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Comment by dreamcompiler

5 months ago

If the goal is only to communicate with people on the other side of the world, HF ionosphere skip can do that with cheap 100-year-old technology (although transistors make it easier).

I assume the goal is to do something cooler than that.

15 comments

dreamcompiler

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kragen  5 months ago

The entire HF band, including the parts already used for something, is only 27 MHz of bandwidth, it's full of noise, and at any given time only a fraction of it can propagate to the other side of the world, dependent on time of day and, literally, sunspots. This antenna has 1100 MHz of bandwidth, the analog front end has 40 MHz for any given conversation, and noise levels are much lower. It could conceivably deliver Shannon bit rates one or more orders of magnitude higher. But it only works when the moon is visible to both sides of the connection.

I discussed these possibilities and some more challenging ones in 02013 in https://dercuano.github.io/notes/ultraslow-radio.html, although I was considering laser moonbounce rather than phased-array microwave moonbounce because of the higher antenna gain available.

  • dreamcompiler  5 months ago

    I didn't see any discussion on your page of the retroreflectors the Apollo astronauts left on the moon. These were put there for distance measuring but they might be useful for laser-based communication too.

    Caveat: Retroreflectors only reflect in the same direction as the incoming beam. But I'd guess that imperfections in their construction together with the roughly 1 degree of arc spanned by 2 stations on opposite sides of the earth might make this idea practical with a better S/N than using only the lunar surface as a reflector. But I don't know. They might be a lot more precise than 1 degree.

lll-o-lll  5 months ago

> I assume the goal is to do something cooler than that.

Yes. Bounce the signal off the moon. The moon.

  • topspin  5 months ago

    Hams bounce signals off of everything. Aircraft scatter, for example. Probably the coolest is using the ion trails of meteorites. When meteors punch through the atmosphere, they create ionized trails. These trails can reflect RF signals. Some of these happen to be optimal at low VFH frequencies, and hams make contacts using frequencies that ordinarily don't work beyond line of sight.