Antenna Theory: Key Parameters, Types, and Applications
Antenna Fundamentals
Emitting vs. Receiving Antennas
There is no significant difference in principle between an emitting and a receiving antenna, as the transformation is reversible. However, emitting antennas often use thicker materials to dissipate the applied power. Receiving antennas undergo strict quality control.
Key Antenna Parameters
- Resonance Frequency: The frequency at which reactive components are canceled out, leaving only resistive components.
- Bandwidth: The frequency range considered valid, typically 70% of the radiated frequencies.
- Impedance: Standardized at 75 Ohms for radio and TV.
- Directivity: Indicated by the radiation diagram.
- Gain: Antennas are often designed for higher gain in a specific direction.
- Beam Opening: Determined by subtracting 3 dB from the maximum level of the main lobe.
- Front-to-Back Ratio: Expressed in dB.
- Performance.
- Wind Load.
Cassegrain Antennas
Using a small Cassegrain antenna is not advisable. The shadow zones created by the secondary reflector introduce significant losses, reducing antenna performance.
Log-Periodic Antennas
A log-periodic antenna is formed by associating medium-sized dipoles with progressively increasing separation. The antenna is fed such that, starting from a single line, the signal is distributed in opposite phases between adjacent dipoles. This results in a selective antenna in the direction of the smaller dipole, resonant over a large frequency range, and producing different minimum impedance with periodic separations.
Hertz Dipole vs. Marconi Antenna
The Marconi antenna can be seen as one of the two rods of the Hertz antenna, with the other replaced by a conductor plane placed perpendicular to it. This allows the antenna to resonate at a quarter of the wavelength. This creates a dipole whose lower half is “virtual,” making it a good alternative for generating or receiving vertically polarized signals. Its characteristics are similar to those of a simple dipole.
High-Performance Satellite Dishes
The offset parabolic antenna offers the highest performance, reaching up to 80%, due to the absence of shadow zones.
Multi-Satellite Devices
Multi-satellite devices can be used to capture signals from satellites whose orbital positions are separated by a maximum of 12 degrees.
Yagi Antenna Elements
The largest element in a Yagi antenna is the reflector, whose size corresponds to half the length of the wave it receives.
Antenna Wind Load
Wind load is a physical parameter that evaluates the antenna’s resistance to wind when mounted on a mast.
This parameter (expressed in kilograms or newtons) must be considered when choosing the appropriate mast to support the antenna and determining the order of location for masts with multiple antennas.
DAB Digital Radio Polarization
Vertical polarization is used to receive DAB digital radio.
Coaxial Cables: Indoor vs. Outdoor
The primary difference between coaxial cables for indoor and outdoor installations lies in the outer sheath material. Indoor cables often use PVC, which degrades under solar radiation. Outdoor cables require a polyethylene cover to withstand weather conditions and prevent degradation over time.