This is actually old news, it happened in March of 2009, 17 full years ago but I just learned about it recently and was embarrassed that I didn't know about it.
A couple of members of the German AMSAT, a club dedicated to working satellites and more difficult modes, such as Earth-Moon-Earth (EME or Moonbounce) successfully bounced signals off the planet Venus and heard their own signals. For comparison, the moon is many times closer than Venus and if you're listening for your own signals bouncing off the moon, they take around 2.6 seconds (based on one way distance to the moon of 250,000 miles and the speed of light being 186,000 miles) per second or 1.3 seconds each way.
It took the signals to and from Venus "a round trip delay of about 5 minutes." Sounds like Venus bounce isn't very practical for conversation, but I think doing Venus bounce will be like Moonbounce with an automated software system the sends a signal report and another small piece of info, maybe taking a minute to reply, we're talking 2-1/2 minutes there, the message time, and 2-1/2 minutes back, maybe a Venus bounce contact (QSO) could take place in under six minutes.
I'm full of questions about this, and the source article at the American Radio Relay League has almost no technical details. The ARRL reports, "According to an AMSAT-DL press release, the team's transmitter was generating about 6 kW CW on 2.4 GHz," and that's about all.
I've written about moonbounce and trying to hear my own signal reflections from the moon before; several times. So let me play with some numbers. The first hurdle is how far we're trying to send a signal to. At it's closest, Venus is 24 million miles while at its farthest, it's 162 million miles. Both Venus and Earth are in elliptical orbits around the sun and we're both constantly moving around the sun. There are more complications there, but I'll skip them for now and use a number that isn't the closest or farthest. Since it's a nice round number, let's say 100 million miles.
The big concern is the same as every communications link everywhere else: the amount the signal attenuates - weakens - over that 100,000,000 miles. The term for this is path loss, and many times I've used a handy equation to calculate it that gets you within less than half a dB of the more theoretically-backed equation. We haven't talked about a frequency, so let me use that 2.4 GHz (2,400 MHz) the Germans used. It's a ham band here in the USA as well.
Path loss in dB = 37 dB + 20log(f) + 20log(d) where,f is the frequency in MHz and d is the distance in miles.
Path loss in dB = 37 dB + 20log(2400) + 20log(108)
Path loss = 264.6 dB
If you're not used to this world, this may shock you, but the impact in path loss isn't terribly big for the return back to you doubling the distance. If you double the 100 million miles in the last term to 200 million (2*108) the increase is 6 dB. Right, 270.6. There's more uncertainties to come than that 6 dB.
The problem with that number is not knowing how much signal you get back because of a big unknown. How much signal is lost in the reflection sending the signal back to you? It seems a safe bet that your signal isn't going to exactly light up the entire earth-facing side of Venus, it'll be wider than the planet out that far, so there will losses I can't guess at - say your signal's diameter at Venus is 10 times the diameter of the planet, yes, you're losing 9/10 of your signal strength in that reflection. The next problem is how much of what hits the planet reflects? Venus is famously clouded over, how much loss is there? Let me PFA some numbers, and I'll say I lose 20 dB in the combination.
What does it mean to increase the path loss from 271 dB to 291 dB?
The real problem, then, is not knowing how much loss there is in those places. I'll use the 6000 watts that the German guys did. That's +67.8 dBm (decibels above 1 milliwatt) If I had no antenna gain the signal I'd get back would be +67.8 dBm minus 291 dB or -223.2 dBm. I don't know of any way to detect and use a signal that weak, regardless of modulation and receiver enhancements.
What if I had an antenna like the one on the right here, with LOTS of gain?
I don't know exactly how big this is or how much gain it would give at 2.4 GHz, but it's a site in New Jersey where a secret military radar project first bounced radar off the moon. Not with that antenna, it's just a nice looking model for a ham radio project. My guess is the AMSAT Germany guys probably had something like that, if not bigger.
There's a lot of what I consider reasonable guesses here, and (as always) some "hand-wavium" but it doesn't sound easy - more like on border of impossible. Good antennas for 2.4 GHz aren't hard to get and will do nothing but help. I'm not sure about the amplifier to get 6 kW, but I think they're out there.
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