Dictionary Definition
radiotelephony n : telephony that uses
transmission by radio rather than by wire [syn: radiotelephone, wireless
telephone]
User Contributed Dictionary
English
Noun
- The transmission of sound (in both directions) using modulated radio waves rather than wires
See also
Extensive Definition
The Marine Radiotelephone Service or HF
ship-to-shore operates on shortwave radio frequencies,
using single-sideband
modulation. The usual method is that a ship calls a shore
station, and the shore station's marine operator connects the
caller to the
public switched telephone network. This service is retained for
safety reasons, but in practice has been made obsolete by satellite
telephones, particularly INMARSAT.
Short wave radio is used because it bounces
between the ionosphere and the ground,
giving a modest 1,000 watt transmitter (the standard power) a
world-wide range.
Most shore stations monitor several frequencies.
The frequencies with the longest range are usually near 20 MHz, but the
ionospheric weather (propagation) can dramatically change which
frequencies work best.
Single-sideband (SSB) is used because the short
wave bands are crowded with many users, and SSB permits a single
voice channel to use a narrower range of radio frequencies
(bandwidth), about 3.5 kHz. In comparison,
AM radio
uses about 8 kHz, and narrowband (voice or communication-quality)
FM uses 9
kHz.
Marine radiotelephony first became common in the
1930s, and was used extensively for communications to ships and
aircraft over water. In that time, most long-range aircraft had
long-wire antennas that would be let out during a call, and
reeled-in afterward.
One of the most important uses of marine
radiotelephony has been to change ships' itineraries, and to
perform other business at sea.
Some ships, including almost all military ships,
carry teletypewriters,
and use them to communicate over short wave. This is called "marine
radiotelegraphy",
but in practice the equipment is a normal shortwave radio with an
attachment that generates and receives
audio tones in order to drive the teletype.