How to get Weather Fax on a Mac

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 one from as far as New Orleans, but none of the foreign stations I tried came in clearly enough.

The setup which I found worked the best on my 2013 MacBook Air:

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.

AirSpy HF+ Discovery literally fits in the palm of your hand.

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 radio 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.)

Soundflower 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 Soundflower was not straightforward. It takes some doing, but is worth it.

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 Sparklemuffin’s 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).

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.

The very first image received. “Pt. Reyes (NMC) Radiofacsimile Schedule Part 1 of 2.” I had started in black and white mode, then changed to greyscale. This image (and all following except for the New Orleans one) are from Pt. Reyes with SDR tuned to 8,680.1 kHz in USB mode.
First complete image. “Pt. Reyes (NMC) Radiofacsimile Schedule Part 2 of 2.”
An actual weather map! “48 Hr Wave Period Fcst/Direction.” Each weather map takes about 10 minutes to receive.
“72 Hr Wave Period Fcst/Direction.”
This one is interesting because it is unlabeled, appears “inverted,” and is an image taken from a satellite. According to the broadcast schedule this is a “NE Pacific GOES IR Satellite Image.”
“Pacific GOES IR Satellite Image.”
“48Hr Surface Forecast” — for the eastern Pacific tropics, at least.
“48Hr Wind(kt)/Wave(ft) Forecast.” There was a noise disturbance in the middle of reception which caused things to get out of sync and shift horizontally. This could be repaired somewhat using image editing software.
“00-Hour 500 MB Forecast.” Note that a 00-hour forecast is NOT the same as an analysis. This is raw output from the GFS model that has not been augmented by human insight. An analysis is generated by a professional meteorologist and is based on additional information available to the analyst. Human-generated maps have a “FCSTR” label and a name.
“Sea-State Analysis (Meters).”
Mostly unintelligible but recognizable image received from New Orleans on 8,502 kHz USB, almost 2,000 miles away.
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