[amsat-bb] Re: was TS-2000X, now AO-40 lids

[Scott Townley <nx7u@xxxxxxx>]

Fred's right, and then some.  Note well:
The uplink is all about EIRP (not TX power or # elements or anything else).  It is power delivered to antenna times (circular) gain.  
The downlink is all about G/T (gain to total system noise temperature).  Nothing else.  With a 100' dish I could use a 2N2222-based downconverter and hear the xpdr noise floor :-)
Of course squint counts too, in an absolute sense.

One thing I might add to Fred's comments is, given that G/T is the determining factor, that even a super-hot d/c may not turn the trick on a smallish dish.  If the dish pattern is such that the antenna picks up a lot of ground noise etc., you can build a 0.01dB NF d/c and it won't do any good.  And clean-enough patterns to minimize ground noise are not trivial.

> 
> From: "Frederick M. Spinner" <fspinner@hotmail.com>
> Date: 2003/02/05 Wed AM 11:51:39 EST
> To: amsat-bb@AMSAT.Org
> Subject: Re: [amsat-bb] Re: Re: was TS-2000X, now AO-40 lids
> 
> 
> Egads.. what a political debate.  Let me state this a little more 
> practically...
> 
> All I can say is I've tried both, and yes I use crappy "surplus" 
> downconverters.  I also drive a
> 1988 vehicle because at present I can't afford or need better.  And it works 
> 75% as well as a new $35,000 one.
> 
> I'll bet you my <$100 cheap downconverter/surplus dish combo side by side 
> with your $300 UEK and high priced Hyperlink BBQ
> I'd do better.  Would it be bigger?  Yes.  In my case I'm on 36 acres, I can 
> care less.
> 
> I don't think anyone is saying your system is a problem.  But there are many 
> people out there trying to use a Drake with a BBQ.  It works only well 
> enough to hear the beacon.
> 
> But I have to strongly disagree with the following comment:
> 
> >I can't tell you the number of people I work using surplus D/C's that can't
> >hear worth a darn when connected to moderate receiving antennas. Connect
> >them to a 3 meter TVRO dish and they "seem to work really well", but it is
> >an illusion. They still don't work worth a darn, but the massive antenna in
> >front of them lets the "system" work well.
> 
> ITS NOT AN ILLUSION.  AS A SYSTEM, THEY HEAR VERY WELL.  This is an 
> acceptable set of trade offs.  I am getting a
> bit tired of you UEK-3000 people saying that spending $300+ dollars on a D/C 
> is the only way to get good signals on AO-40.
> (And I've heard plenty of that, and you are certainly implying that here, 
> and its BS).
> 
> As you yourself said, put the numbers into the spreadsheet.  If the final 
> S/N ratio is good it really doesn't matter how you get there.
> Yeah, your beamwidth is smaller with the larger dish.  Unless it's too 
> small, who cares?  Gain, noise pickup, efficiency all are better.
> These are tradeoffs.
> 
> My point is that if you have a BBQ and can't hear well (and it is obvious if 
> this is the case) try getting a *FREE* Primestar or other VSAT, MUZAK or 
> similar dish-- they are all over the place.   Or a larger prime focus.  If 
> it's a offset, use a 5.25 turn helix, if it is a prime focus, a
> patch.  G3RUH patches are expensive.  W0LMD designs are almost as good and 
> are free with a weekend of effort.  The difference between my BBQ and even 
> my 2 foot prime focus w/patch is about an S-unit.  An S-unit is a lot on 
> AO-40.  For me that was the difference between
> struggling to hear the bird and having enjoyable copy.
> 
> Could I make that up by getting myself a sub 1dB converter?  Yes.  It is 
> worth $300 to me to do this?  No.
> 
> The logic of spending $300 for a D/C and then going minimal on the antenna 
> is like spending $300 on a stereo system and then hooking 3" speakers to it! 
>   It could work, but is this an optimal setup?
> 
> If you don't have an AO-40 setup already, skip the $%#$ BBQ's altogether.  
> Or don't listen to the (radio engineers in the) group and waste $75+ on 
> something that is marginal.
> 
> The BBQ's would have worked GREAT if $hit didn't happen with AO-40.  It did. 
>   What is you system going to be like if the transponder loses another 3-6 
> dB over the years?  This is a common occurance with commercial sats....
> 
> >
> >I really can't emphasize strongly enough sitting down with Gene's
> >spreadsheet and "playing" with all the variables, especially the "front 
> >end"
> >ones like antenna gain, circularity, coax losses, D/C NF and Gain, etc.
> >
> 
> >(Much snipped and attribution removed to protect the guilty.)
> >
> 
> I agree.  Try more dish gain versus NF improvement.  Things move the same 
> way...
> 
> I'm probably one of the leaders in the "try and use what you have" camp in 
> ham radio.   But even in the AO-13 days, hearing the transponder noise floor 
> was a goal of a good system.
> 
> It frankly scares me that so many people think its actually good to have a 
> system with no overhead margin.  And for that matter
> how do you know you are not hearing the noise floor by only 0.5 dB if you've 
> never heard it?  A spreadsheet and models are very good
> and necessary engineering tools, but are only approximations of reality.  It 
> ain't gospel until its actually measured.
> 
> Fred W0FMS
> 
> 
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> 

Scott Townley NX7U
Gilbert, AZ  DM43di

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