Radiation and Propagation of Waves Chapter 10 Standard Text Book Lecture # 4

Objectives Understand wave, electromagnetic waves Radiation and associated phenomenon Explain Reflection Refraction Diffraction Polarization Describe the Propagation of waves Ground Waves Sky waves Space Waves

Wave Wave is a mode of transfer of energy Transverse waves Longitudinal waves

Transverse Waves Transverse waves are those whose direction of propagation is perpendicular to both the electrical field and the magnetic field The electrical field and the magnetic fields lie in planes that are perpendicular to each other. (x and y planes) Thus the direction of propagation will be in the z plane or third dimension

Electromagnetic Waves Consist of Magnetic wave Electrical wave Most of the energy is returned to the circuit. If it isn’t, then some it must be “set free” or radiated. Radiated energy is not desirable. But if such power is “escaped on purpose” then it is said to be radiated

Wave Propagation Example electric field propagation direction magnetic field

Electrical to Magnetic Conversion The antennas are the transducers The transmitting antenna changes the electrical energy into electromagnetic or waves The receiving antenna changes the electromagnetic energy back into electrical energy These electromagnetic waves propagate at rates ranging from 150kHz to 300GHz

Radio-frequency Interference If the radiated energy comes from another radio transmitter, then it is considered radio-frequency interference (RFI) The transmitting antenna should be specifically designed to prevent the energy from being returned to the circuit. It is desirable that the antenna “free” the energy in order that it might radiate into space

Electromagnetic Interference If the energy comes from else where, then it is electromagnetic interference (EMI)

A few Concepts at a glance Free Space Which does not interfere with normal radiation and propagation Point Source A simple point acting like a source radiating in all directions Power density Power per unit area Isotropic source one which radiates uniformly in all directions

Polarization of the Electrical Field The polarization of the electrical field is determined by the direction of oscillations If the oscillations are in the vertical direction then the polarization is said to be vertical If the oscillations are in the horizontal direction then the polarization is said to be horizontal

Polarization of antenna Thus a “vertical” antenna will result in a vertically polarized wave. A vertical antenna is one that consists of a vertical tower, wire, or rod, usually a quarter wavelength in length that is fed at the ground and uses the ground as a reflecting surface.

Wavefronts A wavefront is a plane joining all points of equal phase in a wave Take a point in space. Imagine waves radiating outward in all directions from this point. The result would resemble a sphere. The point of radiation is called the isotropic point source

Isotropic Power Since the power at any point away from the isotropic point is inversely proportional to the square of the distance from the point, then the power decreases rapidly the further away from the point you need. Although the wavefront is curved in shape, from a distance small sections appear planar and can be thought of as plane wavefronts

Reflection Reflection is the abrupt reversal in direction Caused by any conductive medium such as Metal surfaces or Earth’s surface There will normally be a shift in phase Coefficient of reflection is less than 1

Complete Reflection Complete reflection will occur only in perfect conductors and when the electric field is perpendicular to the reflecting element or medium Coefficient of Reflection will be 1 Coefficient of Reflection is the ratio of the reflected wave intensity to the incident wave intensity

Refraction Occurs when the waves pass from one medium to another whose densities are different Coefficient of reflection is less than 1 The angle of incidence and the angle of refraction is related by Snell’s Law

Refraction Sub-Refraction Refraction (straight line) Normal Refraction Earth Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere. Amount of refractivity depends on the height above ground. Refractivity is usually largest at low elevations. The refractivity gradient (k-factor) usually causes microwave signals to curve slightly downward toward the earth, making the radio horizon father away than the visual horizon. This can increase the microwave path by about 15%,

Diffraction Waves traveling in straight lines bend around obstacles Based on Huygen’s principle (1690) Each point on a wavefront can be thought of as an isotropic point or a source of secondary spherical energy Concepts explains why radio waves can be heard behind tall mountains or buildings that are normally considered to block line of sight transmissions

Attenuation and Obstructions Shorter the wavelength (higher frequency) of the wireless signal, the more the signal it is attenuated. Same wavelength (frequency), less amplitude. Longer the wavelength (lower frequency) of the wireless signal, the less the signal is attenuated.

Propagation…..

Ground and Space Waves

Ground-Wave Propagation The curved surface of the Earth horizon can diffract long-wavelength (low frequency) radio waves. The waves can follow the curvature of the Earth for as much as several hundred miles.

Ground-Wave Propagation Results from a radio wave diffraction along the Earth’s surface. Primarily affects longer wavelength radio waves that have vertical polarization (electric field is oriented vertically). Most noticeable on AM broadcast band and the 160 meter and 80 meter amateur bands. Communication distances often extend to 120 miles or more. Most useful during the day at 1.8 MHz and 3.5 MHz when the D-Region absorption makes sky-wave propagation impossible.

Attenuation related to frequency Loses increase with increase in frequency Not very effective at frequencies above 2Mhz Very reliable communication link Reception is not affected by daily or seasonal weather changes

Used to communicate with submarines ELF (30 to 300 Hz) propagation is utilized

Sky Wave Propagation

Atmospheric Regions Region Height Notes Troposphere 7 miles Region where all weather occurs Stratosphere 6 to 30 miles Region where atmospheric gases “spread out” horizontally. The high speed jet stream travels in the stratosphere. Ionosphere 30 to 400 miles Region where solar radiation from the sun creates ions. Major influence on HF radio wave propagation.

Atmospheric Regions

What is the ionosphere? The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. At heights above 80 km (50 miles), the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by nearby ions. This part of the atmosphere is ionized and contains a plasma. In a plasma, negative free electrons and positive ions are attached by the electromagnetic force, but they are too energetic to stay fixed together in neutral molecules.

Sky-wave Propagation Ionization levels in the Earth’s ionosphere can refract (bend) radio waves to return to the surface. Ions in the Earth’s upper atmosphere are formed when ultraviolet (UV) radiation and other radiation from the sun knocks electrons from gas atoms. The ionization regions in the Earth’s ionosphere is affected the sunspots on the sun’s surface

FIGURE 12-9 Sky-wave propagation. Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Sky Wave Propagation

Radio waves radiated from the transmitting antenna in a direction toward the ionosphere Long distance transmissions Sky wave strike the ionosphere, is refracted back to ground, strike the ground, reflected back toward the ionosphere, etc until it reaches the receiving antenna Skipping is the refraction and reflection of sky waves

Ionosphere The layers that form the ionosphere vary greatly in altitude, density, and thickness with the varying degrees of solar activity. The upper portion of the F layer is most affected by sunspots or solar disturbances There is a greater concentration of solar radiation during peak sunspot activity. The greater radiation activity the more dense the F layer and the higher the F layer becomes and the greater the skip distance

FIGURE 12-11 Relationship of frequency to refraction by the ionosphere. Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

FIGURE 12-12 Relationship of frequency to critical angle. Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Terms Critical Frequency: Critical Angle: The highest frequency that will be returned to the earth when transmitted vertically under given ionospheric conditions Critical Angle: The highest angle with respect to a vertical line at which a radio wave of a specified frequency can be propagated and still be returned to the earth from the ionosphere

Maximum usable frequency (MUF) The highest frequency that is returned to the earth from the ionosphere between two specific points on earth Optimum Working frequency: The frequency that provides for the most consistent communication path via sky waves

Quiet Zone or Skip Zone: The space between the point where the ground wave is completely dissipated and the point where the first sky wave is received Fading: Variations in signal strength that may occur at the receiver over a period of time.

Space Wave Two types Direct Ground reflected

FIGURE 12-6 Direct and ground reflected space waves. Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Direct Limited to “line-of sight” transmission distances Antenna height and curvature of earth are limiting factors Radio horizon is about 80% greater than line of sight because of diffraction effects

FIGURE 12-7 Radio horizon for direct space waves. Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Reflected Part of the signal from the transmitter is bounced off the ground and reflected back to the receiving antenna Can cause problems if the phase between the direct wave and the reflected wave are not in phase Detuning the antenna so that the reflected wave is too weak to receive

Tropospheric scattering Signals are aimed at the troposphere rather than the ionosphere 350 Mhz to 10GHz for paths up to 400 mi Received signal = 10-6 th of the transmitted power Fading a problem

Line-Of-Sight Propagation Radio signals travel in a straight line from a transmitting antenna to the receiving antenna. Provides VHF/UHF communications within a 100 miles or so. Signals can be reflected by buildings, hills, airplanes, etc.

Radio Path Horizon The distance D to the radio horizon is greater from a higher antenna. The maximum distance over which two stations may communicate by space wave is equal to the sum of their distances to the horizon.

VHF/UHF Signals Through Ionosphere Sporadic E A type of sky-wave propagation that allows long distance communication on the VHF bands (6 meters, 2 meters and 220 Mhz) through the E region of the atmosphere. Occurs only sporadically during certain times of the year.

Radio Spectrum Symbol Frequency range Wavelength,  Comments ELF < 300 Hz > 1000 km Earth-ionosphere waveguide propagation ULF 300 Hz – 3 kHz 1000 – 100 km VLF 3 kHz – 30 kHz 100 – 10 km LF 30 – 300 kHz 10 – 1 km Ground wave propagation MF 300 kHz – 3 MHz 1 km – 100 m HF 3 – 30 MHz 100 – 10 m Ionospheric sky-wave propagation VHF 30 – 300 MHz 10 – 1 m Space waves, scattering by objects similarly sized to, or bigger than, a free-space wavelength, increasingly affected by tropospheric phenomena UHF 300 MHz – 3 GHz 1 m – 100 mm SHF 3 – 30 GHz 100 – 10 mm EHF 30 – 300 GHz 10 – 1 mm