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Re: I: receiving system design tradeoffs



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



>From: "i8cvs" <domenico.i8cvs@tin.it>
>To: <aa2tx@attbi.com>,"Frederick M. Spinner" <fspinner@hotmail.com>
>Subject: I: [amsat-bb] receiving system design tradeoffs
>Date: Wed, 12 Feb 2003 11:29:13 +0100
>
>Hi Fred and Tony,
>
>Tony is correct because the S/N ratio limiting factor is the equivalent
>noise temperature of the transponder noise.
>
>As soon you start to receive the transponder noise with ((S+N)/N)= 3 dB
>corresponding to S/N= 0 dB there is no way to improve the S/N ratio
>increasing the gain of the antenna or decreasing the receiver NF
>
>I have included the G3WDG spreadsheet for easy calculations on this matter.
>
>73" de i8CVS Domenico
>
>----- Original Message -----
>From: Anthony Monteiro <aa2tx@attbi.com>
>To: <amsat-bb@AMSAT.Org>
>Sent: Tuesday, February 11, 2003 11:12 PM
>Subject: Re: [amsat-bb] 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
> >
> > ----
> > Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> > Not an AMSAT member? Join now to support the amateur satellite program!
> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>
>
>
>
><< noisev1_2.xls >>


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