One of the visitors to this web site sent me a message using the contact form seeking answers to a number of questions regarding British Shipping Company radio communication schedules (skeds) and British collective call signs that were in operation when I was at sea.
My answers to him are incorporated into and elaborated on in this section because it struck me that the arrangements that were in place all those years ago might also be of interest to others wishing to know how Maritime communications between ships belonging to the same shipping or radio company in the 1960’s were conducted. Regrettably any notes that I might have had when I left the sea on the timings, durations and frequencies used for T and Jno Brocklebank skeds were disposed of a long time ago. I must therefore rely on my memory. I apologise if anything I have written below is incomplete or inaccurate in any way. The reason behind the skeds and some of the benefits that were derived from them are described in the GWZM and BBC Overseas Service story on this Web site.
Firstly, skeds took place on both the Medium Wave Band (MF) which occupied the radio frequency spectrum between 410-512 kHz and on the High Frequency (HF) short wave bands of 4/6/8/12/16 and 22 MHz.
The arrangements that are described are specific to those in place for Thos and Jno Brocklebank Ltd which was the shipping company I worked for and applied to their cargo Liners. Even though in many cases Brocklebank ships carried two Radio Officers (R/O’s), the watches they worked related to ships carrying one RO. The arrangements for skeds for ships belonging to other British or Foreign cargo ship owners, oil tankers or passenger ships might have been different.
On single operator ships the R/O kept daily radio watches at sea that consisted of four, two hour watches that were separated by intervals of two hours. International Maritime Regulations mandated when watches started and finished. All radio watches commenced and finished at times related to Greenwich Mean Time (GMT) now known as Universal Time Coordinated (UTC). Local time onboard ship moved as the ship travelled either Westwards (GMT plus local time gained) or Eastwards (GMT minus local time lost).
During each of the four daily watches you would have a single MF and one or possibly 2 HF skeds. I cannot now remember when the second HF sked was held. The HF skeds would be on a different frequency band in each of the four watch periods. One of the HF skeds took place towards the end of the second hour of the 1600 to 1800 hour watch, immediately after the fourth silence period which was at 45-48 minutes past the hour. I cannot remember the timing of the MF sked. Possibly at the same time but in the first hour of the watch.
When I was at sea all of the MF and HF transmitters were crystal controlled ie their final carrier transmission frequency was related to a fundamental quartz crystal oscillator. Non of the transmitters I sailed with had frequency synthesisers (a device that enabled you to dial in the operational frequency).
The ship’s MF and HF transmitters were each fitted with quartz crystals that related harmonically to the final frequencies that the ship was authorised to use. Each ship’s crystals could be used for both calling and working purposes. No specific crystals were designated for sked purposes. Different ships were allocated slightly different frequencies for each of the bands in order to reduce the impact of mutual interference but all frequency assignments ensured that final transmission frequencies fell within a narrow range of frequencies within the frequency band allocated to merchant shipping.
The number of crystals fitted and the harmonic relationship between the fundamental crystal and final transmitted frequency differed between transmitter manufacturers. The same crystals could be used for working on different frequency bands.
For some bands only the harmonic multiplier changed. Careful choice of fundamental oscillator frequencies and frequency multiplier meant that a ship could work the 4/6/8/12/16 and 22 MHz bands with as little as ten crystals. On MF, skeds were conducted on either 410 or 512 kHz. I cannot now remember which.
The Marconi Oceanspan Transmitter which was commonly deployed on British Merchant ships in the 1960’s and which I trained on, was designed to work on the following bands (1)
4,063 - 4,238 kHz
6,200 - 6357 kHz
8,195 - 8,476 kHz
12,330 - 12,714 kHz
16,460 - 16,952 kHz
22,000 - 22,400 kHz
Coast Station broadcast frequencies on HF, for each of the above bands were located in a narrow part of the spectrum at a slightly higher frequency to the ship’s allocation.
Apart from the International distress frequency of 500 kHz on medium wave, where sending and receiving working was single frequency simplex, communications between ships and coast stations on both MF and HF were conducted on the basis of two frequency simplex.
The calling and working frequencies of Coast stations across the World were published for British and Commonwealth ships and those of the Irish Republic in (British) Admiralty publications. These documents in 2023 are rare but occasionally are offered by specialist second hand book sellers. National libraries might also hold some copies.
When initiating a HF sked, the Radio Officer would chose a frequency in the band the sked was to be held on. He or she would then listen to that frequency using the ship’s HF receiver to ensure that it was clear of other traffic. In the case of T and J Brocklebank ships a GWZM call would be initiated along the lines of GWZM GWZM GWZM de GFRG GFRG GFRG k. GWZM was Brocklebank’s collective call sign and GFRG was the callsign of the SS Manaar (my favourite ship).
You would then sweep the narrow frequency band allocated for ships working purposes (in that band) using the ship’s HF receiving equipment to listen to any company ships calling GFRG. The duration of the sked was mandated by the shipping company although if the sked was timed for minutes before the end of the watch it might occasionally overrun! An Internet search for collective call-signs may return a number of hits but the most complete list of British shipping company collective callsigns that I have come across can be found on the Radio Officers Association web site, the link to which is https://www.radioofficers.com/archives/collective-call-signs
Reference (1) Marine Radio Manual. Danielson & Mayoh 1966.
copyright © John Leary