UNIT-5AWP_wave propagation and electromagnetic KEC603.pptx

This pdf give theory of wave

1. Antenna and Wave Propagation(BEC-603) Unit-IV W ave Propagation Dr. Deepak Yadav Asst. Prof. ECE Department AKGEC Ghaziabad

2. Ground wave Propagation and Space wave Propagation

3. Ground wave Propagation and Space wave Propagation

4. Ground wave Propagation and Space wave Propagation In grounded wave propagation , the waves are guided along the surface of the earth just an electromagnetic wave is guided through a waveguide. This propagation exists when transmitting and receiving antennas are close to the surface of the earth. They are produced by vertical antennas, i.e., vertically polarized antenna. In space wave propagation , the waves from transmitting antenna reach to the receiving antenna after reflector (from troposphere of space) and direct. Therefore, the receiving antenna receives the two waves, one is indirect and other is direct. Since, both these waves travel along the different paths, they have different path lengths. S. No. Mode of Propagation Frequency Range 1. Ground Wave Propagation Upto 2 MHz 2. Space wave Propagation UHF and Microwave above 300 MHz 3. Sky Wave Propagation Between 2 to 30 MHz

5. Plane Earth Reflection The wave propagation can be obtained by means of space wave propagation when transmitting and receiving antennas are elevated. This propagation is called as line of sight propagation. The reason is that the two antennas are within the sight of vision. The received resultant signal is a resultant of signal the receiver through a direct path (space wave) and that reaching after being reflected by the ground (surface wave). Fig. (1) shows the direct and reflected waves between transmitter and receiver

6. The transmitted and reflected waves will have the same amplitudes and the earth is considered to be perfect conductor having infinite conductivity . If the surface of the earth is smooth and has finite conductivity , then the amplitude and phase of reflected wave will differ from that of the incident wave. When the surface of the earth is rough , then the reflected wave will be scattered. Their amplitude will be reduced as compared to the amplitude of a smooth surface

7. (ii) At low operating frequencies , very tall antennas should be used. This is because the antenna height should be atleast ( ). (iii) High transmission power is necessary to cover the adequate range. The power in excess of 1 MW is a common thing.  

8. Both the waves undergo attenuation (negligible small) as they travel through space as spreading and rain. It is important to mention here that only high frequency waves are attenuated by rain. The field strength of each wave is inversely proportional to the distance from transmitter provided the transmitter and receiver antennas as far away from each other. Further, the distance d should be greater than the antenna heights. Therefore, the angle of incidence for the ground reflected wave at the surface of earth will be small. At receiving antenna, the magnitude of two waves remains the same but the phase reverses irrespective of the type of polarization.

9. Sky Wave Propagation It is a type of radio wave propagation. The wave which propagate through atmosphere and are reflected back by the ionosphere of earth's atmosphere is called as sky wave propagation. These waves go from transmitter antenna to receiver antenna travelling through sky. The sky waves are of practical importance at medium and high frequency for very long distance radio communication. Frequency communication range (2 and 30 MHz)

10. The waves, which are transmitted from the transmitter antenna, are reflected from the ionosphere. It consists of several layers of charged particles ranging in altitude from 30- 250 miles above the surface of the earth. Such a travel of the wave from transmitter to the ionosphere and from there to the receiver on Earth is known as  Sky Wave Propagation . Ionosphere is the ionized layer around the Earth’s atmosphere, which is suitable for sky wave propagation .

11. Earth’s atmosphere has several layers. These layers play an important role in the wireless communication. These are mainly classified into three layers. Troposphere This is the layer of the earth, which lies just above the ground. We, the flora and fauna live in this layer. The ground wave propagation and LOS propagation take place here. Stratosphere This is the layer of the earth, which lies above Troposphere. The birds fly in this region. The airplanes travel in this region. Ozone layer is also present in this region. The ground wave propagation and LOS propagation takes place here. Ionosphere This is the upper layer of the Earth’s atmosphere, where ionization is appreciable. The energy radiated by the Sun, not only heats this region, but also produces positive and negative ions. Since the Sun constantly radiates UV rays and air pressure is low, this layer encourages ionization of particles.

12. Structure of Ionosphere The ionosphere is composed of three layers designated D, E, and F, from lowest level to highest level as shown in figure, The F layer is further divided into two layers designated F1 (the lower layer) and F2 (the higher layer). The presence or absence of these layers in the ionosphere and their height above the Earth varies with the position of the sun. The D layer ranges from about 50 to 90 km. Ionization in the D layer is low because it is the lowest region of the ionosphere. This layer has the ability to refract signals of low frequencies. High frequencies pass right through it and are attenuated. After sunset, the D layer disappears because of the rapid recombination of ions.

13. The E layer limits are from about 90 to 130km. The rate of ionic recombination in this layer is rather rapid after sunset and the layer is almost gone by midnight. This layer has the ability to refract signals as high as 20 megahertz. For this reason, it is valuable for communications in ranges up to about 1500 miles. The F layer exists from about 140 to 400 km. During the daylight hours, the F layer separates into two layers, the F1 and F2 layers. The ionization level in these layers is quite high and varies widely during the day. At noon, this portion of the atmosphere is closest to the sun and the degree of ionization is maximum. Since the atmosphere is rarefied at these heights, recombination occurs slowly after sunset. Therefore, a fairly constant ionized layer is always present. The F layers are responsible for high-frequency, long distance transmission.

14. Expression for Refractive Index of Ionosphere

15. Mechanism of Wave Propagation

16. Critical frequency For any given time, each ionospheric layer has a maximum frequency at which radio waves can be transmitted vertically and refracted back to Earth. This frequency is known as the CRITICAL FREQUENCY. Radio waves transmitted at frequencies higher than the critical frequency of a given layer will pass through the layer and be lost in space; but if these same waves enter an upper layer with a higher critical frequency, they will be refracted back to Earth. Radio waves of frequencies lower than the critical frequency will also be refracted back to Earth unless they are absorbed or have been refracted from a lower layer. The lower the frequency of a radio wave, the more rapidly the wave is refracted by a given degree of ionization

17. Maximum Usable Frequency For a given angle of incidence and time of day, there is a maximum frequency that can be used for communications between two given locations. This frequency is known as the MAXIMUM USABLE FREQUENCY ( muf ).

18. SKIP DISTANCE

19. Curved surface

20. Wave Characteristics

21. Examples: Q1. Calculate maximum effective aperture of an antenna which is operating at wavelength of 2m and has a directivity of 100. Q2. Calculate the critical Frequency for reflection at a vertical incidence if maximum value of electron density is 1.24 x 10 6 cm -3 . Q3. A high Frequency radio link has to be established between two points at a distance of 2500km on earth’s surface. Considering the ionospheric height to be 200km and its Critical Frequency of 5 MHz. Calculate the MUF for the given path (assume Flat Earth). Q4. The Radiation Resistance of an antenna is 72 ohm and loss resistance is 8 ohm. What is the Directivity, if power gain is 16? Q5. Find the skip distance for waves of frequency 4.6 MHz at a time when the maximum ionization in the E- region has a value of 1× 10 11 e/m 3 at a height of 110 km.

22. THANK YOU Reference: Antennas for all applications by John D. Kraus