A NAVTEX signal received from Singapore
Well it's been quite a while since the Lounge has been in action. There are a few reasons for that which include a trip to Melbourne for work in February (before the entire world went into melt-down), some serious downtime for a personal improvement project, of which I will write about soon, and a lack of motivation.
Anyway, enough of excuses. Despite my overall laziness, one thing I manage to maintain as part of my daily routine is to set up a radio receiver and run some software in order to decode NAVTEX signals. NAVTEX are Navigational Telex messages transmitted to ships at sea to warn them of various maritime conditions. Messages might include warnings of typhoons, persons overboard, navy exercises and most recently COVID-19 related messages.
NAVTEX messages are transmitted on MF (medium frequencies) which are just below your AM radio dial, on both 518 kHz and 490 kHz as well as on high frequency on 4209.5 MHz. The signal to the human ear is unintelligible and sounds like a stream of data noises. There's a technical explanation for the transmission mode which I won't bore you with here. Look it up on the web if you're interested. I run a piece of free software called YaND (Yet another NAVTEX Decoder) which decodes the transmissions and allows the user to log the stations received.
NAVTEX stations are run by governments around the world, but not Australia curiously enough, and are generally situated in a strategic coastal area. Stations are divided into a dozen or so zones around the world and each station in each zone transmits for 10 minutes every four hours. Everything is scheduled (at least in theory) so stations do not transmit over the top of one another.
518kHz is the most regular frequency for me here in Australia and all transmissions on this frequency are in English regardless of the country of origin. 490 kHz is reserved for transmissions in the language of origin. There are fewer of these stations in my part of the Southern Hemisphere.
The photo above shows signal received from Singapore. On the screen in the background are the signal and its trace in the waterfall on my software defined receiver. The YaND software decoding the message sits in the foreground.
The fun starts when radio propagation varies and signals from distant zones are received. This is what us radio lovers call DX (distance). The great thing about it is that it is very hard to predict what might happen so all sorts of surprises await the listener. One advantage with NAVTEX is that I can set the radio to run overnight and I can check the signal received the next day at my leisure. No staying up until 3am to hear that rare station.
Where I am in Australia is well situated to receive signals from across Asia and sometimes beyond. When conditions are favourable I have received signals from far away as India, Honolulu, Bahrein and Vladivostock. My best effort is San Francisco at some 12,200 plus kilometres. NAVTEX stations are generally low power and aiming to only reach nearby ships so receiving DX stations is a treat.
Each day I upload my logs on to a friendly iogroup based in Europe which has about 30 or so regular contributors and these are then uploaded to a worldwide database by an editor based in the Netherlands. I have made some nice friendships through the small NAVTEX DX community and everyone is very supportive of one another. Here's a sample of a recent log for you.
Peppermint Lounger
Receiver: Afedri SDR rev.6, SDR Console v.3.17
Antenna: PA0RDT Mini-Whip (6m)
Software: YaND v.7.0
YYYYMMDD HHMM KHz ID CALL STATION,COUNTRY DIST (Km)
----------------------------------------------------------------
20200323 1231 518 $11C 9VG Singapore ,SNG 3368
20200323 1237 518 $11E PKX Jakarta ,INS 2726
20200323 1251 518 $11F HAS Bangkok (Nonthaburi),THA 4426
20200323 1300 518 $11G JNB Naha ,JPN 4300
20200323 1311 518 $11H JNR Moji,JPN 5146
20200323 1327 518 $11I JGC Yokohama ,JPN 5400
20200323 1343 518 $11K JNX Kushiro ,JPN 6313
20200323 1348 518 $11M XSI Sanya,CHN 4136
20200323 1445 518 $11Q XSG Shanghai ,CHN 4941
20200323 1500 518 $11S 9WH21 Sandakan,MLA 2483
20200323 1510 518 $11T 9WW21 Miri R, Sarawak, MLA 2637
20200323 1814 518 $11N XSQ Guangzhou,CHN 4384
20200323 1851 518 $11R XSZ Dalian ,CHN 5785
20200323 1925 518 $11U 9MG Penang ,MLA 3910
It seems all of us will be spending a lot more time at home in the coming months so we will need something to keep us occupied. Exploring the hidden world of signals in the radio spectrum may just be a good way to remain sane.
Until next time - peace and love.
