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Re: I: receiving system design tradeoffs



Yes, it's a tired thread but I think we are all getting to some points of
agreement here.

What's emerging as a common theme is that the actual power level RECOMMENDED
by the control ops is the power one should use.  If you consider that the
dynamic range of the satellite is basically from the beacon level to the
xponder noise floor, it appears that level is roughly 20 to 23 dB or so from
what I see you guys saying.  So the control operators are recommending that
stations run right in the middle of that dynamic range window.  Gee, what
they say makes sense.

This way, you have your 6 to 10 dB of SNR for SSB coms and you are far
enough below the beacon that you aren't hogging the satellite power.

This was my whole point in that it is silly to run oneself 18 dB below the
beacon.  You limit your own dynamic range and you make it difficult for
others to hear you.  Some people I think do this out of a misplaced feeling
of running the least amount of power they can to hear themselves.  They
think that is following the FCC rules.  But the rules state, "Minimum needed
to carry on communication."  Communication is between two stations and you
can't assume the other guy can hear as good as you by any stretch of the
imagination (even if you both have BUDs, one guy still may not copy as well
due to obstructions, etc).  So if every station were 10 to 12 dB below the
beacon then no one would be stepping on anyone else.

Finally, if AO-40's dynamic range were greater than what I've learned it is,
we could have stations running 20 to 25 dB under the beacon.  But perhaps
the designers limited its range for the very fact that it helps to equalize
between those who have smaller and those who have bigger antennas.

I'll agree completely with Fred that the bigger dishes give more margin.
Squints at apogee have been quite optimal lately.  When the move ALON and
ALAT, those of us with the smaller dishes probably won't be able to hear
very well and will have less usable time on the bird.  The bigger dishes
help too for going through trees, at low elevation angles, etc.  So, Fred,
your advice is well taken.

73,

Jon
NA9D

on 2/13/03 5:53 PM, Edward R. Cole at al7eb@ptialaska.net wrote:

> Hi Fred, Domenico, Tony, any otheres that still care about this tired topic:
> 
> Well repeating my own observations:
> Typical orbit maybe 55,000km and squint 15 or so:
> 
> Assuming the average rig s-meter is 4 dB per s-unit (big reach, I admit),
> 
> I hear the S2 Middle Beacon fairly often at S-5.5
> and the satellite nose floor at S-0.5 (using an 85cm offset feed dish*).
> 
> ...that is a 20 dB range (probably as accurate as one should state for
> using a s-meter).
> 
> On that day a couple weeks back I heard two very loud stations at S-4 (that
> would put them about -6 dB the beacon).
> 
> I heard my own signal (weakly) at S-1 (-18 dB the beacon and only +2 dB
> over noise).  
> 
> So it would seem that running two to three s-units below the becon would be
> correct ( -8 to -12 dB the beacon).  This would correspond to signals 12 to
> 8 dB over noise...should be quite easy to hear and understand if you have a
> receving system that will hear down to the noise floor of AO-40.
> 
> Ed - AL7EB
> since I have only tried an 18-inch DSS dish, a 4-foot commercial dish, and
> a 33-inch sat-TV dish*...I have no idea how that compares to a BBQ dish.
> PS:  been raining for a straight week and my helix feed is not covered so
> not hearing well at present due to rain noise (wet feed).

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."


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