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Re: AO-40 spreadsheet



Hi Gary,

If you have a converter with a good noise figure and adequate gain, as well
as an antenna with sufficient gain, you'll do fine.

A 5 dB NF with a 21 dBi linear antenna is not adequate, no matter what a
person thinks they are hearing.

It isn't about cost, model, or anything else.

On one side we worry about "scaring" people with system requirements too
high.

On the other side we have alligators. The trick is finding the best middle
position one can construct.

People can convince themselves of whatever they want to...I've done it many
times myself. The one thing that is indisputable is that until you can hear
at least a dB or 2 of satellite passband noise at apogee, you haven't hit
the ideal. Is this required? Of course not. Is it desrable? Yes. How close
each of us comes to it, will improve the transponder's performance for
EVERYONE, especially the weaker stations.

It isn't "my NF is better than your NF". If anything, it's "my MDS is better
than your MDS", if people must be competitive. The real question is, "Is my
MDS good enough?"

How do we determine what is good enough?

Here are some (silly)  criteria in rank order:

1. I hear at least 1 dB of passband noise at apogee with 0 squint.
2. I hear nearly every person on the bird, even the weakest stations.
3. I hear most everyone, and I sound pretty decent myself.
4. I hear only the strongest signals, and I'm OK if I crank the power up
enough
5. I only can get others to copy me if I crank my uplink through the roof.
6. I hear a few others, but my signal stinks.
7. I trigger Liela when I try to hear myself.

As I've said before, we have been through this situation several times
before. The rules don't change. The poorer you hear, the more selfish you
must be in order to use the bird. It's unavoidable. The goal should be to
hear as well as you possibly can. This provides the maximum benefit to
everyone.

It doesn't matter how we do it...but it needs to be done. It's not about
being scared. It's about the nature of receiving weak signals at this
frequency. We don't help people with a "this will get you by" approach. We
need to understand what it is the best that can be achieved and how far
short of it we are. Then investigate how to get there, or as close as
possible, given our skill, finances, et al.

The nice thing about the tool that Marcus has given us is that we can plan,
evaluate and change. For example:

You think your downconverter and antenna are doing really well. Measure your
actual S/N ratio on the beacon and compare it to what kind of system would
produce that S/N in Marcus's program. This requires nothing more than a
calibrated attenuator...MFJ sells a really nice one for all of 69 bucks.
S-meters are worthless for this purpose unless they have been calibrated
with an attenuator on the band of interest.

You can prove to yourself how well you system is working (forget about
signals on the bird). Measure the SNR you are getting on the Beacon at a
particular point in time, plug those numbers into the spreadsheet and see
how your system is "really" working. It's a neat, and revealing experiment.

With a system NF of .8 dB, the Beacon SNR should be 26 dB
With a system NF of  3 dB, the Beacon SNR should be 21.1 dB
With a system NF of 5 dB, the Beacon SNR should be  18.0 dB

Since the STRONGEST STATION is supposed to be no more than 8 dB BELOW the
beacon:

Strongest Station:

NF: 0.8 = 18 dB SNR
NF:    3 = 13.1 dB SNR
NF:    5 = 10.0 dB SNR

This means at a NF of 5, the strongest station will only be 10 dB above your
noise floor. That leaves only 10 dB of dynamic range for you to hear other
stations. The classic alligator.

We should look for balance in our approach. We are unlikely to achieve the
ultimate, but we shouldn't settle for guessing.

73

hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Gary Mayfield" <gary_mayfield@hotmail.com>
To: <schiers@netins.net>; <fspinner@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Sunday, May 27, 2001 6:55 AM
Subject: Re: [amsat-bb] AO-40 spreadsheet


>
> >
> >The original "illusion" that these 20 or 30 buck converters with a bbq
> >grill
> >antenna system were going to work just fine has been shattered. There
>
> Is this really true?  I've been quite happy with mine, and I've made quite
a
> number of QSOs.  I think it is more a case of "my noise figure is better
> than your noise figure".  I seem to hear everyone who is on.  I'm sure
they
> are closer to the noise than where some other folks hear them, but once
you
> can hear them good enough to work them how much more signal do you need?
If
> you can copy your own signal well, and don't trip LEILA is that
acceptable?
>
> I have a fear that we will scare more people by implying that you need
> either:
>
> A) a really big investment to get on S-Band (The complaint I already hear
> the most)
>
> B) the skill of a neuro-surgeon along with $250 K of test equipment.
>
> 73,
> Joe
> ka0yos@amsat.org
>
> _________________________________________________________________
> Get your FREE download of MSN Explorer at http://explorer.msn.com
>

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