Sky Wave PropagationS.Mahendrakumar Asst. Prof. (Sr. Gr.) Department of ECE Velalar College of Engineering and Technology, Erode. 1 Content • Sky Wave Propagation • Ionosphere layers • Virtual height • Critical Frequency • Critical Angle • Maximum Usable Frequency & OWF • Skip Distance • Fading • Diversity 2 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. 3 Sky Wave Propagation • The propagation of radio waves reflected or refracted back toward Earth from the ionosphere. • Since it is not limited by the curvature of the Earth. 4 Sky wave propagation can include multiple hops between the Earth and the ionosphere. Frequency range 2 to 30 MHz. Sky Wave Propagation Sky wave also Known as Skip/ Ionospheric / Hop wave. 5 . 6 . from about 50 km to 400 km. • It is ionized by solar radiation. Ionosphere • The ionosphere is a region of Earth's upper atmosphere. Ionosphere • The molecules of the atmosphere are ionized by Ultra violet rays. alpha. • Ionization density depends upon “ the molecular density and energy of solar radiation”. beta & gamma rays from Solar radiation. • Ionization is the process by which a molecule acquires a negative or positive charge by gaining or losing electrons to form ions. 7 . • Snell’s Law of refraction in the ionosphere. Refractive Index of Ionosphere • Reflection of wave from ionosphere is through refraction of waves. n.is angle of refraction 8 .is angle of incident θr.is refractive index θi. Layer 9 . The Ionosphere layers • D – Layer • E – Layer • Es – Layer • F1 – Layer • F2 . The Ionosphere layers 10 . The Ionospheric Layers 11 . • 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. D-Layer • Average height 70km • Average thickness 10km • Its exists only in day time. 12 . 350km 13 . 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. 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 ad how long it will persist. 14 . 000km • Although some HF waves get reflected from it. most of them pass through it • It affect HF waves by providing more absorption. 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. 15 . 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 16 . Virtual height 17 . • Virtual Height > Actual Height 18 .• The virtual height is the height from which the radio wave appears to be reflecting. Critical Frequency (fc) 19 . 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” 20 . Critical Frequency (fc) Angle of incident θi = 0 21 . Critical Frequency (fc) 22 . Critical Frequency (fc) 23 . Critical Angle (θc) 24 . Critical Angle (θc) 25 . Critical Angle (θc) • Critical Angle is defined as the angle of incidence θ< θc which wave will be reflected. 26 . • As the frequency of a radio wave is increased. the critical angle must be reduced. θ> θc which wave will not be reflected. height and frequency of wave. • θc depends on thickness of layer. • The highest frequency that will be returned to earth for a given angle of incidence. 27 . 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. 28 . 1 29 . 2 30 . 31 .Comparing Equ. 1 & 2. Skip Distance 32 . • The size of the skip distance depends on the frequency. along the surface of the earth. at which a wave above the critical frequency will be returned to earth. angle of incidence and ionization. Skip Distance • The SKIP DISTANCE is the distance from the transmitter to the point where the sky wave is first returned to Earth. • The minimum distance from the transmitter. 33 . ds is • h – height of the layer • θc – Critical angle 34 . Skip Distance • The Skip distance. Skip Distance 35 . Optimum Working Frequency (OWF) 36 . Optimum Working Frequency (OWF) • Optimum Working frequency that provides the most consistent communication path via sky waves. 37 . • It is chosen to be about 15% less than the MUF. Fading 38 . • 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 39 . Fading • The random variation in the received signal strength is called fading. Diversity • Diversity reception is used to minimize the effects of fading. • Diversity techniques are – Frequency diversity – Space diversity – Polarization diversity – Time diversity 40 . Q&A 41 . Multiple Choice Questions a b c 42 . c c c 43 . a d b 44 . a a 45 . b a 46 . c 47 . Q&A 48 .