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.