[Lowfer] H.F. Navtex frequency?

[Garry Hess]

J.B., thanks for tweaking my curiosity. HF Navtex is a nice diversion 
for daylight hours when not much is happening on LF.

The usual answer to your query is that the specified frequency 
represents the actual transmitted frequency so you should tune your USB 
receiver down a standardized amount. That amount is 1700 Hz for Navtex, 
so you tune 518 kHz International broadcasts with the receiver set to 
516.3 kHz. But if your decoder lets you center on the received signal 
(like, for example, MultiPSK does) it really doesn't matter what the 
center frequency is, just that the frequency shift is 170 Hz for Navtex. 
Similarly, you would tune the maritime safety information on 4209.5 kHz 
by setting the receiver to 4207.8 kHz, USB. A nice listing of stations 
on other frequencies is at 
http://www.dxinfocentre.conm/maritimesafetyinfo.htm, though it's a bit 
dated. Note that only 4209.5 kHz transmits Navtex; the other frequencies 
use NBDP MSI.

Of more interest might be the meteorological facsimile stations listed 
at http://www.dxinfocentre.com/rafax.htm. An easy station to start with 
for US and Canadian listeners is NMF, Boston, MA because it transmits 
simultaneously on several frequencies over most of the day (for a 
detailed schedule see http://www.nws.noaa.gov/om/marine/hfmarsh_links.htm).

The facsimile text and maps can be displayed using MultiPSK, among 
others. Here the upper tuning bar (the "white" standard frequency) 
responds to mouse clicks on the waterfall and nominally should be 2300 
Hz. HF FAX nominally uses 800 Hz shifts too so tune in NMF on say 12.75 
MHz by setting your dial to 12,750,000-(2300-800/2)=12,748,100 Hz. 
Incidentally, I find errors on the order of 40 Hz or so using an SDR-IQ 
that was recently calibrated. I've cancelled them by adjusting the 
tuning slightly but as mentioned above with MultiPSK you really don't 
need to do that - just click on the actual white frequency.

Others with much more experience at doing this should feel free to 
comment, but hopefully the above gives a good start.

73, Garry, K3SIW, EN52ta, Elgin, IL