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Unit-6Sky Wave PropagationQ. 1 Give the Structural Details of Ionosphere.Ans- 1. The ionosphere is the ionized part of Earth’s upper atmosphere from about 48 km (30 mi) to 965 km (600 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. 2. The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km (31 mi) to more than 1,000 km (620 mi). It exists primarily due to ultraviolet radiation from the Sun.

Figure: Structural Details of Ionosphere3. Layers of IonizationD layer

● The D layer is the innermost layer, 60 km (37 mi) to 90 km (56 mi) above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of nanometre (nm) ionizing nitric oxide (NO).

● In addition, high solar activity can generate hard X-rays (wavelength < 1 nm) that ionize N2 and O2. Recombination rates are high in the D layer, so there are many more neutral air molecules than ions.

● Medium frequency (MF) and lower high frequency (HF) radio waves are significantly attenuated within the D layer, as the passing radio waves cause electrons to move, which then collide with the neutral molecules, giving up their energy.

● Lower frequencies experience greater absorption because they move the electrons farther, leading to greater chance of collisions. This is the main reason for absorption of HF radio waves, particularly at 10 MHz and below, with progressively less absorption at higher frequencies.

● This effect peaks around noon and is reduced at night due to a decrease in the D layer's thickness; only a small part remains due to cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

● During solar proton events, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.

E layer

● The E layer is the middle layer, 90 km (56 mi) to 150 km (93 mi) above the surface of the Earth.

● Ionization is due to soft X-ray (1–10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10 MHz and may contribute a bit to absorption on frequencies above.

● However, during intense sporadic E events, the Es layer can reflect frequencies up to 50 MHz and higher. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination.

● At night the E layer weakens because the primary source of ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

● The Es layer (sporadic E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, rarely up to 225 MHz.

F layer

● The F layer or region, also known as the Appleton–Barnett layer, extends from about 150 km (93 mi) to more than 500 km (310 mi) above the surface of Earth.

● It is the layer with the highest electron density, which implies signals penetrating this layer will escape into space. Electron production is dominated by extreme ultraviolet (UV, 10–100 nm) radiation ionizing atomic oxygen. T

● he F layer consists of one layer (F2) at night, but during the day, a secondary peak (labelled F1) often forms in the electron density profile. Because the F2 layer remains by day and night, it is responsible for most skywave propagation of radio waves and long-distance high frequency (HF, or shortwave) radio communications.

Q.2 Explain in detail Ionospheric Wave Propagation Mechanism.Ans-

Figure: Electron density as a function of altitude and various ionospheric layers1. Radio waves below 40 MHz are significantly affected by the ionosphere, primarily because radio waves in this frequency range are effectively reflected by the ionosphere. 2. The E and F layers are the most important for this process. For frequencies beyond 40 MHz, the wave tend to penetrate through the atmosphere versus being reflected. The major usefulness of the ionosphere is that the reflections enable wave propagation over a much larger distance than would be possible with line-of-sight or even atmospheric refraction effects.3. This is shown graphically in Figure 2. The skip distance dmax can be very large, allowing very large communication distances. This is further enhanced by multiple reflections between the ionosphere and the ground, leading to multiple skips.4. This form of propagation allows shortwave and amateur radio signals to propagate worldwide. Since the D layer disappears at night, the best time for long-range communications is at night, since the skip distance is larger as the E, and F regions are at higher altitudes.5. The ionosphere exists between about 90 and 1000 km above the earth’s surface. Radiation from the sun ionizes atoms and molecules here, liberating electrons from molecules and creating a space of free electron and ions. 6. Subjected to an external electric field from a radio signal, these free and ions will experience a force and be pushed into motion. However, since the mass of the ions is much larger than the mass of the electrons, ionic motions are relatively small and will be ignored here.

.Figure: A single skip of radio wave using ionosphere7. Free electron densities on the order of 1010 to 1012 electrons per cubic metre are produced by ionization from the sun’s rays. Layers of high densities of electrons are given special names called the D, E, and F layers.8. During the day the F layer splits into two layers called the F1 and F2 layers, while the D layer vanishes completely at night.Q.3 Write a note on Reflection by the Ionosphere.Ans- 1. When high-frequency signals enter the ionosphere at a low angle they are bent back towards the earth by the ionized layer. 2. When operating at frequencies just below the MUF, losses can be quite small, so the radio signal may effectively "bounce" or "skip" between the earth and ionosphere two or more times. 3. If the ionization is not great enough, the wave only curves slightly downwards, and subsequently upwards as the ionization peak is passed so that it exits the top of the layer only slightly displaced. The wave then is lost in space. 4. To prevent this a lower frequency must be chosen.

Figure: Reflection by the Ionosphere5. Ionospheric reflection occurs when certain radio waves strike a thin, highly ionized layer in the ionosphere. Although the radio wave are actually refracted, some may be bent back so rapidly that they appear to be reflected.6. For Ionospheric reflection to occur the highly ionized layer can be approximately no thicker than one wavelength of the wave since the ionized layers are often several miles thickQ.4 Explain Refraction in the atmosphere.Ans- 1. When a radio wave is transmitted into an ionized layer, refraction, or bending of the wave, occurs. 2. Refraction is caused by an abrupt change in the velocity of the upper part of a radio wave as it strikes or enters a new medium. 3. The amount of refraction that occurs depends on three main factors: ⎫the density of ionization of the layer, the frequency of the radio wave, and ⎫the angle at which the wave enters the layer.4. When a radio wave is transmitted into an ionized layer, refraction, or bending of the wave, occurs. Refraction is caused by an abrupt change in the velocity of the upper part of a radio wave as it strikes or enters a new medium. 5. The amount of refraction that occurs depends on three main factors: (1) the density of ionization of the layer, (2) the frequency of the radio wave, and (3) the angle at which the wave enters the layer.Q.5 How the ray path occurs with varying angles of incidence explain in detail.Ans-. 1. It is also known as Propagation Path.2. The path that a refracted wave follows to the receiver depends on the angle at which the wave strikes the ionosphere. 3. It may also, reach the receiving antenna over a path involving more than one layer, by multiple hops/skips between the ionosphere and Earth, or by any combination of these paths.4. The various angles at which RF waves strikes the layer are represented by dark lines and designated as rays 1 through 6.

Figure: Ray path with varying angles of incidence5. Ray 1 -- the propagation path is long. 6. Ray 2 and Ray 3-- the rays penetrate deeper into the layer but the range of these rays decreases. 7. When a certain angle is reached (Ray 3), the refraction of the ray is first returned to Earth, its second refraction from the ionospheric layer. 8. Ray 4 and Ray 5--the RF energy penetrates the central area of maximum ionization of the layer. These rays are refracted rather slowly and are eventually returned to Earth at great distances. 9. Ray 6-- the ray is not returned at all, but passes on through the layer.Q.6 Write a note on Critical Frequency in sky wave propagation.Ans- 1. The critical frequency is an important figure that gives an indication of the state of the ionosphere and the resulting HF propagation. It is obtained by sending a signal pulse directly upwards. This is reflected back and can be received by a receiver on the same site as the transmitter. 2. The pulse may be reflected back to earth, and the time measured to give an indication of the height of the layer. As the frequency is increased a point is reached where the signal will pass right through the layer, and on to the next one, or into outer space. The frequency at which this occurs is called the critical frequency.3. Critical frequency is defined as the maximum frequency at which the total internal reflection takes place from the ionosphere. The mathematical representation is given as:

Where,fc: critical frequency in HzNmax: maximum electron density per m34. The mathematical representation of critical frequency as a function of MUF is:

Where,fc: critical frequency in HzMUF: maximum usable frequencyӨ: angle of incidence5. Critical frequency varies depending upon atmospheric conditions, time of the day and the angle of fire of the radio waves by the antenna.Q.7 Explain Maximum Usable Frequency (MUF) in sky wave propagation.Ans- 1, When a signal is transmitted using HF propagation, over a given path there is a maximum frequency that can be used. This results from the fact that as the signal frequency increases it will pass through more layers and eventually travelling into outer space. 2. As it passes through one layer it may be that communication is lost because the signal then propagates over a greater distance than is required. Also when the signal passes through all the layers communication will be lost.3. The frequency at which radio communications just starts to fail is known as the Maximum Usable Frequency (MUF). As a rule of thumb it is generally three (for the F region) to five (for the E region) times the critical and it is true for low angles of incidence, although more exact methods are available for determining this figure.4. It is possible to calculate the relationship more exactly:

Where:
MUF = Maximum Usable Frequency
CF = critical frequency
θ = the angle of incidence.5. The factor sec θ is called the MUF factor and it is a function of the path length if the height layer is known.6. This is the maximum usable frequency, MUF that would permit acceptable operation of a radio service between given terminals under specific working conditions. 7. This form of MUF has the emphasis on the operational acceptability of the circuit. It means that factors such as the antenna, power levels and such like are considered and gives an indication regarding the possibility of real communication at a given station.Q.8 what is mean by Lowest Usable Frequency (LUF) in sky wave propagation, explain it.Ans- 1. As the frequency of a transmission is reduced further reflections from the ionosphere may be needed, and the losses from the D layer increase. These two effects mean that there is a frequency below which radio communications between two stations will be lost. In fact the 2. “Lowest Usable Frequency (LUF) is defined as the frequency at below which the signal falls below the minimum strength required for satisfactory reception.”3. From this it can be seen that the LUF is dependent upon the stations at either end of the path. Their antennas, receivers, transmitter powers, the level of noise in the vicinity, and so forth all affect the LUF. The type of modulation used also has an effect, because some types of modulation can be copied at lower strengths than others.4. In other words the LUF is the practical limit below which communication cannot be maintained between two particular radio communications stations.5. If it is necessary to use a frequency below the LUF then as a rough guide a gain of 10dB must be made to decrease the LUF by 2 MHz. This can be achieved by methods including increasing the transmitter powers, improving the antennas, etc.6. It is found that the LUF actually increases in periods of high solar activity. This is a rising because of the increased levels of solar radiation that give rise to higher levels of ionisation in the D layer. 7. His in turn increases the level of attenuation introduced by this layer. This means that at the peak of the sunspot cycle there is degradation in the performance of the low frequency bands for long distance communications.Q.9 Explain virtual height of sky wave propagationAns- 1. When a wave is refracted, it is bent down gradually, but not sharply. However, the path of incident wave and reflected wave are same if it is reflected from a surface located at a greater height of this layer. Such a greater height is termed as virtual height.

Figure: Virtual Height2. The figure clearly distinguishes the virtual height (height of wave, supposed to be reflected) and actual height (the refracted height). If the virtual height is known, the angle of incidence can be found.Q.10 Define Skip distance. Give the formula for calculation for skip distance.Ans- 1. Skip distance is defined as the minimum distance from the earth’s surface and the point from where the radio signal is been transmitted. For a flat earth skip distance is given as:2. DSKIP = 2h

Where,DSKIP: skip distanceh: height at which reflection happensfMUF: maximum usable frequencyfc: critical frequency

=Figure: Skip distance3. So, the skip distance is the distance over the Earth's surface between the point where a radio signal is transmitted, and the point where it is received having travelled to the ionosphere, and been refracted back by the ionosphere.Q.11 Give the relation between MUF & Skip distance.Ans- 1. Relation between MUF & Skip distance for Flat Earth The ionosphere has many tiny layers, for atmospheric refraction.

Figure: Relation between MUF & Skip distance

Sin

1 =

Sin

……

Sink ● The condition for the wave to return to earth is to have total internal reflection (TIR), which begins when the refracted angle, θr is 900. ● If this happens at the k th layer,

Sin

Sin 900 =

● And since,

Sin

● Sin2

2. Critical frequencyFor a given angle of incidence θi and frequency f, the minimum electron density required to achieve TIR is

Sin2

= 1-

● Where, N is the electron density (m-3),

is the mass of electron at rest,

=9.109

kgw is the angular frequency of the wave, w = 2Пf

is the magnitude of electron charge,

= 1.6021

is the permittivity of free space.

F/m● If the maximium electron density present is Nmax, refraction of the wave at normal incidence (θi=0 & sinθi = 0), the only possible way for the wave to be totally internally reflected is if ε= 0. ● This requires the frequency to be less than the critical frequency f c , given by

= Sin2

= 1-

= 0,

= 1

= 9

Refractive index as the function of frequency

= 1 -

= 1 –

3. For MUF ,

=900,

where 900= 1

= 1- sin2

= cos2

= 9

This value of

is called the maximum usable frequency, and is less than 40 MHz, and can be as low as 25-30 MHz4. Skip distance

Figure: Skip distanceCos

Cos

= =(

+ 1

, h =

, d=

= 2h

h is the height of the layer d is the skip distance on the flat Earth’s surface.( Key takeaways :Critical frequency is

= 9

= 2h

)Q.12 Write a note on Multi hop propagation.Ans-

Figure: Multi hop propagation1. There is a way to exceed the maximum distance allowed by F2 layer is the multi hop propagation. 2.When conditions are in favour of DX, bands open down to QRP station, instead of working a near station in doing only one jump or a single hop via F2 layer, we can reach DX stations in doing several hops and break the skip distance over 1500 km on the higher frequencies.3. In using a very low incidence angle, if the first signal reflects well to the upper layers of the ionosphere and then impacts the ground far from your home, hop after hop this multi hop propagation allows communications with stations located on the other side of earthQ.13 Write characteristics of sky wave propagation.Ans-1. Not limited by the curvature of earth.2. Also known as skip wave/ ionospheric wave / hop wave.3. Frequency range 2 to 30 MHz4. It includes multiple hops between earth and ionosphere.5. It is ionised by solar radiation.6. Does not require high transmission power.7. Does not require large antennas.

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