How Do I Decode a Weather Facsimile (WEFAX) Off of my Shortwave?


While I suppose it could be a case of weak Google-fu, my searches for info on how to decode weather facsimiles off of the shortwave turns up a whole bunch of not much. There’s some very helpful frequency guides and a few decoders, but not much else on how to put it all together. With that in mind, here is my effort to make this seemingly daunting process a little easier to wrap your head around.

First off, keep in mind that this process only SEEMS daunting. In reality, this is a nearly 90 year old technology. People were doing this back in the early days of radio with tube powered equipment and lord only knows what for a printer. With your modern receivers and computer technology, you’ve already got a huge head start over what the earlier experimenters had to work with. After this tutorial, you’ll find wefax decoding to be a piece of cake.

Heeere’s What You Need!

The Perseus in mid decode.
The Perseus in mid decode.

A radio. Yes its an obvious requirement, but you will need a radio capable of receiving upper sideband (USB). Since these signals are transmitted on frequencies not allocated to international broadcasting, it would be best if the receiver is capable of listening in on the entire HF band, and not just shortwave broadcasters. A software defined radio will be a BIG help, but its not necessary. Stand alone radio users will need to come up with a way to feed the signal into your computer, but we’ll talk about that later.

A computer. Mac, PC, Linux, Windows… doesn’t matter. There’s decoders out there for pretty much anything you’re running if you look hard enough.

Decoding software. There’s a lot of weather fax decoders out there, but I’d recommend fldigi to start with. It’s available for Apple and PC, and it does an excellent job. If you can’t run fldigi, give Sorcerer a try. It’s a little less intuitive, but it can decode just about everything on HF that isn’t encrypted, and works well. Just to keep things simple though, this tutorial will only focus on fldigi.

Audio cable. If you’re using a stand alone radio like a Sangean, Tecsun, Sony, or Drake, you will need to get an audio cable to feed the signal from your radio to your computer’s sound card.

You won’t need a separate cable if you have an SDR, but you may need an extra piece of software called Virtual Audio Cable. Configuration of VAC is a little beyond the scope of this tutorial, but there are a lot of other VAC resources out there if you have problems.

Guide to Weather Fax Frequencies. You’re REALLY going to want to download this now. This is a comprehensive guide to all known weather fax transmitters around the world, and is very helpful when it comes to finding weather fax frequencies.

Putting It All Together

The first thing you’ll need to do is connect the radio to the computer. The actual connection can be as simple as running a cable from the headphone jack of the radio to the microphone input of the computer, but keep in mind you might need a stereo to mono adapter to make it all work. Your mileage may vary.

Next, if you haven’t already installed your decoder software, go ahead and do that now. Once it’s installed, and assuming that you’re using fldigi, go to the Op Mode section at the top, go down to wefax, and select WEFAX IOC-576. The only difference between the two (that I know of) is that the other standard gives you smaller maps, so stick with IOC-576 for now.

Fldigi software and the three panes: top, middle, and bottom.
Fldigi software and the three panes: top, middle, and bottom.

Fldigi software is broke up into three separate ‘window panes’. The top pane is the view of what you’re receiving at this moment, sort of a sneak preview of what is being decoded. The pane below that is a viewing window where you can see previous faxes you’ve already decoded. The bottom one is a view of the signal as it arrives, and is where you can make fine adjustments to the tuning. We’ll talk more about this in a bit.

Now that the radio is connected, turn it on and see if there’s any trace of a signal on your decoding software. If the bottom pane of fldigi goes from black to yellow and blue, you’re in luck! Your computer is hearing your radio, and you’ve succeeded in getting the signal from one into the other. Go ahead and switch the radio into upper sideband if you haven’t already, and lets try decoding some faxes!

Now, take a look at the guide to wefax frequencies and find a station relatively close to you. Here in North America, I’d recommend New Orleans, Port Reyes, or Boston. All should work though, just choose your frequency based upon the time of day. For our example, we’ll use Port Reyes on 12786.

Its important to keep in mind that a lot of radios have what is called an ‘offset’ in sideband modes. Without getting into the nitty gritty details of radio waves and sidebands work (although that might make a good future blog entry), just remember to aim low. For example, if we are trying to tune into a fax station on 12786, you will want to enter in something like 12784 into your radio and start tuning around. Also remember that you might have caught the station between faxes, which means you might be waiting a while for another transmission. You can either try another transmitter site when that happens, or check out the schedule for the next transmission and wait it out.

Once you do hear a transmission though, which should sound something like this, you should see something like two yellow and red streaks running down the bottom pane of fldigi, and a red box sort of thing. That red box is your fine tuning, and can be moved around with your mouse. Go ahead and line up those two red lines onto the centers of the red and yellow streaks, and wait for the magic to happen. If you can’t move the box around with your mouse, look down in the lower right hand corner of the program for a button labelled AFC. If there’s a green light in that box, click on it to turn it off. You should now be able to move the red box to wherever you need.

An example of what a properly tuned wefax transmission should look like in the tuning pane of fldigi. Note the red box and how its sides are alligned with the red/yellow stripes.
An example of what a properly tuned wefax transmission should look like in the tuning pane of fldigi. Note the red box and how its sides are alligned with the red/yellow stripes.

After a while, you should start to see your results appear in the preview screen. The first one might be off centered, but don’t worry about it. It will synch up on the next transmission. While your first fax is coming down, this would be a good time to tell the program where you’d like them saved. To do this, go to the Configure drop down box and select modems. Navigate to the ‘Wefax’ tab, and select the directory where you’d like your faxes saved. I have a folder on my desktop called, originally enough, Wefax Decodes where all of my faxes get saved.

So you’ve got everything worked out. There’s signal from the radio to the computer, you’ve tuned into a transmission, and you’re waiting with wide eyed anticipation as your first fax materializes in front of you. There’s just one problem… Why is it crooked??

Time to correct the slant
Time to correct the slant

Don’t worry, this can be corrected. Just below the first pane in fldigi you will find a box labelled Slant with an arrow to either side. Use these arrows to straighten out your fax while it is decoding. The solid black line on the side of the fax is a big help with this. Once your lines are straight, you probably won’t have to do this again. In my case, the slant is set to .008 and I haven’t had to adjust it since it was first calibrated.

So there you have it, a semi-brief primer on how to decode weather faxes. Hopefully you’ll find them as enjoyable and addictive as I do.

Hurricane Gonzalo heads towards Bermuda.

14 thoughts on “How Do I Decode a Weather Facsimile (WEFAX) Off of my Shortwave?”

  1. Still find JVcomm32 the best performing and solid wefax decoding software. Have tried many others during the past decade like Fldigi, code32, MultiPSK, ZornsLemma, Bonito Radiocom and a few others.

  2. Still having an issue with decoding WEFAX using my Perseus and an i5 computer with 12 GB ram. The black bar, about every ten minutes and lasting for about 90 seconds will start to curbe and then continue vertical from the location where the curving ends. Please note that this is not a slant issue. I cannot find a cure for this issue. When using my Kenwood TS940 transceiver for the WEFAX signal, this issue does not occur. Anyone have any ideas?

    George, NJ3H

    nj3h at yahoo dot com

  3. I had a simular problem a number of years ago with my perseus. At the end the embedded sound board of the motherboard was the problem because it frequency clock was unstable. I installed a ASUS Xonar D2 soundboard in a PCI slot and never looked back since. Sometimes the VAC can cause havoc to by vertical jumping a few pixels from time to time. By enlarging its buffer can bring relief.

    Good luck,

    1. Hi Ron,

      Thanks for your feedback. I will order an ASUS card when I get resettled. My wife and I just sold out home here in Virginia and will be moving to Oregon.

      The thing that I find interesting is that when using my Kenwood TS940, the problem doesn’t exist. However, I have read that SDRs and sampling rates can cause issues with graphical digital modes.

      Sorry you had an issue, so glad I am not the only one. If you provide me your email address, I can send you a picture of the curved line issue.

      Regards and thanks again,

      George, NJ3H

  4. Hi Ron,

    Checking my Pc, it looks like I will need a PCI Express card.

    The expansion slots I have are:

    Expansion Slots:

    1 PCI Express x16 (Gen2.0)

    3 PCI Express x1 (Gen2.0)

    1 PCI Express Mini Card x1

    So the D2 won’t fit. Any recommendations as to which card to go with now?

    George, NJ3H

  5. There’s nowhere near enough info here to make a cogent answer. What receiver (portable, desktop, sdr) are you using? What software? How are you feeding audio to your PC? Do you hear the grinding sound in your PC’s speakers (if not, you won’t get anything)? Did you go into your Control Panel and make sure that your speaker (or mic input, if that’s what you are using) is turned on?

    Too many questions…Mike

    1. It sounds like you’re using an SDR, so you will need some sort of virtual audio cable to feed the signal into the decoding software. The whole tutorial is built around using fldigi as the decoder, so I would recommend using that. Yes, you will hear the signal in your PC speakers, but without the virtual audio cable you will receive no images.

  6. – don’t spend money for a commerical wefax program… open-source software works very well, even with old rigs such as the SGC SG-2020 (which has notorious drift on startup)

    – you’re not stuck w/MSFT software; there are programs which work on many different operating systems

    – you can find the schedule for worldwide faxes from many places on the internet…

    – for example, frequences for the Carribean are:

    4316 kHz
    8502 kHz
    12788 kHz

    – if you must spend money and have an IOS tablet, it’ll cost you US$4.99 for an app – either place the tablet’s mic near your rig, or better, attach a bluetooth dongle to your rig’s speaker jack and use bluetooth to receive your wx info/maps

  7. Thanks for the tutorial.
    Working great with fldigi on Win10 with audio coming from SDR#, and using an Airspy HF+ with Youloop antenna.
    The only thing I would like to add is that I expected the bandwith to be 1.900 kHz, as you offset tune 1900 kHz below the broadcast frequencies using USB. But I ended up having to use 2.500 kHz to capture all data.
    Also, disableing AFC is a must, as fldigi kept putting the box too low, resulting in a very low contrast image.

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