I’ve witnessed tropospheric ducting between Hawaii and Southern California on VHF and UHF on several occasions. That’s about 4000 km.
That’s true, but I can pick up enough Chinese to tell that they were talking a lot about Beijing, including weather, so I thought it safe to assume that’s where it came from. Anyway, Beijing is one of the closest places in China from where I live anyway.
Some posters mentionned tropospheric ducting, a term I was not familiar with. I searched around a bit and found a site (in Japanese) that collects occurences of unusual radio transmition. There have been submissions from people who received radio and television signals from China, North and South Korea, Okinawa, as well as eastern Russia. It is somewhat reassuring to know I’m not alone in hearing these distant voices.
The site attributes most of these occurences to tropospheric ducting. According to them, there are even enthusiasts who actively search for these stations, trying to guess when TD is likely to happen.