[amsat-bb] Re: NASA's American Student Moon Orbiter...
nss at mwt.net
Fri Jul 4 13:38:13 PDT 2008
The whole part that is confusing me on all this power budget stuff is
the to me, the seemingly HIGH budget.
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. I assume it reflects
radio waves not too far efficiently than it reflects sunlight. And it
only reflects 6% of the energy it gets.
I haven't found any numbers of the moos efficiency at reflecting radio
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
or what am I missing?
Edward Cole wrote:
>At 05:42 AM 7/4/2008, i8cvs wrote:
>>Only considering the 2 meters downlink suppose to put AO40 at 400.000
>>km with the antennas pointing at the earth with low squint angle let say
>>less than 10 degrees.
>>The gain of the AO40 2 meters antennas was 10 dBi and we put your
>>10 watt on it.
>>Suppose that your 2 meter antenna has a gain of 13 dBi and the overall
>>noise figure of your receiving system is NF= 0,7 dB = 51 kelvin so that
>>the noise floor into a CW passband of 500 Hz with the antenna looking
>>at the moon (200 kelvin) is about -178 dBW
>>Suppose that the station in QSO with you has a 70 cm EIRP capability to
>>get the full 2 meters 10 watt from the transponder only for you and we
>>can calculate it later on.
>>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 noise floor
>>of the receiving system increases by log (2500/500) = 7 dB i.e.
>>it becames about -171 dB and the SSB signal will be received with a
>>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 that the
>>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 70 cm
>>EIRP capability in order to get 10 watt from the 2m transponder only
>>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 considering that
>>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 +8.5 dB
>>in SSB and the same is true for the other 3 users.
>Good example of path link analysis, keeping it simple!
>But the trick is limiting input to four stations with a linear
>transponder and they all running an equal uplink. Reality is this
>doesn't happen so the shared portion of downlink power may and most
>likely will be less with reduced S/N. My experience with AO-40 was
>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 was
>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 running up
>to 60w at a 16.5 dBdc antenna (18.6 dBic). Most of the time I was
>good with about 5-10w if the satellite was lightly loaded. But with
>high numbers of stations trying to operate I needed the full EIRP =
>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 the
>antenna (M2-436CP42UG) for uplink. The 2.4 GHz downlink was a
>33-inch dish with helix feed+MKU-232A2 preamp+Drake converter+FT-847
>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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