The smudge

My first encounter with 6m smudge was in March this year. The band was wide open to Japan – a typical early night opening over the equator. Each decoding pass yielding 20, 30, or more callers.

But all of a sudden, within a few minutes, WSJT-X stopped decoding. The signals were still visible on the spectrum, and plenty of them, but they were not decoded.

I was frustrated. My first thought: external sound card problem. It felt like the wrong sampling rate, or maybe a signal overload, or like a reverb was introduced into audio line. But I didn’t want to miss the opening so the antenna was quickly switched from IC-7700 to SUN SDR transceiver.

Different radio, different computer – and still no decoding.

It had become obvious that the signal degradation was external. Looking carefully at the waterfall, it was noticeable that FT8 50Hz wide signals looked smudged and wide. Some of them 70Hz, others almost 100Hz wide! Obviously WSJT can not decode such signals. They reminded me of weak and disturbed FT8 signals on HF which travel over polar paths, but even those polar signals are only marginally wider on the spectrum.

The smudge lasted for a solid 30 minutes. Then slowly, some decodes started to appear. Eventually, things reverted to normal with signals resuming a strict 50 Hz width. I was back in business.

Since then, the TEP smudge manifested itself a few more times. The pattern is always the same: an hour or two after sunset, heavy disturbance which lasts around half an hour, then gone. Interestingly, the Australian Bureau of Meteorology site has a few articles which explain TEP, recognising two distinctive propagations: an afternoon TEP [ aTEP] and evening TEP [eTEP]: “Evening TEP propagation is not as well understood as aTEP but it is believed to take place via a whispering gallery or field-guided mode which relies on the existence of ionospheric bubbles, tubes or plumes that have an electron concentration lower than the surrounding area. Rays are reflected from the surfaces of the bubble walls, at all times staying within the ionosphere until they finally emerge on a path down to the ground.”

What is causing the disturbance? Who knows.

Last night it happened again and I was able to capture the screenshots of my smudged waterfall, as well a spectrum screenshot once decodes resumed. Quite amazing.

Also, for the first time, I continued to transmit ‘through’ the smudge. To my amazement, my signal seemed to be unaffected at the Japanese end with two dozen reporters decoding me just fine. Last night, at least, the disturbance manifested itself as “one way propagation”, affecting only signals entering the TEP from the northern hemisphere.

No doubt, the smudge has been around for ever, but thanks to WSJTx, dozens and dozens of stations transmitting within a narrow bandwidth, and real-time signal reporting networks, we are now able to see how it manifests itself. Radio amateurs in action, observing a phenomenon in real time, as it happens. The very frontier of 6m exploration.

Screenshot 1: The smudged non-decodable 6m signals during eTEP opening to Japan.