-=jd=- wrote:

I have no idea if it's an actual fact or not, but back in the mid 70's I
was third party to a conversation in which it was mentioned that the moon
missions communicated on something like 10 watts. I didn't believe it at
the time - not that I would have known any better... Perhaps someone in
here has the scoop on it?

10 watts is pretty typical for downlink for a space mission. High power
transmitters are difficult in space: even if the power budget allows,
it's hard to get rid of the heat without air to help. We tend to go with
modest power and put in enough antenna gain to make the link work.
NASA's Deep Space Network has some enormous dishes, more than adequate
for a 10 watt voice link from the moon.

One thing that helps is that losses in space are very small: you don't
have ground absorption and ionospheric absorption is slight at the
frequencies we use. I'm sitting here watching a 300 bps link from the
HETE-2 satellite. Half a watt gives us 2000 km range using
non-directional antennas at both ends.

-jpd