Yesterday I received my first weather fax! I might be a nerd (ok, replace “might be” with “am”), but it was pretty exciting.
The clearest ones came from the Pt. Reyes station, which is closest. I did get ones from as far as New Orleans and Honolulu, but none of the foreign stations I tried came in clearly enough.
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The setup which I found worked the best on my 2013 MacBook Air:
- AirSpy HF+ Discovery – SDR (software-defined radio) hardware device, $169 as of writing
- Gqrx SDR software – free software
- Black Cat HF Weather Fax – $20 as of writing
Soundflower – free audio routing softwareUpdate 19 June 2021: I’m no longer recommending the use of Soundflower. See the extended update at the bottom of the page for details.
- Loopback – virtual audio cable, $99
- Amphetamine – keeps the computer from sleeping which would interrupt reception
- SMA male to SO-239 female cable
- Backstay HF antenna
Disclosure: I have no associations or affiliations with the above products. The links do not earn me anything. These are just what I personally tried and found to work well.
The AirSpy HF+ Discovery is an amazing little device that lets you receive radio signals on your computer. You can think of it as taking the radio signal, moving it down in frequency, then sampling it with an analog-to-digital converter. This enables digital decoding of all kinds of radio signals on the computer.
Using the AirSpy and my computer, I’m able to tune into and hear all kinds of radio signals — marine VHF channels, AM and FM radio, NOAA’s weather stations, WWV time, even distant foreign shortwave broadcasts. (I can see that there’s television broadcast signals, too, though I haven’t tried to decode them yet.)
It’s pretty incredible and the possibilities are fun to think about. Here’s a video where someone uses an AirSpy to decode and display the PAL video signal from their 8-bit Nintendo on their Mac. And then there is the mind-blowing How to Pull Images from Satellites in Orbit.
There are less expensive SDR options available, though I opted for the AirSpy HF+ Discovery because it was designed specifically for receiving HF radio without additional hardware (like an upconverter) and reviews indicated it was excellent for its purpose. If you end up playing with a different SDR for weather fax, I’d love to hear how it goes!
While exciting, this is all relatively new, and in the software arena at least, it still feels very much “wild west.” There isn’t a single solution that just “does it all” and getting something working and viable involves much fiddling, experimentation, and tolerance of “quirks.” It is not for the faint of heart. But getting it going is satisfying and rewarding. I plan to use weather fax reception to augment weather data received via satellite while under way at sea.
(In this post I consider and discuss only Mac OS X. Other software options will apply to Windows and Linux.)
Gqrx is a free and open source software program that, basically, takes the raw radio signal from the SDR and demodulates it into audio that you can hear. By now I’ve tried a few different software options, and found Gqrx to be by far the simplest and easiest to use. Other options were overwhelming in their complexity and requirements for in-depth knowledge of HF radio.
Once the weather fax signal is in human audible form, you’ll need a way to decode the aural warbles into a readable weather fax. That’s where Black Cat HF Weather Fax comes in. It “listens” to a weather fax broadcast and translates it into the fax image. In fact, even without an SDR, you could use a standard shortwave radio to tune in to a broadcast, and Black Cat HF Weather Fax can decode it by listening through the computer’s microphone. (You’ll get better quality by using an SDR though.)
Loopback is the “missing link” between Gqrx and Black Cat HF Weather Fax. It allows you to route audio losslessly between software programs. This way, the audio flows perfectly from Gqrx to Black Cat HF Weather Fax. It can be done by having Gqrx go to the speaker and Black Cat HF Weather Fax pick it up on the mic, but the quality won’t be as good. Installation of Loopback is straightforward and simple. At $99 it’s not cheap, but it’s worth it.
If the computer sleeps it’ll interrupt reception, so I use Amphetamine to prevent sleeping while HF Weather Fax is in use. To set up Amphetamine
- Go to Preferences > Triggers
- Add a new trigger and call it “Weather Fax”
- Set Application to “Black Cat HF Weather Fax”
- Uncheck – ❌ Allow display to sleep
- Check – ✅ Allow system to sleep when display is closed
- Uncheck – ❌ Allow screen saver to run after…
There’s a lot of jargon and technical speak in HF radio and I found it challenging — even as a technically capable, but new to this arena individual — to find answers to very basic questions. I found this YouTube video series helpful in this regard.
I’m far from expert, but my experience was that 90% of the success is in the antenna. I have a telescopic whip antenna for the AirSpy, but it failed to receive ANY meaningful shortwave signal either at home or on the boat at the marina. (VHF was fine.) Unfortunately there is a lot of RF noise in this frequency range these days. A shortwave reel antenna helped when at home, but even then the WWV time signal could barely be made out, and there was no hope for radiofax.
Connecting the SDR to a backstay antenna made a night-and-day difference. I found that a roughly 15-20 dB difference between the noise floor and the peaks of the radiofax tones resulted in very clear images.
A schedule of worldwide weather fax transmission can be found at Worldwide Marine Radiofacsimile Broadcast Schedules (NOAA).
Does receiving weather fax, synoptic charts, and satellite imagery while at sea appeal to you? The Weathermuffin app enables you to do exactly this—without the highly technical setup—and with additional benefits like clearer pictures, weather charts and imagery overlaid on a global map, and your current position indicated, too.
What’s been your experience with receiving weather fax at sea? With what antenna(s) have you had the best reception? Do you have any questions? Leave a comment! I’d love to hear from you.
Samples of faxes received appear below.
Update 19 June 2021: No Longer Recommending Soundflower
After a recent outing I discovered two issues with receiving weather fax.
1. Poor horizontal sync. Sometimes the horizontal sync was lost and the rest of the image becomes shifted. This is not too onerous if it only happens once in an image; but when it happens multiple times it becomes too tedious to try to repair the image.
2. Loss of audio input. Occasionally, HF Weather Fax would stop receiving audio input. When this happened, the image was filled with grey (note the flat FFT display in the screen shot), and I needed to quit and re-open HF Weather Fax for it to have audio input again. I can’t baby sit weather fax reception, so unfortunately when this happens I usually lose several hours worth of images before I notice.
After discussing these issues with Chris at Black Cat Systems, maker of the HF Weather Fax software, he suggested I try a different virtual audio cable. Early results with Rogue Amoeba’s Loopback are promising. In an overnight trial, issue #2 did not occur at all. Issue #1 still occurred, but significantly less frequently.