Radio wave propagation S

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Information about Radio wave propagation S

Published on November 7, 2007

Author: Ariane


Propagation of the Radio Wave:  Propagation of the Radio Wave Electromagnetic Waves:  Electromagnetic Waves In order to give a better understanding of the ionospheric measurements by means of ionosonde (HF-radar) or other radio techniques, it is useful to give a short description of the electromagnetic radiation ( e.m. wave). Electromagnetic wave consists of oscillating electric and magnetic fields in certain directions with propagate . Electromagnetic Radiation:  Electromagnetic Radiation Includes radio waves, light, X-rays, gamma rays VLF 3 – 30 kHz LF 30 – 300 kHz MF 300 – 3000 kHz HF 3 – 30 MHz VHF 30 – 300 MHz UHF 300 – 3000 MHz Radio waves of our interest TEM Propagation:  TEM Propagation Radio waves in space are transverse electromagnetic waves (TEM) Electric field, magnetic field and direction of travel of the wave are mutually perpendicular Waves will propagate through free space and dielectrics Conductors have high losses due to induced current Propagation Velocity:  Propagation Velocity Speed of light in free space: 3  108 m/s In dielectric and plasma the velocity of propagation is lower: Electromagnetic Waves:  Electromagnetic Waves Wavelength is : Where, Vp is the phase velocity is the wavelength f is the frequency Electric and Magnetic Fields:  Electric and Magnetic Fields For waves we use the following units: Electric field strength E (V/m) Magnetic field strength H (A/m) Power density PD (W/m2) Ohm’s law holds if characteristic impedance Z of medium is used For free space, Z = 377 Ohm Ohm’s Law in Space:  Ohm’s Law in Space Power Density:  Power Density Plane and Spherical Waves:  Plane and Spherical Waves Waves from a point in space are spherical Plane waves are easier to analyze At a reasonable distance from the source, spherical waves look like plane waves, as long as only a small area is observed Spherical waves:  Spherical waves Isotropic antenna radiating equally in every direction Free-space Propagation:  Free-space Propagation Assume an isotropic radiator at the center of a sphere Let receiving antenna be on surface of sphere As we move farther from transmitter the amount of power going through the surface remains the same but surface area increases Power flux density:  Power flux density Power flux density= E X H Geometrical loss:  Geometrical loss Because of the power P on the spherical surface is constant for every spherical surface (4π r2 ) we consider, the power flux density at the distance r from the isotropic antenna must decrease as 1/4πr2. If an isotropic antenna radiates 10 W of power at the distance of 1 km the power flux density (PD)is about 0.796 microW/m2 Attenuation of Free Space:  Attenuation of Free Space Power stays the same but power density is reduced with increasing distance r Power density is total power divided by surface area of sphere Unit: watts/meter Free Space Electric Field:  Free Space Electric Field Electric field strength is relatively easy to measure Often used to specify signal strength Unit: volts/meter Transmitting Antenna Gain (G):  Transmitting Antenna Gain (G) Gain is achieved by radiating more energy in some directions than others Total radiated power cannot be more than power input to antenna Gain is usually expressed with reference to an isotropic radiator By definition G = PD/P (Isotropic radiator) Antenna gain:  Antenna gain Antenna gain:  Antenna gain Power Density at distance r including antenna Gain:  Power Density at distance r including antenna Gain Receiving Antenna Effective Area:  Receiving Antenna Effective Area The receiving antenna can be considered to absorb all the power passing through a certain area This is the antenna’s effective area Effective area is related to wavelength and gain Received Power:  Received Power Calculation of Effective Area:  Calculation of Effective Area Path Loss:  Path Loss Friis’s Formula expresses the attenuation of free space in a convenient decibel form Units are typical engineering units, not basic units like Hz and meters Loss is in dB, distance in km, frequency in MHz, gains in dBi (decibels with respect to an isotropic radiator) Friis’s Formula:  Friis’s Formula Reflection:  Reflection Specular reflection: smooth surface Angle of incidence = angle of reflection Diffuse reflection: rough surface Reflection in all directions because angle of incidence varies over the surface due to its roughness Specular Reflection:  Specular Reflection Polarization:  Polarization Polarization of a wave is the direction of the electric field vector Linearly polarized waves have the vector in the same direction at all times Horizontal and vertical polarization are common Circular and elliptical polarization are also possible Circular polarization:  Circular polarization linear polarization:  linear polarization Cross Polarization:  Cross Polarization If transmitting and receiving antennas have different polarization, some signal is lost Theoretically, if the transmitting and receiving polarization angles differ by 90 degrees, no signal will be received A circularly polarized signal can be received, though with some loss, by any linearly polarized antenna Diffuse reflection:  Diffuse reflection Refraction:  Refraction Occurs when waves move from one medium to another with a different propagation velocity Index of refraction n is used in refraction calculations Snell’s Law:  Snell’s Law Angles are measured with respect to the normal to the interface refraction:  refraction Angle of Refraction:  Angle of Refraction If n1<n2 then ray bends toward the normal (away from the interface) If n1>n2 then ray bends away from the normal (toward the interface) Diffraction:  Diffraction Occurs when radiation passes an object with dimensions small compared with wavelength The object appears to act as a source of radiation Allows radio stations to be received on the shadow side of obstacles Terrestrial Propagation:  Terrestrial Propagation Propagation over earth’s surface Different from free-space propagation Curvature of the earth Effects of the ground Obstacles in the path from transmitter to receiver Effects of the atmosphere, especially the ionosphere Ground-Wave Propagation:  Ground-Wave Propagation Happens at relatively low frequencies up to about 2 MHz Only works with vertically polarized waves Waves follow the curvature of earth range varies from worldwide at 100 kHz and less to about 100 km at AM broadcast band frequencies (approx. 1 MHz) Ionospheric Propagation:  Ionospheric Propagation Useful mainly in HF range (3-30 MHz) Signals are refracted in ionosphere and returned to earth Worldwide communication is possible using multiple “hops” Ionospheric Layers:  Ionospheric Layers D layer: height approx. 60-90 km E layer: height approx. 90-150 km F1 layer: height approx. 150-250 km F2 layer: height approx. 250-400 km D, E layers disappear at night F layers combine into one at night Ionospheric Activity:  Ionospheric Activity More ionization causes signals to bend more Ionization caused by solar radiation greater during daytime greater during sunspot cycle peaks (we are about at a decreasing value now-2004) D,E layers are less highly ionized than F layer and usually just absorb signals Refraction of Signals:  Refraction of Signals Bending of signals by atmosphere decreases with increasing frequency Bending of signals by atmosphere increases with increasing ionization Daytime Propagation:  Daytime Propagation D and E layers absorb lower frequencies, below about 8-10 MHz F layers return signals from about 10-30 MHz Nighttime Propagation:  Nighttime Propagation D, E layers disappear F layer returns signals from about 2-10 MHz Higher frequencies pass through ionosphere into space Ionospheric Sounding:  Ionospheric Sounding Transmit signal straight up Note the maximum frequency that is returned This is the critical frequency Important Frequencies in HF Propagation:  Important Frequencies in HF Propagation Critical frequency Highest frequency that is returned to transmitter Maximum Usable Frequency (MUF) Highest frequency that is returned at a given point Optimum Working Frequency (OWF) 85% of MUF for more reliable communication Skip Zone:  Skip Zone Region between maximum ground-wave distance and closest point where sky waves are returned from the ionosphere,

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