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What phenomenon occurs when a semirough surface reflects as if it were a smooth surface based on the angle of incidence and wavelength?
Rayleigh criterion
What is the term for the modulation or redistribution of energy within a wavefront when it passes near the edge of an opaque object?
Diffraction
What term describes the zone where diffraction allows secondary waves to 'sneak' around the corner of an obstacle?
Shadow zone
What principle states that the total voltage intensity at a given point is the sum of individual wave vectors?
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What are Terrestrial Waves also known as?
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Ground waves are limited to very low, low, and medium frequencies. Is this statement true?
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What is the Propagation Constant used to express?
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Polarization vector remains constant.
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What is the dielectric constant?
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______ loss is the inherent unavoidable loss in a transmission line.
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Match the following types of polarization with their descriptions:
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What is the woven stranded mesh that surrounds some types of coaxial cables?
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What is the term for having one layer of foil insulation and one layer of braided shielding?
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What are the types of coaxial cables made of either rigid air-filled or flexible material?
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What is the term for signal power being lost due to conductor loss, radiation loss, and dielectric heating?
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What is the impedance seen when looking into an infinitely long transmission line?
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What reflects the incident power back to the source in a transmission line?
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What is the term for a technique used to locate an impairment in a metallic cable?
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Study Notes
Transmission Lines
- Transmission lines are metallic conductor systems used to transfer electrical energy from one point to another using electrical current flow.
- Propagation Constant is used to express the attenuation and phase shift per unit length of a transmission line.
- Velocity factor is the ratio of the actual velocity of propagation of an electromagnetic wave through a given medium to the velocity of propagation through a vacuum.
- Dielectric Constant is the relative permittivity of a material.
Types of Transmission Lines
- Single-ended or Unbalanced Transmission Lines: one wire is at ground potential, and the other wire is at signal potential.
- Balanced Transmission Lines: with two-wire balanced lines, both conductors carry current, but one conductor carries the signal, and the other conductor is the return path.
- Open-wire Transmission Line: consists of two parallel wires, closely spaced and separated by air.
- Twin-lead Transmission Line: same as open-wire transmission line, except that the spacers between the two conductors are replaced with a continuous solid dielectric.
- Twisted-wire pair Transmission Line: formed by twisting two insulated conductors around each other.
- Shielded-cable Transmission Line: is a parallel two-wire transmission line consisting of two copper conductors separated by a solid dielectric material.
- Concentric Transmission Line: often used for high data transmission rates to reduce losses and isolate transmission paths.
Transmission Line Losses
- Five types of transmission line losses: Conductor loss, Radiation loss, Dielectric heating loss, Coupling loss, and Corona.
Electromagnetic Wave Propagation
- Propagation of electromagnetic waves is often called radio-frequency (RF) propagation or simply radio propagation.
- Electrical energy that has escaped into free space is called electromagnetic wave.
- Polarization is the orientation of the electric field vector in respect to the surface of the Earth.
- Types of polarization: Linear Polarization, Circular Polarization, and Elliptical Polarization.
Electromagnetic Waves
- Waves propagate through free space, they spread out, resulting in a reduction in power density.
- Power density is inversely proportional to the square of the distance from the source (Inverse Square Law).
- Refraction, Reflection, Diffraction, and Interference are optical properties of Radio Waves.
Refraction and Reflection
- Refraction is the bending of the radio wave path.
- Refractive index is the square root of the dielectric constant and is expressed in: n = (k).
- Reflection is the ratio of the reflected to the incident voltage intensities.
- Reflection Coefficient is the ratio of the reflected voltage to the incident voltage or the reflected current to the incident current.
Diffraction
- Diffraction occurs when an incident wave front strikes an irregular surface, it is randomly scattered in many directions.
- Diffraction occurs around the edge of the obstacle, which allows secondary waves to “sneak” around the corner of the obstacle.
Terrestrial Waves
- Electromagnetic waves traveling within Earth’s atmosphere are called Terrestrial waves.
- Communications between two or more points on Earth using Terrestrial waves are called Terrestrial radio communications.
Surface Waves
- Surface waves are Earth-guided electromagnetic waves that travel over the surface of the earth.
- Relative Conductivity of Earth Surfaces affects surface wave propagation.
Ground Waves
- Ground waves are limited to very low, low, and medium frequencies.
- Disadvantages of surface waves: requiring relatively high transmission power, limited to low frequencies, requiring large antennas, and ground losses vary considerably with surface material and composition.
- Advantages of ground wave propagation: given enough transmit power, ground waves can be used to communicate between any two locations in the world, and ground waves are relatively unaffected by changing atmospheric conditions.
Space Waves
- Space wave propagation with direct waves occurs when the curvature of the Earth presents a horizon to space wave propagation.
- Radio Horizon is the horizon to space wave propagation.
- Duct propagation occurs when the density of the lower atmosphere is such that electromagnetic waves are trapped between it and Earth’s surface.
Ionosphere
- The ionosphere is the lowest layer of the atmosphere and is located approximately between 30 miles and 60 miles above Earth’s surface.
- The ionosphere is made up of several layers, including the D Layer, E Layer, and F Layer.
- Ionospheric Sounding is a measurement technique used to determine the critical frequency.
Sky Waves
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Sky waves are used for high-frequency applications.
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Maximum Usable Frequency (MUF) is the highest frequency that can be used for sky wave propagation between two specific points on Earth’s surface.
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Operating at a frequency of 85% of the MUF provides more reliable communications (Optimum Working Frequency).### Fading and Transmission Media
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To accommodate temporary fading, an additional fade margin loss is added to the normal path loss, calculated as Fm = 30 logD + 10log (6ABf) – 10log (1-R) – 70.
Guided Transmission Media
- Guided transmission media provide a conduit in which electromagnetic signals are contained.
- Examples of guided transmission media include cable transmission and transmission lines.
Types of Transmission Media
- Metallic conductor system used to transfer electrical energy from one point to another using electrical current flow is a transmission line.
- Two basic kinds of waves are longitudinal and transverse waves.
Cable Transmission
- Cable transmission is used to propagate electromagnetic signals between two locations in a communications system.
- Most common means of interconnecting devices in local area networks use cable transmission systems.
- Examples of metallic cables used to interconnect data communications systems and computer networks include parallel-conductor transmission lines, coaxial transmission lines, and twisted-pair cables.
Twisted-Pair Cables
- Twisted-pair cables are formed by twisting two insulated conductors around each other.
- Types of twisted-pair cables include unshielded twisted pair (UTP) and shielded twisted pair (STP).
- The EIA specifies a standard color code for CAT-5 cable, including pair 1: blue/white stripe and blue, pair 2: orange/white stripe and orange, and so on.
Coaxial Cables
- Coaxial cables are used for high data transmission rates to reduce losses and isolate transmission path.
- Types of coaxial cables include rigid air-filled, solid, and flexible cables.
- Coaxial cables have a woven stranded mesh that surrounds the inner conductor, known as shielding, which can be single or dual-layered.
Transmission Line Characteristics
- Distributed parameters refer to the uniform distribution of transmission characteristics throughout the length of the line.
- Secondary constants refer to the transmission characteristics of a transmission line.
- Surge impedance is the impedance seen looking into an infinitely long line.
- Propagation constants express the attenuation and the phase shift per unit length of a transmission line.
- Velocity factor is the ratio of the actual velocity of propagation of an electromagnetic wave through a given medium to the velocity of propagation through a vacuum.
Signal Loss
- Signal power is lost in several ways, including conductor loss, radiation loss, dielectric heating, and coupling loss.
- Incident voltage is the voltage that propagates from the source toward the load, while reflected voltage is the voltage that propagates from the load toward the source.
- Reflection coefficient is a vector quantity that represents the ratio of reflected voltage to incident voltage or reflected current to incident current.
Transmission Line Conditions
- A matched line is a transmission line with no reflected power, where the incident power is absorbed by the load.
- An unmatched or mismatched line is a transmission line where incident power is returned (reflected) to the source.
- A standing wave is an interference pattern set up by two travelling waves, with a standing-wave ratio that is the ratio of the maximum voltage to the minimum voltage or the maximum current to the minimum current.
Other Transmission Media
- Microstrip is a flat conductor separated from a ground plane by an insulating di-electric material.
- Stripline is a flat conductor sandwiched between two ground planes.
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Description
Quiz on transmission lines, including propagation constant, velocity factor, and Transverse Electromagnetic Waves. Understand the concepts of electromagnetic wave propagation through transmission lines.
