[amsat-bb] Fw: Re: NASA's American Student Moon Orbiter...
domenico.i8cvs at tin.it
Sun Jul 6 01:53:45 PDT 2008
----- Original Message -----
From: G0MRF at aol.com
To: nss at mwt.net
Cc: domenico.i8cvs at tin.it ; kl7uw at acsalaska.net
Sent: Saturday, July 05, 2008 1:21 AM
Subject: Re: [amsat-bb] Re: NASA's American Student Moon Orbiter...
Looking at 70cm up and 2m down, Dom has shown the 2m path can be achieved with a 13dB gain antenna on the ground and 10 Watts on the satellite. That's a very small bandwidth transponder
For the uplink, it's not quite as easy.
I've just run a link budget calculator and it looks like you need about 80 Watts and 20dB of antenna gain on 70cm. That produces around 5000 Watts ERP (allowing for losses)
As Dom pointed out, you dont just need a signal thats just above the noise level at the satellite. Up there you need enough of a signal at the receiver so that the satellites transmitter can generate a decent amount of power on 2m.
The 5kW ERP generates a signal around 16dB above noise at the satellite. With luck, that will be sufficient to produce enough power on the 2m downlink.
Of course there are a lot of variables and 'detail' . but 5kW ERP up and a 13dB gain antenna on the downlink are 'In the ballpark' for a minimal working U/V system.
Hi David, G0MRF
Looking at the specifications of AO40 I found that at 435 MHz the equivalent noise temperature of the RX is
TN (k) = 500 kelvin
The Prx (23) i.e. the power needed by the receiver to develope a signal of 23 dB over the noise is -145 dBW
The 435 MHz satellite antenna gain is 14 dBi
It is not specified the transponder output power with a S/N=23 dB because it depends on what transponder is intended to be used for the downlink.
Probably -145 dBW at the RX input is the power needed to get a S/N = 23 dB at the IF input before to be switched by the matrix to different transponders.
By the way the attenuation earth to moon at 435 MHz for 400.000 km is 197 dB and going back to calculations
we get :
TX power at 435 MHz on earth........................ + 20 dBW (100 W)
435 MHz antenna gain on earth ....................... + 18 dB
EIRP of the ground station................................ + 38 dBW ( 6.3 kW)
Attenuation 435 MHz for 400.000 km .............. -197 dB
Power received on a 435 MHz isotropic........... - 159 dBW
antenna at a distance of 400.000 km
Satellite antenna gain ......................................... + 14 dBi
Received power at RX input for S/N=23 dB.... - 145 dBW
It seems that both calculations match and 5 kW to 6 kW EIRP up on 70 cm and 13 dBi antenna gain on the 2 meters downlink are a minimum necessary provided that the satellite antennas orbiting around the moon are constantly directed toward the earth but this is a very difficult task.
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