Re: I: [amsat-bb] receiving system design tradeoffs

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

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).

 


At 09:25 PM 2/13/03 +0000, Frederick M. Spinner wrote:
>That is true, and it didn't really dawn on me that as a system AO-40 didn't 
>have better than this for dynamic range, but thinking about
>it-- you guys are correct as far as power levels go there is really quite a 
>small range of signasl then that are in the "acceptable range".
>And actually that is good, we should all strive to be at a consistant level 
>of -10dB MB.   This really means that the "really weak" station
>argument doesn't hold much weight either.  So if the beacon is +21 dB over 
>the xpndr floor, and 6 dB is it for SSB... then
>the acceptable range is 10-15 dB under the beacon.  5 dB range isn't really 
>very much.
>
>(And I've never ever seen the numbers 10-15 dB as a range before, so maybe
>we accomplished something with this argument finally!)
>
>And yeah it, makes sense in general as radio sigs typically don't ever do 
>better than about 20 dB of dynamic range anyway, thats why
>16QAM modems and the like don't work well over radio systems....Cant squeeze 
>that portion of Shannon's law out of the ether...  :O)
>
>I stand corrected on that assumptions, as I didn't work it (or gave it alot 
>of thought) out before I argued it.  But the rest of the statements prior to 
>that are correct.
>
>I've done all of the spreadsheet calcs a LONG time ago, and have tested 
>several different types of antennas in the past.  I'm just trying to
>back up my experience with numbers as it seems very few hams who haven't 
>tried switching from a BBQ believe the difference.
>
>But since the amount of gain on the ground varies with range, squint, 
>mystery effects, damage to the sat, etc.  I'm still going to
>say that a bigger system is better due to these facts.
>
>And after thinking about it more, really the BBQ case  I've been trying to 
>make applies at Apogee (with good squint) or during less than favorable 
>squints.  Certainly when the sat is at half it's range and good squint, it 
>should be possible to hear the xpdr noise floor just
>fine.  Which of couse, is better than nothing and probably good for portable 
>or emergency situations.
>
>I just wanted to try to educate the group in what the limitations of the 
>BBQ's are.  I have been one to hear about how "bad" AO-40 is at hamfests and 
>the like, and I have to agree it's people who've only tried BBQ's that tend 
>to tell me that.
>
>I'm saying, get a surplus dish, build a feed for it and see the difference.  
>Then you can have a system that can work 100% of the time that the sat is 
>workable.  And it's fun to build the dish and feed up from scratch.
>
>If you are satisfied with a system that is good 75% of the time fine.  But 
>please don't attempt to operate at the other times!
>
>This is giving me a headache, and I wouldn't have even replied to this, but 
>since I was wrong I figured I should.
>
>
>Fred W0FMS
 

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