Understanding Signal Acquisition Systems: Antennas and Preamplifiers
Signal Acquisition Systems: Antennas and Preamplifiers
Antennas
An antenna is an open oscillatory circuit that transforms energy into electrical oscillations. It transmits and receives electromagnetic waves, converting electrical signals into waves and vice versa. It acts as the final element in wave emission and the initial element in signal reception.
Types of Signals
- Radio & TV (Analog and Digital)
- Analog FM Radio (designed for mobile reception with high power)
- DAB Digital Radio (designed for stationary reception)
Antenna Construction
Antennas operate based on a dipole element, typically made of aluminum or duralumin due to their light weight, durability, conductivity, and affordability.
Key Components:
- Dipole: Collects electromagnetic waves. Consists of two conductive wires or rods arranged end-to-end in opposite directions. Length is half the radiated wavelength.
- Reflector: Passive element behind the dipole, reflecting energy back to increase signal strength (3dB gain).
- Directors: Passive elements in front of the dipole, narrowing the reception lobe and enhancing signal capture from greater distances.
Dipole Types:
- Half-wave Dipole: Length matches half the wavelength. Fed at the center, creating standing waves of voltage and current.
- Folded Dipole: Higher bandwidth and multichannel reception capabilities.
Characteristics of an Antenna
- Gain: Measures the increase in signal level compared to a reference antenna (expressed in decibels).
- Directivity: Ability to capture signals from a specific direction.
- Forward/Backward Ratio: Attenuation of the posterior lobe, minimizing interference from opposite directions.
- Bandwidth: Frequency range over which the antenna operates effectively.
- Impedance: Facilitates efficient energy transfer between the antenna and the cable.
Antenna Types
- Yagi: Consists of a dipole, directors, and a reflector. Suitable for broadband, narrowband, and multiband applications.
- PRO: Variant of the Yagi with angled director arrangement for improved bandwidth and gain.
- Logarithmically Periodic: Multiband antenna with a group of active dipoles.
- Panel Antenna: Array of vertically arranged dipoles for medium gain and good bandwidth.
- DAT Antenna: Designed for digital signals with angled director stacking.
- Electronic Antenna: Integrated into a circuit board with an amplifier stage.
Preamplifiers
Preamplifiers boost the signal received by the antenna, ensuring a clear TV picture. They are essential when signal levels are low and are typically installed on the antenna itself.
Types of Preamplifiers
- Mast-mounted Preamplifiers:
- Broadband amplifiers with single input and output.
- Broadband amplifiers with multiple inputs for amplifying and mixing signals from different bands.
Characteristics of Antenna Amplifiers
- Gain: Difference in signal level between input and output.
- Noise Factor: Level of noise introduced by the amplifier (manifests as snow or spots on the TV).
- Maximum Input Voltage: Voltage level beyond which intermodulation occurs.
- Intermodulation: Signal distortion due to high input voltage.
- Minimum Input Voltage: Minimum voltage for acceptable signal-to-noise ratio.
- Output Voltage: Maximum signal level the amplifier can deliver without distortion.
- Input/Output Return Loss: Measure of impedance matching between the amplifier and the distribution network.