Sky Wave Propagation

Content Sky Wave Propagation Ionosphere layers Virtual height Critical Frequency Critical Angle Maximum Usable Frequency & OWF Skip Distance

Ground Wave Propagation Disadvantages Requires relatively high transmission power. Frequencies up to 2 MHz. Require large antennas. Ground Wave get attenuated. Attenuation increases with frequency.

Sky Wave Propagation The propagation of radio waves reflected or refracted back toward Earth from the ionosphere. not limited by the curvature of the Earth.

Sky Wave Propagation Sky wave also Known as Skip/ Ionospheric / Hop wave. Sky wave propagation can include multiple hops between the Earth and the ionosphere. Frequency range 2 to 30 MHz.

Ionosphere The ionosphere is a region of Earth's upper atmosphere, from about 50 km to 400 km. It is ionized by solar radiation.

Ionosphere The molecules of the atmosphere are ionized by Ultra violet rays, alpha, beta & gamma rays from Solar radiation. Ionization density depends upon “ the molecular density and energy of solar radiation”. Ionization is the process by which a molecule acquires a negative or positive charge by gaining or losing electrons to form ions.

Refractive Index of Ionosphere Reflection of wave from ionosphere is through refraction of waves. Snell’s Law of refraction in the ionosphere. n- is refractive index θi- is angle of incident θr- is angle of refraction

The Ionosphere layers D – Layer E – Layer Es – Layer F1 – Layer

The Ionosphere layers

The Ionospheric Layers

D-Layer Average height 70km Average thickness 10km Its exists only in day time. It is not useful layer for HF communication It reflects some VLF and LF waves Its electron density N=400 electrons/cc fc=180kHz Almost no refraction (bending) of radio waves. Its virtual height is 60 to 80km.

E-Layer Average height 100km Average thickness 25km Its exists only in day time It reflects some HF waves Its electron density N=5×105 electrons/cc fc=4MHz Its virtual height is 110km Maximum single hop range 2,350km

Es-Layer Its exists in both day and night It is a thin layer Its height normally 90 to 130km Its electron density is high It is difficult to know where and when it will occur and how long it will persist.

F1-Layer Average height 180km Average thickness 20km It combines with F2 layer at night fc=5MHz Its virtual height is 180km Maximum single hop range 3,000km Although some HF waves get reflected from it, most of them pass through it It affect HF waves by providing more absorption.

F2-Layer Average height 325km in day time Thickness is about 200km It falls to a height of 300km at nights as it combines with F1 layer It offers better HF reflection Its electron density N=8×1011 electrons/m3 fc=8MHz in day & fc=6MHz in night Its virtual height is 300km Maximum single hop range 4,000km

Virtual height

The virtual height is the height from which the radio wave appears to be reflecting. Virtual Height > Actual Height

Critical Frequency (fc)

Critical Frequency (fc) Critical frequency for a given layer is the highest frequency that will reflected to earth by that layer at vertical incidence. Higher frequencies “escape”

Critical Frequency (fc) Angle of incident θi = 0

Critical Frequency (fc)

Critical Frequency (fc)

Critical Angle (θc)

Critical Angle (θc)

Critical Angle (θc) Critical Angle is defined as the angle of incidence θ< θc which wave will be reflected, θ> θc which wave will not be reflected. θc depends on thickness of layer, height and frequency of wave. As the frequency of a radio wave is increased, the critical angle must be reduced.

Maximum Usable Frequency Maximum Usable Frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere at a specified time. The highest frequency that will be returned to earth for a given angle of incidence.

1

2

Comparing Equ. 1 & 2.

Skip Distance

Skip Distance The SKIP DISTANCE is the distance from the transmitter to the point where the sky wave is first returned to Earth. The size of the skip distance depends on the frequency, angle of incidence and ionization. The minimum distance from the transmitter, along the surface of the earth, at which a wave above the critical frequency will be returned to earth.

Skip Distance The Skip distance, ds is h – height of the layer θc – Critical angle

Skip Distance

Optimum Working Frequency (OWF)

Optimum Working Frequency (OWF) Optimum Working frequency that provides the most consistent communication path via sky waves. It is chosen to be about 15% less than the MUF.

Electrical Properties of Ionosphere VARIATIONS IN THE IONOSPHERE Regular or cyclic variations- can be predicted in advance Irregular variations – abnormal behaviour of Sun- cannot be predicted in advance.

Regular Variations Daily Seasonal 11-year 27-day

Daily variations Due to 24 hour rotation of earth around the sun. D layer- disappears at night. Reflects VLF waves- important for VLF comm. – refracts IF and MF waves for short range comm. Absorbs HF waves; little effect on VHF and above

Daily variations E layer- refracts HF waves during day. Ionization- reduced at night. F layer- Ionization density depends on time of the day and angle of the sun. Has one layer at night and 2 layers F1 and F2 during day.

Seasonal Variations Are due to the Earth revolving around the sun. Ionization density - D, E and F1 – greatest – summer. F2- greatest- Winter. Operating freq- higher in winter

Eleven Year Sun Spot Cycle SUNSPOTS- phenomena of appearance and disappearance of dark, irregularly shaped areas. Life span of individual sunspots is variable; however, a regular cycle of sunspot activity has also been observed. This cycle has both a minimum and maximum level of sunspot activity that occur approximately every 11 years.

Maximum sunspot activity, the ionization density of all layers increases. Absorption in the D layer increases - the critical frequencies for the E, F1, and F2 layers are higher. At these times, higher operating frequencies must be used for long distance communications.

27-DAY SUNSPOT CYCLE. As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, the app. period required for the sun to make one complete rotation. The 27-day sunspot cycle - variations in the ionization density of the layers on a day-to-day basis.

Abnormal Variations Sudden Ionospheric Disturbance Ionospheric storms Sporadic E layer reflection Tides and winds Fadings Whistlers

Sporadic E Sporadic E- Irregular cloud-like patches of unusually high ionization at heights greater than normal E layer- some times thin layer – other times heavily ionized thick layer. fc – very high- long distance transmission occurs at unusually high freq. Form and disappear short time either day or night.

Sudden Ionospheric Disturbance(SID) Sudden appearance of Solar flares. Causes complete fading- Dellinger fade-out. D-layer Not found in E, F1 and F2 layers.

Whistlers Whistlers are transient electromagnetic disturbances which occur naturally. Consist of EM pulses of Audio Frequency radiation along the direction of the magnetic field of the earth between conjugate points in the northern and southern hemispheres. Types- Long whistlers, short whistlers, noise whistlers.

Tides and Winds Tides and Winds are common in the atmosphere. Winds in the ionosphere are caused by tides. Winds –due to motion of turbulence in F2-layer.

Free-Space Propagation Radio waves propagate through free space in a straight line with a velocity of the speed of light (300,000,000 m/s) There is no loss of energy in free space, but there is attenuation due to the spreading of the waves

Attenuation of Free Space An isotropic radiator would produce spherical waves The power density of an isotropic radiator is simply be the total power divided by the surface area of the sphere, according to the square-law:

Transmitting Antenna Gain In practical communication systems, it is important to know the signal strength at the receiver input It depends on the transmitter power and the distance from the transmitter to the receiver, but also upon the transmitting and receiving antennas Two important antenna characteristics are: Gain for the transmitting antenna Effective area for the receiving antenna Antennas are said to have gain in those directions in which the most power is radiated

Receiving Antenna Gain A receiving antenna absorbs some of the energy from radio waves that pass it A larger antenna receives more power than a smaller antenna (in relation to surface area) Receiving antennas are considered to have gain just as transmitting antennas do The power extracted from a receiving antenna is a function of its physical size and its gain

Path Loss Free-space attenuation is the ratio of received power to transmitted power The decibel gain between transmitter and receiver is negative (loss) and the loss found this way is called free-space loss or path loss

Fading

Fading The random variation in the received signal strength is called fading. The main causes of fading are Multipath propagation Variation in ionosphere condition Types of fading Multipath fading Selective fading Interference fading Polarization fading Skip fading Absorption fading

Diversity Diversity reception is used to minimize the effects of fading. Diversity techniques are Frequency diversity Space diversity Polarization diversity Time diversity

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