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@eskerridge.com http://www.megalink.net/~wejones
303rdBG page http://www.eskerridge.com/bj/303rdbg/wwii.html