[amsat-bb] Re: NASA's American Student Moon Orbiter...
bruninga at usna.edu
Fri Jul 4 19:09:16 PDT 2008
> I've done moon bounce. And many of these
> numbers seem to be not too far from Moonbounce
> numbers, and that is a horrid dead piece of rock
> reflector. that has a efficiency of a wet sponge.
> ...And it only reflects 6% of the energy it gets.
My guess is ... That 6% is an awful lot of power considering the
3.6 million square miles of surface doing the reflecting.
Conversly, any amateur transmitter at the moon would have a much
smaller receiving/transmitting antenna. Though lots more
So what you gain in changing from a 1/R^4 to a 1/R^2 path loss
you lose a lot of it in the loss of signal receive aperture. Or
something like that maybe.
> but i would think anything there that is active
> circutry is a thousand times more efficient at
> sendinga signal back as compared to the moons
> >>2 meters downlink budged calculation:
> >>Satellite power ................................... + 10 dBW
> >>Satellite antenna gain.......................... + 10 dBi
> >>Satellite EIRP..................................... + 20
> dBW (100 W EIRP)
> >>2 m isotr. attenuation 400.000 km.. -188 dB
> >>power density received on a ground
> >>isotropic 2 meters antenna..................-168 dBW
> >>2 m ground station antenna gain.........+ 13 dBi
> >>Power density at 2 m RX input...........- 155 dBW
> >>2 m receiver noise floor......................- 178 dBW
> >>Received CW signal S/N.................... + 23 dB
> >>If we increase the BW to 2500 Hz for a SSB QSO than the
> >>of the receiving system increases by log (2500/500) = 7
> >>it becames about -171 dB and the SSB signal will be received
> >>S/N ratio = 23-7 = 16 dB wich is a very strong SSB signal.
> >>Be aware that the above figures are based on the assumption
> >>satellite antennas are pointig toward the earth wich is not
> the case with
> >>a moon orbiting satellite.
> >>In addition we assume that the station in QSO with you has a
> >>EIRP capability in order to get 10 watt from the 2m
> >>for you.
> >>On the other side if a fixed 10 dBi 2 meters antenna is
> placed over the
> >>moon and it is oriented toward the earth could easily cover
> the inclination
> >>X libration window without any adjustement and only from
> the point of
> >>view of the downlink with 10 watt it can be easily used for
> a transponder
> >>on the moon.
> >>If you make again the downlink budged calculation
> >>the 2 meter transponder will develope only 2.5 watt for
> you then you
> >>will realize that the transponder will accomodate 3 more
> stations if each
> >>one is getting 2.5 watt as well.
> >>In this case your S/N ratio will be still +15.5 dB on CW and
> >>in SSB and the same is true for the other 3 users.
> >>73" de
> >>i8CVS Domenico
> >Good example of path link analysis, keeping it simple!
> >But the trick is limiting input to four stations with a
> >transponder and they all running an equal uplink. Reality is
> >doesn't happen so the shared portion of downlink power may
> >likely will be less with reduced S/N. My experience with
> >that to have a reasonably good SSB contact you needed at
> least S/N of
> >10-dB. In fact that resulted in a fairly weak signal which
> >difficult to copy. 20-dB S/N made for arm-chair reception.
> >Not discussed were the 70cm uplink requirements. I suppose
> one could
> >run high power to achieve that. My AO-40 experience was
> >to 60w at a 16.5 dBdc antenna (18.6 dBic). Most of the time
> >good with about 5-10w if the satellite was lightly loaded.
> >high numbers of stations trying to operate I needed the full
> >72x60 = 4320w or in dB: 18.6 + 47.8 = 66.4 dBW
> >My AO-40 mode-US station consisted of a FT-847+60w linear at
> >antenna (M2-436CP42UG) for uplink. The 2.4 GHz downlink was
> >33-inch dish with helix feed+MKU-232A2 preamp+Drake
> >(on 123-MHz).
> >I'm not going to go into those calculations.
> >Sent via AMSAT-BB at amsat.org. Opinions expressed are those of
> the author.
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