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Re: receiving system design tradeoffs



Hi Fred,

This is not technically correct and here is why.

Let's say you have a really big ugly dish (say +35 dB gain!)
and an excellent low noise pre-amp (say 0.4 dB NF.)

Even though you can easily hear the transponder noise floor
at close to 10dB over the thermal and sky noise, you still
hear the beacon at +21 dB signal to noise ratio because of
the noise from the satellite.

If you transmit your uplink at -20dB compared to the beacon,
no one will be able to copy your 1 dB signal to noise ratio
signal, not even with the biggest BUD.

You should adjust your uplink to be about 10 dB below the
beacon giving about an 11 dB or so signal to noise ratio
so you can be heard. Having a bigger dish doesn't change this.

You can verify this for yourself by playing with the ao40eval.xls
spreadsheet.

OK?

Tony AA2TX
---
At 09:01 PM 2/11/03 +0000, Frederick M. Spinner wrote:

>Sorry I've been really busy selling a house and at work, and not checked 
>my email in days.
>
>I have a HF dipole at about 30' right now!  It's better than I've had with 
>my deed restricted house before.  But I'm putting up a tower eventually.
>
>Understand that the problem with using a marginal system on a satellite is 
>this:
>
>If MB -10db is suggested max power level, than a good assumption is that 
>-20 dB to -10 db (10 dB range) is a reasonable power range.
>
>If you can hear the stations at -10dB, but weakly, yes you can work them.
>The stations at -20 dB you will not hear at all.  Not only
>will you miss them-- you risk stomping right on top of them in QSO.  And 
>they are really not wrong to work at -20 dB under the beacon, and I'd 
>personally not consider that QRP level.
>
>It's like having the HF antenna at 20 feet, but running legal limit 
>through it all the time.  That's the HF analogy.
>
>It gets worse though, because on a satellite you are expected to be able 
>to hear everything in the passband since it is possible to do this.  On HF 
>it is not possible due to propagation.  So on HF, it's normal to get 
>stomped on (but in many cases still not right), but it's not normal
>on satellites.
>
>So this is why an "alligator" is defined as: "not hearing well before 
>talking" and not "running too much power".
>
>If you run the BBQ you better be aware that you could kill ongoing QSO's 
>by people that are running at legimate power levels.
>If you don't tha'ts fine, but how could you know if you can't hear 
>them?  I guess by people complaining on an "acronmonious" amsat-bb 
>thread... :O(
>
>This has been my logic all along, and is my final comment on this subject.
>I've simply tried to get this point across to you and the others in the group.
>
>Good Luck,
>
>Fred W0FMS

----
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