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My 11-year-old nephew Mikey was smiling at the array of master and slave clocks in my shack.   Mikey did once proudly tell the curator at the Malta National War Museum at Valletta that “Uncle Steve has an RAF sector clock” but this time his interest was piqued by my ship’s radio room slave clock pair which are a Seiko QC-6MS and a Citizen TX5 5S being driven by the Power Station Synchronome master and he was asking why these clocks had different sectors to the RAF clock so as I was telling him why, I realised that others may appreciate the history lesson too.

Before you start with the hopefully non-turgid history lesson, perhaps you may like to peruse the horological loveliness shown HERE or HERE

 Many of you will know that these coloured segments have something to do with maritime distress call monitoring and may believe that such monitoring on the 600M waveband – or 500 KHz first started after the sinking of the Titanic – which is only partially true. The use of this frequency was actually initially standardized during the second Berlin International Radiotelegraphic Convention, which was signed on November 3, 1906 and became effective on July 1, 1908 – some four years earlier than the Titanic disaster.

The second part of this Convention designated 500 KHz as one of the standard frequencies to be employed by shore stations, specifying that "Two wave lengths, one of 300 meters (1 MHz) and the other of 600 meters (500 KHz), are authorized for general public service. Every coastal station opened to such service shall use one or the other of these two wavelengths."

 International standards for the use of 500 KHz were further expanded by the Third International Radiotelegraphic Convention, which was held after the sinking of the RMS Titanic. This Convention produced a signed agreement on July 5, 1912 which became effective on July 1, 1913 thus firmly establishing 500 KHz as the primary frequency for seagoing communication. Communication was generally conducted by CW Morse code, initially using spark-gap transmitters.  Most two-way radio contacts were to be initiated on this frequency, although once established, the participating stations could shift to another frequency to avoid the congestion on 500 KHz.  To facilitate communication between operators speaking different languages, standardized abbreviations were used including a set of "Q codes".

The Service Regulations required that whenever an SOS distress call was heard, all transmissions unrelated to the emergency must immediately cease until the emergency was declared over.  As usage of 500 KHz as a common frequency often led to heavy congestion, especially around major ports and shipping lanes, it was possible that any distress message may be drowned out by the myriad of background radio traffic so the Service Regulations further specified that: "Coastal stations engaged in the transmission of long radiograms shall suspend the transmission at the end of each period of 15 minutes, and remain silent for a period of three minutes before resuming the transmission. Coastal and shipboard stations working under the conditions specified must suspend work at the end of each period of 15 minutes and listen in on 500 KHz during a period of three minutes before resuming the transmission."

During distress working all non-distress traffic was banned from 500 KHz and adjacent coast stations then monitored 512 KHz. as an additional calling frequency for ordinary traffic.

Hence the RED bands on the marine clock dial shown at h+15’ to h+18’ and h+45’ to h+48’ do represent a radio silence listening band listening band on the 600M waveband – or 500 KHz

 This arrangement continued unchanged until the 1947  International Telecommunications Union Radio Conference (USA Atlantic City) where an additional international radiotelephone distress frequency of 2182 KHz was designated and a corresponding set of markings was marked on marine radio room clock dials as a radio silence listening band only this time in GREEN at h+00’ to h+03’ and h+30’ to h+33’.

The use of 2182 KHz has two distinct advantages over 500 KHz, firstly, it is ideally suited for Upper Sideband reception as well as having between three and six times the range of a 500KHz signal with the approximate reliable range on 2182 KHz being between 50 to 150 nautical miles during the day and 150 to 300 nautical miles at night.

Maritime coastal stations used to maintain 24-hour watches on both these frequencies, staffed by highly skilled radio operators.   As stated above, a ship's radio room clock would have the 500 KHz 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 were marked in GREEN, represented the corresponding silence period for 2182 KHz.

 Anyone breaking the rules would soon hear "QRT SP" in Morse Code, meaning "STOP SENDING - SILENT PERIOD!"

By the year 2000, distress transmissions on 500 KHz had been almost completely replaced by the Global Maritime Distress Safety System (GMDSS).  Beginning in the late 1990s, most nations ended monitoring for distress transmissions on 500 KHz with the he nearby frequencies of 518 KHz and 490 KHz being allocated to the Navtex component of GMDSS. Proposals to allocate frequencies at or near 500 KHz to amateur radio use resulted in the creation of the 600-meter amateur radio band.  Similarly, effective as of 1 August 2013, the US Coast Guard terminated its radio guard of the international voice distress, safety and calling frequency 2182 KHz and the international digital selective calling (DSC) distress.

 The remaining global distress frequencies still monitored are:-

 406 MHz / 406.1 MHz - Cospas-Sarsat international satellite-based search and rescue (SAR) distress alert detection and information distribution system.

243 MHz - military aircraft emergency frequency

156.8 MHz - Marine VHF radio channel 16, short range maritime use.

121.5 MHz - civil aircraft emergency frequency


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