Remember when all ships had real radio operators? If you do you'll know that the ops had to maintain "silent periods" when they wouldn't transmit, instead listening for distress calls on 500kHz and 2182kHz. For this they had a special radio room clock with sectors marked on it.
After the loss of the Titanic, the radio frequency of 500 kHz became an international calling and distress frequency for Morse code maritime communication. For most of its history, the international distress frequency was referred to by its equivalent wavelength, 600 meters, or, using the earlier frequency unit name, 500 kilocycles [per second] or 500 kc.
2182kHz was added later and transmissions on 2182 kHz commonly use single-sideband modulation (SSB) (upper sideband only). However, amplitude modulation (AM) was often used in some parts of the world.
Maritime coastal stations used to maintain 24 hour watches on these frequencies, staffed by highly-skilled radio operators.
As a reminder, a ship's radio room clock would have the 500kHz 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, which is the corresponding silence period for 2182 kHz.
Anyone breaking the rules would soon hear "QRT SP" in Morse Code, meaning "STOP SENDING - SILENT PERIOD!"
I've always wanted to have one of these clocks for my own shack, but couldn't find one. So … I decided to make one. I spent two days with a graphics program recreating a radio room clock from the Winthrop Clock Company of Boston, Ma., USA. It wasn't easy, but I then took the finished result, printed it and pulled apart a quartz clock that I bought for the job and installed it - what a lot of work!
The end result is shown in the photograph above and creates quite a lot of attention in my shack. It also keeps time really well too!
There is even The Radio Maritime Day each April where you have to obey the silent periods, so it comes in useful
Anyway, after all that work it seemed a shame to waste the artwork. I then found that I could upload it to a company called CafePress, letting people buy their own clock. While I was there I also designed some T-shirts, mugs, mouse mats and other goodies.
These products, including a reproduction radio room clock, let you relive those bygone, halcyon days when "sparks" ruled the maritime airwaves. So your shack can now look like a ship's radio room!
I have one of the new Cafe Press clocks - it is about 10 inches in diameter and very striking. It is quartz powered so keeps good time and the tick isn't too loud either. Looks very nice on the wall and not too expensive either.
But remember QRT SP!