Antenna amplifier
An antenna amplifier is an amplifier that amplifies the received signal coming from an antenna . It differs from the input amplifier of a radio receiver in that it is usually broadband and sits at the beginning of a cable.
Antenna amplifiers are used to
- to use a receiving antenna for several receivers (community antenna system)
- to at least compensate for the attenuation of the antenna cable up to the receiver
- to supply a receiver with low input sensitivity
A so-called active antenna has a broadband amplifier directly next to the antenna, which, however, is part of the antenna and cannot be separated from it. Such amplifiers are therefore not explicitly referred to as antenna amplifiers. Active antennas are u. a. Common in passenger cars to amplify the weak signal from an antenna integrated into the windshield, for example, so that it can reach the radio undisturbed.
Antenna amplifiers are installed as close as possible to the antenna so that a signal that is not additionally attenuated or disturbed by the cable can be processed. For this purpose, they are usually supplied with their operating voltage via the antenna cable. A direct current switch is used for this.
Multi-range antenna amplifiers for radio reception have separate selective amplifiers for all television bands (Band I, Band III and Band IV / V) as well as for several radio areas (long / medium / short wave and ultra short wave). Such amplifiers consist of bandpass filters for connecting the respective antennas and an attenuation adjuster arranged in front of the amplifier in order to be able to adapt the individual reception levels regionally. The aim is on the one hand to amplify weak signals and on the other hand to avoid excessively high levels. The outputs of the individual amplifiers are brought together to one output by means of a switch . From there, a cable supplies one or more users, where the frequencies are again separated and passed to the respective receivers.
The level in a communal antenna system must be significantly higher than it would be necessary for individual receivers, since not only the sum of all receivers is supplied, but the subscriber attenuation of the antenna sockets must also be compensated.
Antenna amplifiers should be low-noise , low- intermodulation and low-distortion even at high levels. Non-linear distortion and cross-modulation can result from clipping. For this reason, antenna amplifiers often specify the maximum level up to which they can amplify. Linear distortions are caused by frequency-selective elements.
The use of a broadband amplifier for all reception frequencies merged by means of a crossover can lead to mutual interference if a strong transmitter is also received.