Wave Propagation PDF

Wave Propagation Engr. Harry Bert G. Rolle Modes of Propagation Ground Wave or Surface Wave Propagation Radio waves that travel or propagate along the earths surface. This must be vertically polarized, to prevent the short-circuiting the electric component....

Wave Propagation Engr. Harry Bert G. Rolle Modes of Propagation Ground Wave or Surface Wave Propagation Radio waves that travel or propagate along the earths surface. This must be vertically polarized, to prevent the short-circuiting the electric component. Conductivity and permittivity of the surface play an important part in ground wave or surface wave propagation as the wave will introduce both displacement and condition current in the surface. Modes of Propagation Because the wave induces current to the ground, some of its energy is lost due to absorption. As one moves away from the transmitter, the ground wave eventually disappears due to tilting. Ground losses increase rapidly with increasing frequency. Ground wave propagation is employed on the MF, LF, VLF, and ELF bands. Modes of Propagation Field Strength (E) in V/m E = 120πIhT/λd where: I = Antenna Current in A hT = effective height of transmitting in m λ = signal wavelength in m d = distance from transmitting antenna in m Modes of Propagation Received Voltage at the receiving Antenna (VR) in V VR= 𝐸ℎ𝑅 where : hR = effective height of recieving antenna in m E = field strength in V/m Modes of Propagation A 125 m antenna transmitting at 1.5 MHz has an antenna current of 8A, What is the field strength and voltage is received by the receiving antenna 40 km away with a height of 2m? Modes of Propagation Sky Wave or lonospheric Propagation Sky waves are radio waves that are radiated from the transmitting antenna in a direction that produces a large angle with reference to the earth. The sky wave strikes the ionosphere and is refracted from it back to the ground. The ground may reflect the signal back to the ionosphere. The ionosphere is a region in the earth's atmosphere where the air pressure is so low that free electrons and ions can move about for some time without getting close enough to recombine into neutral atoms. Ultraviolet radiation from the sun is the primary cause of ionization. Sky wave or ionospheric propagation is employed on the HF band. Modes of Propagation 1. Atmospheric Layers a. Troposphere It is the lowest layer of the atmosphere where all weather disturbances take place. It extends to a height of approximately 8 to 10 miles above sea level. b. Stratosphere It is a region directly above the troposphere where no weather is seen but circulation does occur. It is also called Isothermal Region because it has a constant temperature. It is not subjected to temperature inversions, nor does it cause significant refraction. It extends above the troposphere at about 40 miles. C. lonosphere It is the region in the atmosphere above the stratosphere where several ionized layers of low-density gas are found. It extends from 40 to about 250 miles above the ground. Modes of Propagation 2. Ionospheric Layers a. D Layer It is the lowest known ionized region of the ionosphere In this region, atoms that are broken up into ions by sunlight recombine quickly, so the ionization level is directly related to sunlight. lonization begins at sunrise, peaks at local noon, and disappears at sundown. This layer is ineffective in bending HF waves back to earth, but id does refract low frequency signals At very low frequencies, the D Layer and the ground combine to act as a huge waveguide, making worldwide communication possible with large antennas and high power transmitters. Modes of Propagation b. E Layer It is the region above the D Layer and is the lowest portion of the ionosphere that is useful for long distance communication. In this region, ionization varies with the angle above the horizon. Ultraviolet radiation, solar x-rays, and meteors entering this portion of the earth's atmosphere play important parts in its ionization process lonization increases rapidly after sunrise, reaches maximum around noon, and drops off quickly after sundown. Minimum ionization is after midnight. This layer aids MF surface-wave propagation a little and reflects some HF waves in daytime This layer is also known as Kennelly-Heaviside Layer Modes of Propagation c. F Layer It is the region where most of our long-distance communications capability stems. In this region, ions and electrons recombine more slowly, so the observable effects of the sun angle develop more slowly This region holds its ability to reflect wave energy back to earth well into the night. lonization is at its maximum during the afternoon hours Atoms in this layer remain ionized for a longer time after sunset. During the day, the F region may split into two layers. The lower and the weaker layer is the F, Layer, while the layer which doesn't disappear at night is the F₂ Layer. Modes of Propagation Layer Height (Km) Thickness (km) Single Hop Range (km) D 50 -90 10 2350 E 110 25 3000 F1 175-250 20 3840 (daytime) F2 250-400 200 1430 (nighttime) Modes of Propagation a. lon Density The refractive ability of the ionosphere increases with the degree of ionization. The bending of a wave at any given frequency or wavelength and angle of radiation will increase with an increase in ionization density. The degree of ionization is greater in summer than in winter and is also greater during the day than at night. Modes of Propagation b. Frequency (f) or Wavelength (λ) The bending of a wave at any given ionization density and angle of radiation will increase with an increase in wavelength (or with a decrease in frequency). The lower the frequency, the more easily the signal is refracted. The higher the frequency, the more difficult is the refracting or bending process. c. Angle of Radiation (0) The bending of a wave at any given frequency and ionization density will increase with an increase in the angle of radiation (that is, the wave is farther from the horizon). Modes of Propagation 4. Factor Affecting Optimum Operating Frequency a. Location and Geography Intensity of ionizing radiation varies with locations and altitudes. The intensity is greatest in equatorial regions, where the sun is more directly overhead than in the higher latitudes. b. Seasonal Variations Seasonal variations are variations brought about by the revolution of the earth around the sun. Our e arth orbits the sun with an orbital period of 1 year bringing about our seasons: spring, summer, autumn, and winter. The sun is the major controlling element on the behavior of the ionosphere thus ionization is stronger in summer than in winter. Modes of Propagation C. Diurnal Variations Diurnal variations are variations brought about by the rotation of the earth around its axis. The earth rotates around its own axis within a 24-hr period that is broken up into three distinct time periods: day. transition, and night. Transition periods occur twice once around sunrise and again once around sunset. lonization is maximum during daylight hours and minimum during the hours of darkness. Modes of Propagation d. Cyclical Variations Cyclical variations are variations brought about by the solar cycle like the sunspot activities. Solar activities are characterized by sunspot numbers. Sunspots appear on the sun’s surface and are tremendous eruptions of whirling electrified gas. Modes of Propagation 6. Ionospheric Irregularities a. Sudden lonospheric Disturbances (SID's) They are also called Dellinger Fadeouts or Mogel Dellinger Fadeouts, SID's are caused by solar flares, which are gigantic emissions of hydrogen from the sun. b. Traveling lonospheric Disturbances (TID's) They are disturbances that seriously affect the accuracy of high-frequency direction finders due to irregularities of electron densities in the ionosphere. Modes of Propagation c. lonospheric Storms They are caused by particle emissions from the sun generally Alpha and Beta rays. At these conditions, the ionosphere behaves erratically causing signal strengths to drop and fluctuate rapidly. They take about 36 hrs to reach the earth. d. Fading It is the fluctuation of signal strength at the receiver. It can occur because of interference between the lower and the upper rays of a sky wave, between sky waves arriving by a different number of hops or different paths, or even between a ground wave and a sky wave especially at the lower end of the HF band. Modes of Propagation i. Interference Fading - It is the most common type of fading caused by mixing of two or more signal components propagating along different paths ii. Polarization Fading - It is a type of fading caused by the so-called Faraday Effect or Faraday Rotation iii. Focusing and Defocusing - they are mainly due to atmospheric irregularities Deformed layers can focus or defocus a signal wave if they encounter deformities that are concave or convex respectively. The motion of these structures can cause fadeouts iv. Absorption Fading - It is a type of fading caused by solar flare activities and particularly affects the lower frequencies v. Selective Fading It is a type of fading having different effects on different frequency ranges. This happen because ray paths in the ionosphere are different for different frequencies, and not all will necessarily experience a disturbance at a given region.