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

Edward R Cole kl7uw at acsalaska.net
Thu May 27 08:37:59 PDT 2010

At 03:44 AM 5/27/2010, Bill Jones wrote:
>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
>303rdBG page
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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What I recall from  installing c-band dishes in the mid-1980's is 
that adjacent satellites were parked in orbit spaced approx. 4 
degrees apart.  They then polarized even numbered channels 90-degrees 
from odd numbered channels.  The early satellite receivers had to be 
programmed for each satellite and polarity adjusted for best even and 
odd channel.  If you had polarity off, the receiver could pick up a 
transponder (channel) from an adjacent satellite.  It would not be 
clear but it would show a blurry picture and even sound.  Usually, 
once you adjusted the polarity and were pointed dead on a satellite 
the correct channel for that satellite would be received.  In some 
cases satellites had inactive channels and there would be bleed-over 
from an adjacent channel in the same satellite.  Most of these 
satellites had individual transponder amplifiers that could be 
individually adjusted in power by ground station commands.  If you 
had a smaller home dish that sometimes meant not all channels would 
be received clearly.

However, the polarity angle of one satellite would not be the same as 
another so even if they had opposite polarity channel schemes they 
might only be 45 or 60-degrees vs 90-degrees.  At 45-degrees the 
signal is only 3-dB down vs about 20-dB at 90-degrees.

Here in Alaska, the first satellite dishes had to be 14 to 16 foot 
for good reception.  Initially, only Galaxy-V "Aurora" was available 
with two channels: the RATNET (Rural Alaska TV Net) and an 
educational net.  Most rural schools put up 16-foot dishes to receive 
this (my 16-foot eme dish is a surplus one of these).  By mid 1980's 
commercial home dishes became widely available and new satellites 
were launched with enough signal in AK for 10-12 foot dishes.  In 
1985 HBO was first to scramble (encoded) their programs requiring a 
subscription and descrambler box.  In the mid-1990's Ku-band 
satellite service (Direct-TV, Dish-Net) became available but not for 
HI or AK. But enterprising AK dealers offered 6-foot dishes to 
receive the weak signal at the edge of the footprint.  Today, a 
30-inch dish is standard in AK though many opt for 1m (39-in) or 1.2m 
(4-ft) dishes to minimize rain fade.

The 4-MHz gaps in the channels were guard bands and not notched to my 
knowledge.  Often narrow-band subcarriers carried commercial data not 
viewed by home customers.

A standard tool in a satellite installer's kit back in those days was 
a chainsaw, since trees did not pass c-band microwaves.  I figure 
that I installed somewhere about 50-70 dishes from 1985-1991.  The 
Exxon-Valdez Oil Spill in 1989 gave me 13 months of work and I did 
not go back to TV dishes full-time after that.  I just retired after 
15 years as a comm tech for an AK Oil Spill Recovery Org.

73, Ed - KL7UW, WD2XSH/45
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com

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