[amsat-bb] Re: 'Zombie-sat' and the clever orbital dance

[Bill Jones]

On 26 May 2010 at 23:06, Auke de Jong wrote:

> If Galaxy 15's transponders are all of opposite polarities, then wouldn't 
> the potential interference be cancelled-out by around -60dB due to the 
> polarisation mis-match on both the uplink and downlink paths?
> 
> Auke


I think the transponders on both sats are 36MHz wide, spaced every 40 MHz. AMC-11 
has vertical transponders at 3720, 3760,.....4120,4160  and horizontal from 
3740,3780,...4140,4180.  There is about a 4 MHz gap between each transponder, but 
I am not sure just how sharp the dropoff of the bands are, and how effective this 
4 MHz gap really is.  The Galaxy 15 transponders are at the same places, but the 
vertical are where the AMC11 horizontal are, and visa-versa.  For narrower 
signals which aren't centered at the transponder center, are within the bandwidth 
of both horizontal and vertical transponders. For example the QVC shopping 
channel is an approximately 15 MHz wide vertical signal centered at 3928. It's on 
the 3902-3938 "3920" transponder on AMC-11, but would also almost fit into the 
3922-3958 "3940" transponder on Galaxy-15. 
   Most of AMC-11's transponders however are centered on the nulls between the 
Galaxy-15s transponders. I am not at all clear with respect to what this would do 
to the signal. Sort of seems like the 4 MHz gaps would be like a notch filter, 
and would be somewhat similar to removing the carrier when generating a SSB 
signal. These digital signals look very broad and rectangular in a spectrum 
analyzer.  Most are 20 or 30 MHz wide, and if you remove a 4 MHz wide segment 
from the middle, I assume that it would result in significant distortion, 
regardless of whether you leave the AMC-11 transponders on or off, but I really 
don't understand much at all about how removing part of the envelope of a broad 
digital signal like this affects the reception. Since these are mostly QPSK an 
8PSK signals, it doesn't take much interference from similar symbol rate signals 
to mess up the reception of these things, particularly because the phases will be 
different from G-15. 

 
 


Bill Jones         N3JLQ
Sweden Maine
wejones at eskerridge.com
http://www.megalink.net/~wejones 
 
303rdBG page 
http://www.eskerridge.com/bj/303rdbg/wwii.html