3rd long Flashcards
Propagation of electromagnetic waves often called radio-frequency (RF) propagation or simply radio propagation.
Free-space
Electrical energy that has escaped into free space.
Electromagnetic wave
The orientation of the electric field vector in respect to the surface of the Earth
Polarization
Polarization remains constant
Linear Polarization
Forms of Linear polarization
Horizontal Polarization and Vertical Polarization
Polarization vector rotates 360? as the wave moves one wave-length through the space and the field strength is equal at all angles of polarization.
Circular Polarization
Field strength varies with changes in polarization.
Elliptical Polarization
Used to show the relative direction of electromagnetic wave propagation.
Rays
Formed when two points of equal phase on rays propagated from the same source are joined together.
Wavefront
A single location from which rays propagate equally in all directions.
Point source
Invisible force field produced by a magnet: such as a conductor when current is flowing through.
Magnetic Field
Strength of a magnetic field (H) produced around a conductor is expressed mathematically as
H=1/2pi d
Invisible force fields produced by a difference in voltage potential between two conductors.
Electric fields
Electric filed strength (E) is expressed mathematically as:
E=q/4pi e d^2
Dielectric constant of the material separating the two conductors.
Permittivity
The permittivity of air or free space is approximately
8.85 x 10 -12 F/m
The rate at which energy passes through a given surface area in free space.
Power density
Intensity of the electric and magnetic fields of an electromagnetic wave propagating in free space
Field intensity
Mathematically power density is expressed
P = ÛH W/m2
The characteristic impedance of a
lossless transmission medium is equal to the square root of the ratio of its magnetic permeability to its electric permittivity
Zs = (?o0)1/2
Point source that radiates power at a constant rate uniformly in all directions
Isotropic radiator
Power density is inversely proportional to the square of the distance from the source.
Inverse Square Law
Propagation medium
Isotropic medium
Waves propagate through free space: they spread out: resulting in a reduction in power density
Attenuation
Reduction of Power.
Absorption Loss
Reduction in power density with distance is equivalent to a power loss
Wave attenuation
Spherical spreading of the wave
Space attenuation
One with uniform properties throughout.
Homogeneous medium
Absorption coefficient varies considerably with location: thus creating a difficult problem for radio systems engineers.
Inhomogeneous medium
Optical properties of Radio Waves
Refraction: Reflection: Diffraction and Interference
Bending of the radio wave path
Refraction
Square root of the dielectric constant and is expressed in:
Refractive index; n = (k)
(k) Equivalent dielectric constant relative to free space (vacuum).
K = (1- 81N/f2)1/2
Boundary between two media with different densities
Plane
Imaginary line drawn perpendicular to the interface at the point of incidence
Normal
Angle formed between the incident wave and the normal.
Angle of Incidence
Angle formed between the refracted wave and the normal.
Angle of Refraction
Ratio of velocity of propagation of a light ray in free
space to the velocity of propagation of a light ray in a given material.
Refractive Index
Perpendicular to the direction of propagation (parallel to the waveform)
Density gradient
To cast or turn back.
Reflect
Ratio of the reflected to the incident voltage intensities.
Reflection Coefficient
Portion of the total incident power that is not reflected.
Power transmission coefficient
Fraction of power that penetrates medium 2
Absorption coefficient
Incident wave front strikes an irregular surface: it is
randomly scattered in many directions.
Diffuse reflection
Reflection from a perfectly smooth surface.
Specular (mirrorlike) reflection
Surfaces that fall between smooth and irregular
Semirough surfaces
Semirough surface will reflect as if it were a smooth surface whenever the cosine of the angle of incidence is greater than ?/8d: where d is the depth of the surface irregularity and ? is the wavelength of the incident wave.
Rayleigh criterion
Cos ?i > ?/8d
Modulation or redistribution of energy within a wavefront when it passes near the edge of an opaque object.
Diffraction
Diffraction occurs around the edge of the obstacle: which allows secondary waves to ÒsneakÓ around the corner of the obstacle
Shadow zone
States that the total voltage
intensity at a given point in space is the sum of the individual wave vectors
Linear Superposition
Electromagnetic waves travelling within EarthÕs atmosphere.
Terrestial waves
Communications between two or more points on Earth.
Terrestial radio communications
Used for high-frequency applications.
Sky waves
Earth Ðguided electromagnetic wave that travels over the surface of earth.
Surface wave
Travel essentially in a straight line between the transmit and receive antennas
Direct waves
Space wave propagation with direct waves.
Line-of-Sight (LOS) transmission
The curvature of Earth presents a horizon to space wave propagation.
Radio Horizon
Occurs when the density of the lower atmosphere is such that electromagnetic waves are trapped between it and EarthÕs surface
Duct propagation
Lowest layer of the ionosphere and is located approximately between 30 miles and 60 miles (50 km to 100 km) above EarthÕs surface
D Layer
Located approximately between 60 miles and 85 miles (100 km to 140 km) above EarthÕs surface
E Layer
The upper portion of the E layer.
Sporadic E layer
Made up of two layers: F 1 and F 2 layers.
F Layer
Highest frequency that can be propagated directly upward and still be returned to Earth by the ionosphere.
Critical frequency
Maximum vertical angle at which it can be propagated and still be refracted back by the ionosphere.
Critical Angle
A measurement technique used to determine the critical frequency.
Ionospheric Sounding
Height above the EarthÕs surface from which a refracted wave appears to have been reflected
Virtual Height
Highest frequency that can be used for sky wave propagation between two specific points on EarthÕs surface.
Maximum Usable Frequency (MUF)
Operating at a frequency of 85% of the MUF provides more reliable communications.
Optimum Working Frequency (OWF
Minimum distance from a transmit antenna that a sky wave at a given frequency will be returned to Earth
Skip distance
The area between where the surface waves are completely dissipated and the point where the first sky wave returns to Earth.
Quiet: or skip: zone
Formed by the ionosphere is raised: allowing sky waves to travel higher before being returned to Earth.
Ceiling
Define as the loss incurred by an electromagnetic waves as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objects.
Free-space path loss
Occurs simply because of the inverse square law.
Spreading loss
Variation in signal loss
Fading
