A broadcast transmitter (broadcast transmission system) in the technical sense is a transmission system for the generation and emission of electromagnetic waves . The of these radio waves transmitted radio - or television signal, the radio program is for the public is determined and on radio receivers ( radios or TV playing).
The carrier frequencies of the LW to SW transmitters are mostly amplitude modulated . The long-wave and sometimes also the medium-wave signals follow the curvature of the earth, so that they can be received worldwide with the appropriate transmission power. The waves of the higher medium wave band and short waves, on the other hand, are reflected between the boundary layers of the atmosphere and the earth's surface, so that they can also be used to transmit around the globe - with significantly lower transmission power. In this case, however, the signal can be received several times, which leads to fading . In addition, the amplitude modulation is very susceptible to interference from atmospheric charges and discharges ( e.g. thunderstorms ). Radiation is carried out with long-wire antennas in various designs (for example box antennas or long-wire antennas with coil extensions), which form one side of a dipole . The other side is the earth.
VHF radio transmitters work with frequency modulation and transmit via real dipoles with a coordinated length. The waves in this area are not reflected by the earth and the ionosphere , so that overreaching hardly occurs. In addition, the frequency modulation allows for interference-free transmission. Since an VHF radio transmitter has a significantly higher bandwidth available compared to a transmitter on long, medium and short wave, which allows a significantly better signal quality and, among other things, also enables the transmission of stereo broadcasts , work is predominantly carried out in the VHF range today.
A radio transmitter usually emits its electromagnetic waves in an undirected manner. This means that the signals can be received at an angle of 360 ° around the transmitter. This achieves good area coverage, which enables the reception of the programs for large parts of the population. In border regions, due to international agreements, the transmission may have to be directed, i.e. the transmission of the transmission signal in a certain direction is suppressed as far as possible.
The power range of a radio station can range from a few watts to a few megawatts.
Some radio stations also transmit special information that cannot be received with conventional receiving devices. Thus, the transmitting station Allouis of France Inter after AMDS process time signal. According to this procedure, the German broadcasting station Zehlendorf also transmitted press releases for shipping.
Furthermore, in the case of some strong long-wave radio transmitters in particular, the carrier frequency is derived from an atomic clock and can thus serve as a calibration frequency.