500 KHz - Later Policies

Later Policies

The silent and monitoring periods were soon expanded and standardized. For example, Regulation 44, from the July 27, 1914 edition of "Radio Communication Laws of the United States", stated: "The international standard wave length is 600 meters, and the operators of all coast stations are required, during the hours the station is in operation, to 'listen in' at intervals of not more than 15 minutes and for a period not less than 2 minutes, with the receiving apparatus tuned to receive this wave length, for the purpose of determining if any distress signals or messages are being sent and to determine if the transmitting operations of the 'listening station' are causing interference with other radio communication."

International refinements for the use of 500 kHz were specified in later agreements, including the 1932 Madrid Radio Conference. In later years, except for distress traffic, stations shifted to nearby "working frequencies" (425, 454, 468, 480, and 512 kHz) to exchange messages once contact was established. Twice each hour, all stations operating on 500 kHz were required to maintain a strictly enforced three-minute silent period, starting at 15 and 45 minutes past the hour.

As a visual memory aid, a typical clock in a ship's radio room would have the silence periods marked by shading the sectors between h+15 to h+18 and h+45 to h+48 in RED. Similar sectors between h+00 to H+03 and h+30 to h+33 are marked in GREEN which is the corresponding silence period for 2182 kHz. In addition, during this silent period all coastal and ship stations were required to monitor the frequency, listening for any distress signals. All large ships at sea had to monitor 500 kHz at all times, either with a licensed radio operator or with equipment that detected an automatic alarm signal. Shore stations throughout the world operated on this frequency to exchange messages with ships and to issue warning about weather and other navigational warnings. At night, transmission ranges of 3,000–4,000 miles (4,500–6,500 kilometers) were typical. Daytime ranges were much shorter, on the order of 300–1500 miles (500–2,500 kilometers). Terman's Radio Engineering Handbook (1948) shows the maximum distance for 1 kW over salt water to be 1,500 miles, and this distance was routinely covered by ships at sea, where signals from ships and nearby coastal stations would cause congestion, covering up distant and weaker signals. During the silence, a distress signal could more easily be heard at great distances.