Srednji val ponuja kar nekaj zanimivih SWL DX možnosti. Ena izmed njih so signali NAVTEX. To so navigacijska, vremenska in opozorilna sporočila, namenjena predvsem pomorskemu prometu. "Glavna" mednarodna frekvenca oddaje je 518kHz (sporočila so tukaj skoraj izključno v angleškem jeziku). Na 490kHz pa veliko število postaj oddaja sporočila predvsem v lokalnih jezikih. Poleh teh dveh frekvenc uporabljajo Japonci 424kHz, nekaj postaj pa lahko slišimo tudi na KV in sicer 4209,5kHz.

Teleprinterski NAVTEX signal je v osnovi SITOR-B oziroma AMTOR-FEC. Hitrost prenosa je 100 baudov, frekvenčni zamik pa 170Hz. Sprejem na ladijskih terminalih običajno poteka povsem samodejno. Za "amaterski" sprejem torej potrebujemo radijsko postajo, ki zmore poslušati RTTY (SSB) na 518 in 490 kHz, ustrezno anteno ter računalnik in dekodirni program. (Frisnit NAVTEX, SeaTTY, MixW, itd.). Oddajne moči NAVTEX postaj naj bi običajno bile okoli 1kW (mednarodna priporočila pravijo, da naj bo oddajna moč med 100W in 1kW, oziroma največ kilovat podnevi in 300W ponoči). Realna oddana moč je verjetno v veliki meri odvisna tudi od izvedbe in učinkovitosti oddajne antene.

Za prvi poskus sem tako priključil postajo kar na inverted-V za 3,5MHz ter nastavil frekvenco na približno 518 kHz. Kot dekodirni program sem uporabil Frisnit NAVTEX dekoder, kateri je brezplačno dosegljiv na spletu. Čakal sem močan signal postaje Trieste Radio – IQX, računal sem, da kaj dosti več s tako "primitivno" in neprilagojeno anteno ne bom slišal. Poleg tega imam na tej frekvenci tudi lepo zaznavne motnje bližnjega oddajnika radia Koper na 549kHz. No, na moje presenečenje sem takoj dekodiral naslednje sporočilo belgijske postaje:

MW offers some interesting SWL options. One of them are the NAVTEX signals. Those consist of navigational, weather and warning messages, primarily intended for maritime use. The "main" international frequency is 518kHz (traffic here is almost always in english language). Another frequency is 490kHz, where we can hear mainly messages in local languages. The Japanese are using 424kHz, and some traffic can be heard on SW also at 4029,5kHz.

NAVTEX signals are basically RTTY SITOR-B (AMTOR-FEC) at 100 baud, with a frequency shift of 170Hz. Reception on terminals on ships is usually completely automated.
In order to receive NAVTEX signals we need a receiver capable of listening RTTY (SSB) on 518 and 490 kHz, some kind of antenna and a PC and decoding software (Frisnit NAVTEX, SeaTTY, MixW, etc.). NAVTEX transmitting power should usually be around 1kW (international reccomandations says between 100W and 1kW, respectively max 1kW during daytime and 300W at night. The real ERP is probably affected by the actual implementation of the transmiting antenna system also.

For the first try I set my FT100d on app. 518kHz and connected it to the 80m band inverted vee. I used Frisnit NAVTEX as decoding software. I was waiting for the strong nearby signal of Trieste Radio – IQX - with such a "primitive" antenna i was not really expecting to receive anything more distant. Apart from that, I also have clearly noticeable splatter of the nearby 549kHz MW broadcasting station. To my surprise, I immediately managed to decode the following message from the Belgian station:

NAVTEX sporočilo se vedno začne z nizom "ZCZC" ter konča z nizom „NNNN“. Za "ZCZC" je niz dveh črk in dveh številk (v gornjem primeru "TA65"). Prva črka nam nekako poda "identifikacijo" radijske postaje oziroma bolje rečeno tej postaji dodeljeno "časovno okno" oddaje. Druga črka nam pove tip sporočila, sledi pa zaporedna številka sporočila.
Svet je razdeljen v NAVAREA (Navigational Area) cone. V vsaki taki coni je praviloma prva črka gornjega niza dodeljena določenemu NAVTEX radijskemu oddajniku. Ta ista črka hkrati določa desetminiutno "časovno okno" oddaje (perioda se ponovi po štirih urah, postaja "T" iz zgornjega primera bo tako lahko oddajala ob 0310, 0710, 1110, 1510, 1910 in 2310 uri GMT). To pa pomeni, da v nekem trenutku lahko sočasno deluje več oddajnikov iz različnih con. Če preverimo v seznamu oddaj, lahko za časovno okno ob 2310 (ko sem prejšnji primer sprejel) vidimo, da so takrat oddajale postaje Oostende (Belgija), Cagliari (Italija), Iqualit (Kanada) in Miri (Malezija). Črka "T" torej ne določa nedvoumno oddajnika – katerega smo sprejeli, je pač odvisno od trenutnih pogojev (propagacije, oddaljenosti, itd.). Dokončno identifikacijo radijske postaje je tako praviloma potrebno poiskati v vsebini sporočila samega.

NAVTEX messages start with the "ZCZC" and ends with a "NNNN" pattern. "ZCZC" is followed by a set of two letters and numbers ("TA65" in the example above). We can roughly identify the transmitting station from the first letter, as it determines the transmitting "time window" assigned to a station . The second letter determines the message type. The numbers that follow are the message sequence number.
The world is divided in NAVAREAs (Navigational Areas). In each area a letter is assigned to a NAVTEX transmitting station. The same letter determines the ten-minute transmitting time window for that station (the perion repeats every four hours). The station "T" from the example above may transmit at 0310, 0710, 1110, 1510, 1910 and 2310 GMT). This means multiple stations from different NAVAREAs can be on-air at the same time (same time window). By checking the transmitting schedules for the time window at 2310 (when I received the example above) we can see that the following NAVTEX stations were possibly transmitting: Oostende (Belgium), Cagliari (Italy), Iqualit (Canada) in Miri (Malaysia). We see the letter "T" does not unambiguously determine the transmitting station. Usually we have to search for the station identification in the message itself.