[amsat-bb] Re: NASA's American Student Moon Orbiter...
kl7uw at acsalaska.net
Fri Jul 4 09:43:01 PDT 2008
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.
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