Fax and Navtex by SSB

[Benbow]

I have just spent ages trying to receive fax and navtex via SSB, I seemed to be getting an encouraging burble from the radio but the computer programme just sat there. In desperation I scanned the web and found a reference to tuning 1.9MHz below the stated frquency. I tried this and instant sucess. What's that all about ?! Should I always go exactly 1.9MHz below the published freq for all freq's ? Probably a stupid question but why publish the frequency 1.9MHz above the one you should tune to !

Or am I just being stuoid.
Thanks



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[MainlySteam]

Hi Benbow

This is something that confuses even people reasonably familiar with radio. The frequency they publish is the carrier frequency (bear with me on that). I tried to write a simple explanation on this but failed completely.

However, the problem exists at the recieving end in that different decoders rely on different received audio tones for their decoding (usually refered to as their offset) - as you can see the effect of offset by offsetting and varying your radio's tuned frequency off to one side of the published frequency (the carrier frequency). For Navtex, for example, you can change the tone you hear by the amount you tune away from the published frequency and the offset you use is that required such that the tones fall within the range required by your decoder.

So, it is not possible to publish the frequency at which you should tune your own radio and even if they did so the frequency would be further complicated as different offsets may well apply to different modes (eg fax, Navtex, Pactor).

Trust that is of some help. If you are at a loss, don't fret, most are. For the knowledgable out there, please don't knock me - if you can give a simpler and still accurate explanation please do so - I understand the shortcuts I have taken in my attempt to explain simply.

John

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[Benbow]

Thanks for that. I tried tuning in CW mode and then switching to USB, that drops the freq but not enough. I obvously need to read-up some theory.

Anyway if I understand you correctly, once I have set the system up for each freq, message type and decoder (I am using a shareware prog called SeaTTY via a sound card) it will stay constant.


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[MainlySteam]

Sorry about the delay responding -I rattled off my previous post just before catching a plane and have now just go back.

Yes, I believe that your understanding is correct.

John

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[Oldhand]

If your receiver can be tuned from the PC via a serial port (such as the ICOM PCR1000 can) why not buy Bonito RadioCom software? This application already has a database of all weatherfax and SYNOP stations and you just click on the desired station and waveband and the receiver is automatically set-up correctly for the required reception. RadioCom also allows you to pre-program the stations and times you wish to receive. See www.bonito.net.


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