I can see right from the start we're headed in the wrong direction, FM is not considered a weak signal mode. Space communications are conducted using extremely high gain steerable antenna arrays consisting of several stacked circularly polarized beams rather than extreme transmitter power levels alone, getting a signal up there is only half the battle, getting a usable one down is the other half.
As for weak signal work (CW, SSB and now a few digital modes) still a whole lot of transmitter power is seldom used, usually <100W and typically 50W into a high gain horizontal Yagi specifically designed for performance on the bottom of the band where weak signal modes are found is the preferred method. Then there are those who for whatever reasons "big gun" operation is precluded so they use at least one omni delta loop, preferably 2 or better yet 4 stacked and do pretty well with them.
Now to the major point in all of this, a high gain low noise band specific preamp preferably mast mounted always helps. After all if you can't hear them you can't work them. Oh and almost an afterthought, preamps suck raw eggs on FM. Experience of myself and others has shown all they do is raise the S meter reading and do absolutely nothing to improve the signal to noise ratio which is crucial. A strong signal swamped by noise is as useless as straining to hear a weak one, again if you can't hear them you can't work them.
As an aside, oh now wouldn't you just LOVE to get your hands on the Arecibo PR dish and its 70cM planetary radar mapping array? Don't look now but a couple of the scientists are hams and every once in a while announce when they'll be playing around with it for the benefit of terrestrial hams. Even with the transmitter throttled back to the legal 1500W with all that antenna gain the ERP is still well into the terawatt range, the loudest EME returns on a ham band for sure. I remember a few years ago they did planet bounce, for months the standing joke was "Hey, I just worked Uranus!"
Now shut up, don't even go there! (;->)