Radiotelegraph Communications

telecommunications by means of radio waves used for transmitting discrete messages (letters, numerals, or characters). During the first 20–30 years after the invention of radio in 1895, the transmission of messages in radiotelegraph communications was carried out manually using the Morse code, and the reception was auditory; these methods are still being used today in amateur radio communications. High-speed automatic transmitters and recording receivers (siphon recorders) later came into use, giving radiotelegraph communications a capacity of 250–300 words per min. However, this made it necessary to record the message on a perforated tape before transmitting and to decode the message from the tape of the siphon recorder after receiving. These drawbacks were subsequently eliminated by the use of type-printing telegraph apparatus.

In contrast to communications using wired telegraphy, radiotelegraph communications can be established over long distances without any repeaters. In addition, radiotelegraphy permits communications with objects that move on the earth’s surface or above the earth. However, radio interference strongly influences reception quality on long links, particularly with operating wavelengths in the decameter (shortwave) range; the level of interference may sometimes exceed that of the signal. For this reason noise-immune methods of transmission employing phase and frequency modulation are being used more and more frequently.

In addition to conventional typeprinting apparatus, use is also made of equipment capable of detecting and correcting errors. Error correction is done by an automatic inquiry directed to the transmitting point and automatic retransmission of each distorted character or group of characters. Such equipment introduces a redundancy into the messages being transmitted, which involves an increase in signal power, signal bandwidth, or the duration of transmission. In the last case particularly, correcting codes, such as a seven-digit unit-distance code, may be used, or the transmission speed may be lowered significantly.