREASES WITH HEIGHT) TILL END OF INVERSION LAYER AT WHICH POINT THE TEMPERATURE DROPS RAPIDLY LEADING TO SUPER REFRACTION.
- RADIO SIGNALS CAN BE REFRACTED DOWN FROM THIS WARM /MOIST LAYER AND THEN REFLECTED BACK FROM SURFACE OF EARTH THUS GIVING FREAK RANGES IN VHF
OCCURS WHEN :
- WITH IN HEIGHT BAND EITHER TEMP INCREASES OR HUMIDITY INCREASES AT RATES GREATER THAN CERTAIN CRITICAL VALUE.
- FOR TEMP CRITICAL VALUE IS APPROX 4 C/100 FT & HUMIDITY 0.5GM/KG/100FT.
- FREAK RANGE OF SEVERAL HUNDRED MILES (IN VHF) . INVERSION LAYER FORMS DUE TO :
- ( a ) WARM, DRY AIR BLOWING OVER COLD SEA.
- ( b ) SUBSIDENCE ( c ) PRONOUNCED RADIATION COOLING
THIS PHENOMENON IS USUALLY FOUND IN TROPICAL & SUBTROPICAL LATITUDE
Factors Affecting HF Range. The factors affecting HF range are:
- Transmission power.
- Time of day, as it affects the electron density.
- Season of the year also affects the electron density.
- Any disturbances in the ionosphere (solar flares, etc.).
- Geographical location.
- Frequency in use which determines the critical angle and the depth of ionospheric penetration.
The waves reaching a receiver in a straight line (line-of-sight) are called direct waves. All frequencies can be received along direct waves. Signals are attenuated by spread out in accordance with the inverse square law, such that if the range from a transmitter is reduced to half, the signal strength received becomes four times. In addition, as wavelengths reduce into the SHF and EHF bands, water drops and then the gas molecules in air can scatter and absorb progressively more of the signal. Direct waves are regarded as the sole means of propagation of all signals in the VHF band and higher frequencies, and allow lower frequency signals to be received at short range.
Waves can be reflected by any object whose size is more than half their wavelength. This is usually a hindrance to efficient propagation, but radar of course uses the principle of reflection to work. It will be appreciated that when signals are being received from two directions as above, the receiver output will be the vector addition of the two, giving maximum strength when the two signals are in phase, and reduction of signals when the signals are out of phase. The phase difference between the two signals is governed by the lengths of the two paths and the phase shift at the reflecting point. This phase difference, in its turn, depends on the angle of incidence, polarisation of the incident signal and the conductivity of reflecting surface.
Direct waves and waves reflected from the ground are together called ‘space waves’.
Ranges. VHF and frequency bands above VHF are straight-line propagation. However, the actual range is slightly better than mere optical range. The distance to the horizon is given in the formula D = 1.05 √H. The improvement to this range is from the refraction or curving of the waves in atmosphere, due to ‘atmospheric refraction’. The refractive index of the atmosphere ‘n’ is a function of pressure, temperature and humidity. These elements vary significantly in the vertical plane giving rise to diminishing density with increasing height. This means that the refractive index decreases with height. The result is that the radio wave curves away from it towards the regions of higher density, that is, towards the surface. Thus, signals in VHF and above will be received beyond the optical horizon and the working formula for calculating maximum ranges is:
D = 1.25 √ HT + 1.25 √HR
Where D is the range in nm
HT is height of transmitter in feet AMSL
HR is height of receiver in feet AMSL
The term ‘ground wave’ is used to describe all types of propagation except sky waves. Thus, a surface wave is also a ground wave, so is a space wave.
Direct wave + Ground reflected wave and Surface wave = Space wave = All together is Ground wave