HF Communication Through Sky Waves


HF Communication Through Sky Waves For Part 2 of RTR (A)

Generally it has been noticed that this is one of the favourite question of the examiners…Explain HF Communication

Basically while answering this question, one has to understand that HF propagation takes place through SKY Waves. So what it means is you should have the full knowledge of Sky Waves.

The principle of efficient HF communication relies on choosing a frequency appropriate for a given set of ionospheric conditions that will produce the first return at the required skip distance from the transmitter. If the height of the refracting layer is known, the signal’s path from the transmitter to the receiver via the ionosphere can be plotted and from this, the angle of incidence the signal makes at the ionosphere can be calculated. An operator can use the angle of incidence to find the frequency whose critical angle that equates to. That frequency is the maximum usable frequency, which will give communication at the estimated range, given the prevailing ionospheric conditions.

If we use a frequency higher than this maximum usable frequency, the signal will return beyond the receiver. At the maximum usable frequency itself, any ionospheric disturbance may increase the skip distance and cause the signal to be lost, so a slightly lower frequency is used. As we lower the frequency, attenuation increases and we need more transmitter power to produce an acceptable signal, until we are unable to produce enough power. When this limit is reached, we have reached the minimum usable frequency (LUHF or lowest usable high frequency).

In practice, graphs and nomograms are made available to the radio stations from which these values are directly extracted. The graphs take into consideration such factors as the station’s position in latitude and longitude, time of the day, density of the ionosphere and any abnormal condition prevailing, and the distance at which the first sky return is required. Nowadays, of course, computers make the calculations, and can automatically select the optimum frequency for communication between the aircraft and any required ground station.

Because of the diurnal variation in the ionospheric density, if transmission is continued at night on a daytime frequency, a longer skip distance will result, leaving the receiver in the ‘dead space’. This is because at night, as we saw in the previous chapter, the electron density decreases and the signal travels higher in the ionosphere before refraction, and is refracted less. For these reasons, the working frequency is lowered at night. This lowering of the frequency adjusts the skip distance because the lower frequencies are refracted from lower levels and require smaller critical angles. Despite the lower frequency the attenuation is less because the electron density is less. In practice the night time frequencies are approximately half of the day time values.

The HF frequency band allocated to commercial aviation ranges from 2 MHz to 22 MHz, but in practice it is only used up to around 18 MHz. The Flight Information Publication (FLlP) lists each Air Traffic Control Centre (ATCC) or Area Control Centre (ACC) ground station with the frequencies available, which aircraft can use to communicate with them. The transmissions are amplitude modulated and a single sideband (SSB) emission, coded J3E, is used to economise on power and bandwidth or channel space.

 

In the early days when MF and HF wireless telephoney was in the forefront, aircraft were equipped with a trailing aerial. It consisted of a coil of wire, which was wound out and held downwards by a weight. Normally it disappeared at the first sight of a thunderstorm, either by the pilot for safety or in the turbulence. In another system, a permanently fixed wire was used, stretching along the length of the fuselage. These aerials have now been replaced by recessed aerials electronically adjusted and conveniently located to give all-round reception from the ground stations. To give an indication of power required, a mere 100 W transmitter can provide transatlantic voice communication.

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.
HF Communication

Ionosphere Layers

 

 

 

 

 

 

 

 

 

 

  • 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 
Sky Wave Propagation

Skip Distance

 

 

 

 

 

 

 

 

  • . THE IONOSPHERE
  • – ELECRICALLY 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
  • — REFLECTING HT MOVES DN
  • SEASONAL ACTIVITY : MAX — EARTH CLOSEST TO SUN. CAUSES APORADIC 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
  • 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–GREATERATTENUATION
  • 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
  • ii) DEPTH OF PENETRATION
  • iii) CRITICAL ANGLE — MAX RANGE BY WAVE LEAVING TANGENTIAL TO EARTH
  • NOTE : RANGES AT NIGHT ARE GREATER THAN IONIZATION LAYER HT
  • DAY TIME————
  • DEPTH OF PENETRATION
  • FOR A GIVEN FREQ, SKIP DIST VARIES WITH TIME OF THE DAY ( AND ALSO SEASONS).
  • DEAD SPACE POSSIBLE IN HF. AS FREQ TO MF/LF , REFLECTION OCCURS FROM LOWER LEVELS ( RANGE     ) AND SURFACE WAVE RANGE    .            THERE MAY BE NO DEAD SPACE.
  • 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

 

Duct Propagation/ Super Refraction in VHF

Duct Propagation/ Super Refraction 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.

 

 

 

 

 

 

HF Communication

HF Antenna

 

 

 

 

 

 

 

 

 

 

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