Understanding TV Waves: Transmission, Propagation, and Reception
TV Waves: Transmission, Propagation, and Reception
What are TV Waves?
TV waves are electromagnetic waves that travel at the speed of light (300,000 km/s). They are generated by the propagation of an electrostatic field associated with an electric field in empty space. These fields are perpendicular to each other, forming a 90-degree angle.
Key Features of TV Waves
- Can travel in a vacuum.
- Speed depends on the physical environment.
- Comprised of an electric field and a magnetic field, defined by orthogonal vectors.
- Characterized by amplitude, frequency, wavelength, and velocity of propagation.
- Polarization depends on the direction of the electric field vector.
- Subject to interference and noise.
Polarization of Electromagnetic Waves
The trajectory described by the electric field vector associated with electromagnetic wave propagation, permanently maintaining orthogonality with the magnetic field.
TV Bands and Channels
TV channels are grouped into bands:
- BI (VHF 2-4, 47-68)
- BIII (VHF 5-12, 174-230)
- BIV (UHF 21-37, 470-606)
- BV (38-69, 606 — 862)
Means of Spreading the TV Signal
The TV signal is spread in the form of concentric circles from the broadcast antenna. The signal strength decreases as the distance from the transmitting antenna increases.
Propagation Systems
Propagation by Air
TV waves can propagate through the air in several ways:
- Direct ground waves: These waves travel directly from the transmitting antenna to the receiving antenna without encountering any obstacles.
- Ground reflected waves: When the direct signal is blocked, the signal can be reflected by buildings, mountains, or the land surface. If the antenna picks up both the direct and reflected signals, it can result in a double image, with the weaker reflected signal appearing as a shadow of the received image.
Length Limit of Electromagnetic Waves on Earth
The maximum range of TV waves on Earth is limited by the line of sight between the transmitting and receiving antennas. When the distance between the antennas exceeds the line of sight, signals are typically not received. However, diffraction and reflections in clouds can increase the maximum range.
Electromagnetic Waves in Space
TV waves can also propagate through space or the atmosphere. As they travel, they can be affected by obstacles, causing some waves to return to the Earth’s surface, allowing signal reception beyond the direct optical horizon.
Tropospheric Waves
Clouds in the troposphere (a layer of air located at 10 to 12 km above the Earth’s surface) can reflect some types of electromagnetic waves. This can cause the waves to be scattered, allowing them to cover large geographical areas with flat or sea surfaces. Directional antennas and high power are used for this type of propagation.
Ionospheric Waves
The ionosphere, located between 35 and 400 km above sea level, is a highly ionized layer due to the sun’s rays. It can be divided into three layers that affect waves differently:
- Layer 1: Up to 70 km above sea level. Ionization is very small. Waves with frequencies around 0.15 to 0.285 MHz (long wave) are reflected.
- Layer 2: Between 80 and 140 km above sea level. Higher ionization allows reflections to achieve distances up to 2,000 km, with frequencies of 0.57 to 1.605 MHz (medium wave).
- Layer 3: Between 150 and 500 km above sea level. This layer is used for long-distance shortwave broadcasts from 2.30 to 26.1 MHz. The maximum range of an electromagnetic wave is influenced by factors such as the day of the year, frequency of waves, solar activity, time of day, and angle of incidence.
Electromagnetic Wave Transmission Through Cable
Coaxial cables are used to transmit TV signals. They carry electrical currents with lower voltage and current values than power lines, but with much higher frequencies.
Characteristics of Coaxial Cables
- Operating Frequency Range: The frequency range over which the cable maintains its properties (typically between 0-3 GHz).
- Impedance: A key parameter for energy transfer. If the impedance of the signal source, load line, and cable are equal, energy transfer is maximized. If the voltage and resistance are in phase, the impedance is purely resistive.
- Standing Wave Ratio (SWR): Measures the mismatch between the ideal impedance (75 ohms) and the actual impedance. It is also defined as the ratio between the maximum and minimum values of voltage or current. The amplitude of a standing wave has maximum points called antinodes and minimum points called nodes. SWR is related to efficient energy transmission, impedance mismatch, and the appearance of reflected waves.
- Voltage Standing Wave Ratio (VSWR): Indicates the ratio of the reflected signal to the incident signal. In practice, VSWR has a value between 6 and 25 dB.
- Reflection Coefficient: The relationship between the voltage of the reflected wave and the incident wave at a specific point on the line. Impedance matching occurs when the reflection coefficient is 1, meaning the line is closed for its ideal impedance.
- Screenings: Determines the cable’s ability to radiate or be immune to external electromagnetic waves. It depends on the construction of the outer mesh.
- Wire Resistance: Determines the voltage drop when power devices are connected to the cable.
- Speed of Propagation: Determined by the type and construction of the dielectric. Measures the relationship between the speed of propagation in the cable and in a vacuum.
Types and Characteristics of Distribution and Installation
Terrestrial TV reception installations consist of elements that receive, transmit, and transform electromagnetic waves to appear on TV. The goal is to ensure the best possible viewing experience.
Individual Installations
are those designed to serve a single dwelling. The signal must be distributed by a line less than 40m. These facilities are typically used only mixing systems when the waves come from different bands. (UHF and VHF). In the event that the signal has a low base, will amplify the signal. When signal levels are quite different from other channels were equalized by amplifiers or attenuators. If several doses must comply with the regulations (ICT). Facilities collective phenomena are avoided interference that occurred between antennas, also avoiding disturbances in a home can move to another. In buildings with more than 10 houses the antenna must be collective. The demands on demand to a facility shared dishes are: “That’s signal levels in quality and intensity between the margins are required by law. “That is the proper impedance for the TV input 75 ohms.