Fundamentals of EM Waves
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This document discusses electromagnetic wave propagation. Electromagnetic waves consist of oscillating electric and magnetic fields that propagate through space at the speed of light. They include radio waves, light, X-rays and gamma rays. Radio waves propagate through space as transverse electromagnetic waves and their speed depends on the medium. The wavelength is determined by the frequency and phase velocity. Power density decreases with distance from the source according to the inverse square law. Reflection, refraction, diffraction and polarization affect wave propagation. Terrestrial propagation is influenced by the curvature of the Earth and the atmosphere, especially the ionosphere.
Fundamentals of EM Waves
- 2. Electromagnetic Waves Electromagnetic wave consists of oscillating electric and magnetic fields in certain directions with propagate . Propagate through free space at the velocity of light
- 3. 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
- 5. 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
- 7. Propagation Velocity Speed of light in free space: 3 108 m/s In dielectric and plasma the velocity of propagation is lower: r c v
- 8. Electromagnetic Waves Wavelength is : fV p / Where, Vp is the phase velocity is the wavelength f is the frequency
- 9. Ohm’s Law in Space HEZ /
- 10. 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
- 12. 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
- 14. 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
- 15. Power flux density Power flux density= E X H
- 16. Geometrical loss 2 4πr PPD 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
- 17. Attenuation of Free Space Power density is reduced with increasing distance r Power density is total power divided by surface area of sphere Unit: watts/meter 2 4 r P P t D
- 18. Free Space Electric Field Electric field strength is relatively easy to measure Often used to specify signal strength Unit: volts/meter r P E t 30
- 19. Absorption No absorption in free space EM wave are absorbed in atmosphere as energy is transferred to atoms and molecules Electromagnetic waves are absorbed in the atmosphere according to wavelength. Two compounds are responsible for the majority of signal absorption: oxygen (O2) and water vapor (H2O). Absorption below 10 Ghz is quit insignificant
- 21. 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
- 24. 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 It is a physical orientation of radiated waves in space
- 28. 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
- 29. Refraction Refraction takes place when EM wave pass from one medium to another medium with diff density.
- 30. Atmospheric density changes with height Slight refraction of wave Increases Radio horizon
- 31. Refraction Occurs when waves move from one medium to another with a different propagation velocity Index of refraction n is used in refraction calculations r n
- 32. Snell’s Law Angles are measured with respect to the normal to the interface 2211 sinsin nn
- 33. refraction
- 34. 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)
- 35. 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
- 39. 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
- 40. 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)
- 42. 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”
- 43. 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
- 44. 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
- 45. Refraction of Signals Bending of signals by atmosphere decreases with increasing frequency Bending of signals by atmosphere increases with increasing ionization
- 46. Daytime Propagation D and E layers absorb lower frequencies, below about 8-10 MHz F layers return signals from about 10-30 MHz
- 47. Nighttime Propagation D, E layers disappear F layer returns signals from about 2-10 MHz Higher frequencies pass through ionosphere into space
- 48. Ionospheric Sounding Transmit signal straight up Note the maximum frequency that is returned This is the critical frequency
- 49. Important Frequencies in HF Propagation Critical frequency Highest frequency that is returned to earth Maximum Usable Frequency (MUF) Highest frequency that is returned at a given point MUF= fcsecθ Optimum Working Frequency (OWF) 85% of MUF for more reliable communication
- 50. Skip Zone Region between maximum ground-wave distance and closest point where sky waves are returned from the ionosphere,