Understanding Radio Wave Transmission, Antennas, and Satellite Reception
The Waveform
The waveform for radio transmission is sinusoidal for both electric and magnetic fields. A complete cycle includes a rise to a positive peak, descent to zero, a further descent to a negative peak, and a rise back to zero. A half-cycle is a semiciclo. The period is the duration of one cycle (s). Wavelength is represented as λ. A half-wave is half of this measure. Fundamental frequency indicates how many times a cycle repeats per unit of time (Hz) for non-sinusoidal signals.
Electromagnetic Waves
Electromagnetic waves propagate TV and radio signals through space (v = 300,000,000 m/s). They are composed of perpendicular electric and magnetic fields, both perpendicular to the direction of wave propagation.
Radio Spectrum
The radio spectrum encompasses frequencies between 3000 Hz and 3000 GHz, including AM, FM, satellite broadcasts, and terrestrial TV broadcasts.
The Propagation of Electromagnetic Waves
Electromagnetic waves travel from a sender to one or more receivers. This can be through space (satellites) or via terrestrial repeaters. Terrestrial propagation follows a straight line, but Earth’s curvature limits reception range. Antennas are placed as high as possible to overcome this.
The Antenna
The antenna is crucial for transmitting and receiving electromagnetic waves. It’s present in both transmitters and receivers with similar characteristics. An antenna is essentially a wire or metal rod. In a transmitter, oscillating current creates an electromagnetic field. In a receiver, the wave induces current in the wire. A dipole antenna is fed current through a coaxial cable, consisting of two conductors. A long antenna connects one conductor to a horizontal rod and the other to ground. A half-dipole connects each external element to a coaxial cable conductor. A folded dipole has a T-shape. The effective length of a dipole is where resistance (R) is optimal, typically around 75 Ω.
Reflectors and Directors
- Directors are metal elements enhancing antenna directivity. Placed in front of the dipole, they focus the signal into a main lobe.
- Reflectors are positioned behind the dipole to reduce unwanted signal reception.
Antenna Parameters
Antennas both receive and transmit. Key parameters include:
- Directivity: Indicates signal concentration.
- Front/Back Ratio: Compares front and back lobes.
- VSWR (Voltage Standing Wave Ratio): Measures impedance matching.
- Polarization: Refers to the orientation of the electric and magnetic fields. TV antennas often use horizontal polarization. Satellite TV uses horizontal/vertical (FSS) or RHCP/LHCP (DBS).
The Cable
Coaxial cable distributes TV signals in community and individual installations. It has low losses and a concentric structure minimizing radiation losses. It consists of two concentric conductors (live and mesh) separated by a dielectric.
Composition of a Community TV Receiving Facility
A community TV system includes a receiving system, headend equipment, and a distribution network. Amplifiers boost received signals for distribution. There are power amplifiers and single-channel amplifiers. Amplifiers are single units with multiple inputs depending on the antennas.
Realization of an Installation
Installation involves calculating signal distribution, using serial (step attenuators) or parallel (splitters) configurations. Splitters are preferred for their better performance and independence of each dwelling. Calculations must account for signal losses. Losses include cable attenuation, splitter insertion loss, step attenuator loss, and tap-off loss. The choice of components depends on the desired channels and signal origin.
Satellite Reception
Satellites for radio and TV broadcasting are located in geostationary orbit (36,000 km above the equator), in the Clarke belt. They move in sync with Earth. The Effective Isotropic Radiated Power (EIRP) is the power a satellite would need to radiate from an isotropic antenna to produce the same effect as its actual antenna.
Parabolic Dish
The parabolic dish focuses the signal onto a point called the focus. Its shape determines the focal point’s location. Types of parabolic dishes include:
- Prime Focus: Dipole at the focal point, commonly roof-mounted.
- Offset: Dipole reflector is outside the dish surface, avoiding shadows and improving performance. The focus is shifted from the center.
- Cassegrain: Uses a secondary reflector (subreflector) with a hyperbolic shape. The LNB faces the subreflector, requiring two signal reflections. Used in radio astronomy with large diameters.
Converter or LNB
The Low-Noise Block downconverter (LNB) is the main part of the dish, containing the electronic circuitry and dipole. It translates the signal to a lower frequency. It consists of a Low-Noise Amplifier (LNA) and a Low-Noise Converter (LNC). The LNB is powered through the coaxial cable, with voltage switching the dipole’s polarity (12-14V for vertical, 15-20V for horizontal).
Orientation of Satellite
Satellite dish orientation requires finding the azimuth and elevation. Azimuth is the horizontal angle (east to west), and elevation is the vertical angle from the ground to the satellite.
Indoor Unit
The indoor unit processes the signal from the antenna, adapting it to the intermediate frequency (IF) and the distribution type.
Installing the Parabolic Dish
Dish size and type are determined by the EIRP from the satellite’s footprint. The LNB choice depends on the desired frequency range and dish type.