[amsat-bb] Re: Fw: Re: NASA's American Student Moon Orbiter...
John B. Stephensen
kd6ozh at comcast.net
Sun Jul 6 05:24:18 PDT 2008
UV is a problem as hams in much of the U.S. are limited to 1 kW EIRP on 70
cm. The military is the primary user of the band. Recently, some repeaters
near PAVE PAWS sites have had to reduce power to 5 W due to interference
with the return signal from the 4 GW EIRP RADAR.
----- Original Message -----
From: "i8cvs" <domenico.i8cvs at tin.it>
To: "AMSAT-BB" <amsat-bb at amsat.org>; <kl7uw at acsalaska.net>; "Joe"
<nss at mwt.net>; "G0MRF David Bowman" <g0mrf at aol.com>
Sent: Sunday, July 06, 2008 08:53 UTC
Subject: [amsat-bb] Fw: Re: NASA's American Student Moon Orbiter...
> ----- 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...
> Hi Joe.
> 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
> 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
> 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
> 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.
> 73" de
> i8CVS Domenico
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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