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.

The SDR I’ve Been Waiting For?

A somewhat crappy photo of the Elad FDm-S2 sitting on my desk at work. There's a  lot of radio inside that tiny box.
A somewhat crappy photo of the Elad FDm-S2 sitting on my desk at work. There’s a lot of radio inside that tiny box.

When I bought my 1st SDR back in May of 2012, the Bonito 1102s, I really expected it to be a revolution in how I listened to the radio. It wasn’t. Don’t get me wrong, the Bonito is a great little radio, but it didn’t really do it for me. It’s limited spectrum bandwidth and somewhat overly complicated software severely limited it in my mind.*

About a year later, I decided to pick up a Perseus while I was at Dayton. I had a lot pepole tell me this was the best receiver in the world, and that I would find it to be vastly superior to my existing boat anchors. Well, I didn’t. Again, while I think the Perseus is a fine radio, it just didn’t grab me. The software was kind of buggy, mine never seemed to be the super hot performer I was expecting. The whole thing left me wondering if maybe I wasn’t an SDR guy afterall?  Maybe I really am one of those luddites who believe real radios not only glow in the dark, but they have knobs and tuning dials as well.

It is too early to tell whether or not I am still that guy or not,but I can tell you that the Elad FDM-S2 has made me rethink my attitude towards the SDR. I had one of these delivered on Friday, and I can honestly say this is the revolutionary device I was waiting for back in 2012. It seems to hear about as well as anything else in the shack, maybe even better. More importantly, it allows me to ‘see’ more station on the waterfall than I ever could before, which means I’m hearing more. It’s not an exaggeration to say that having this radio in the shack has given me an opportunity to become re-acquainted with the HF spectum in a way I haven’t been since I got my first digital readout receiver. It may not exactly be bandscanning in the traditional sense of the word, but it is very similar. In other words, the FDM-S2 has changed the way I listen to the radio.

A screenshot of the Elad software at work.
A screenshot of the Elad software at work.

One of these days, probably in a month or so, I hope to be able to sit down with all three of these SDRs and give you a thorough head to head comparison of the three. Until then though, I will continue to put the new Elad through its paces. There’s still a lot of learning curve to climb with this radio, and I’m having a lot of fun wading through everything it can do.

*Bonito has recently performed a major update to their software, which in my opinion has added a lot of functionality and eliminated some of the complications. All in all, a very worthy upgrade.

Just the WeFax, Ma’am…

It’s kind of interesting how entries for this blog will sometimes just jump out at you out of nowhere. Weather faxes are a great example of this.

Weather fax, also known as wefax, is one of those interesting technologies that’s been around forever on the shortwave bands. Back before satellites and the internet, this could’ve been the only way for a ship at sea to get any kind of weather forecast maps. It’s also something that’s never worked for me.

I have been trying to decode weather faxes on and off now since 1993. It’s not like I’ve tried for 20 years straight either, but every time I have given it a whirl I’ve failed. Chalk it up to crappy software, a poor interface, misconfigured computers, or just plain old operator error (my bet is on the latter), but I’ve never been able to pull it off.

Now, armed with a Perseus SDR and a program called SeaTTY, I decided to give it another whirl. Much to my amazement, I ended up with the following on my screen:


Holy crap, it’s working!

I think the key to getting it to work this time is a combination of the SeaTTY software and the Perseus SDR. The SeaTTY software is nice in that it has a built in automatic frequency control (AFC), which makes tuning the signal a relatively simple procedure. Just get the station in the pass band, switch the program over to WEFAX mode, and off you go. The Perseus also takes some of the guess work out of the process by letting you see the signal as well as hear it. Being an SDR, it’s also a lot easier to get the signal from the radio into the computer. With the Perseus, it’s just a matter of configuring the virtual audio cable software once and you’re good to go for just about every digital mode, whether it be PSK31, slow scan TV (SSTV), or weather faxes.

Obviously the image quality isn’t all that great. All in all, it looks just like what it is: a fax. You’ll also see that there’s some work that needs to be done on some of these images after you’ve grabbed them. Sometimes the ‘skew’ can be off, and the image will need to be adjusted. You can see a litle of that in the image above, but this one wasn’t too bad. Others have required some TLC in Photoshop, but it didn’t take too long to put them back in to a coherent image. here’s a couple that went through some time in Photoshop:

That blob of clouds off the Northern coast of South America is a baby picture of tropical storm Chantal, which could be affecting the US sometime later this week.


All of these weather faxes originate from here in the US, but there are other applications out there. Supposedly there is a Japanese station that’s broadcasting newspapers in both English and Japan. I haven’t found that one yet, but I’ll keep you posted. Until then, here’s a list of weather fax stations around the world, courtesy of NOAA, in pdf form.