Abstract: In an AM broadcast transmitter having a switching amplifier as main modulator (12), an additional modulator (18) is used in the radio frequency circuit in order to achieve a steady 100% modulation at the modulation minima, with the aid of which additional modulator the carrier signal is also modulated at the modulation minima. The LF signal needed for controlling the additional modulator (18) passes through a delay circuit (16) and a correction circuit (17).

Abstract: A compact and inexpensive LF power amplifier is disclosed. A plurality of pulse trains which are phase shifted relative to each other are width-modulated by an input LF signal. The modulated pulse trains are individually amplified, preferably by means of a plurality of respective power transformers and switch elements. The switch elements include any suitable combination of electron tubes and semiconductor switches. The keying ratio of the amplifier is improved by associating the pulse trains in pairs, the two members of each pair being phase shifted 180.degree. relative to each other. The amplified pulse trains are superimposed to form a single combined amplified pulse train.

Abstract: A wall lead-through structure for live feeders, particularly for use in antenna matching systems. The feeder is led through a wall opening which is covered on its outer side by an insulating mounting fixture which provides a centrally located opening for the feeder lead-through. The surface defining the wall opening is covered with a sheet metal lining, preferably sheet copper which serves as a corona protector, and a metallic corona protector encloses and is secured to the live feeder where it passes through the central opening in the insulating mounting fixture.