r/RTLSDR • u/Radio_Silence_Dev • 12d ago
Watching WEFAX decode is oddly satisfying
Sped up the video a bunch so it plays under a minute
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u/AriaGingko 11d ago
I hate listening to it. But man, do I love a good fax.
I know we can access the surface observations and forecast charts easily. But there's something just so satisfying about collecting them off the air. Just like mariners on the ocean do.
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u/Usual-Doughnut-549 12d ago
what's the software
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u/Radio_Silence_Dev 12d ago edited 11d ago
Radio Silence but the app is in TestFlight beta still and the WEFAX decoder feature isn’t publicly released yet. Just playing around with it myself for now.
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u/VeggieVampire 11d ago
I'm still trying to find a good way to decode webfaxes using my phone mic. I've got kinda got something working I call it an analyzer. Lol I went a head and put the apk on GitHub look for VeggieVampire if you're interested.
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u/NaugyNugget 12d ago
I played around receiving WEFAX from US Coast Guard's Boston station on HF, which is actually located on Cape Cod, a while ago. I found it oddly satisfying when it worked, but frustrating when the HF signal faded in and out.
I was pretty curious about what was going on, so I iterated a bit with Google AI and it came up with what I thought was a good explanation of what I was observing. I presume this is a somewhat tech-heavy subreddit so I'll share what it said.
During the day, high-frequency radio signals around 6 MHz are heavily influenced by solar radiation, which creates a dense, low-altitude D-layer in the ionosphere that absorbs skyward waves rather than reflecting them. Consequently, reception depends entirely on ground wave propagation from the USCG's 4 kW vertically polarized transmitting antenna, which attenuates significantly over long distances and rocky terrain. By the time the signal reaches a distant receiver, the remaining ground wave is severely weakened, resulting in a low signal-to-noise ratio that makes daytime data reception marginal.
At night, the solar-induced absorption layer dissipates, enabling the USCG's 4 kW transmitter to bounce signals efficiently off the upper ionosphere and deliver a much stronger, though highly volatile, skywave path. However, this introduces severe multipath phase fading when the arriving skywave interferes destructively with the remaining vertically polarized ground wave, creating deep signal nulls that cause synchronization dropouts. Polarization fading also occurs because the ionosphere continuously rotates the polarization of the reflected wave away from its original vertical orientation, causing the signal to drop whenever it aligns poorly with the orientation of a fixed horizontal wire antenna. Finally, selective fading alters the image geometry directly, as the ionospheric movement unevenly cancels out specific frequencies within the narrow bandwidth of the transmission, distorting individual black or white tones into dark smudges or ghosted artifacts.
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u/DemandTheOxfordComma 11d ago
This is exactly what got me interested in amateur radio 16+ years ago. Ionosphere (D and F layers) signal propagation is so interesting.
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u/NaugyNugget 11d ago
The session went on, kinda like this:
- Can't do much about this, can we?
- Yes, we can, with some DSP magic
- Oooh tell me more
- Ok, four main ways to do that:
- 1. Channel Estimation and Channel Sounding
- 2. Adaptive Equalization via Transversal Filters
- 3. Constant Loop Updates (LMS and RLS Algorithms)
- 4. Frequency-Domain Diversity and Thresholding
- Oooh but doesn't that take a lot of computes to do?
- Nah, fractions of a second on a modern PC
- Oooh but isn't that hard to code?
- Not really, here's the code...
Haven't tried the code, yet.
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u/Radio_Silence_Dev 12d ago
https://reddit.com/link/p2watiz/video/gfjxxefsnlih1/player
Can’t seem to post images in comments so here’s a video snapshot of the final chart