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Re: AO-40 spreadsheet



Greg, please take this in the spirit it is offered. It's not personal, but
technical.

I think you may be misapplying, if not misinterpreting, the spreadsheet.

1. Antenna: yes, very important, but there are only so many things you can
do. 21 and 24 dBi BBQ style parabolic section dishes are pretty common.

2. MOST CRITICAL: Noise Figure of the Preamp (with very low loss in the coax
between the antenna and the preamp). Gain of preamp IS important. Without
the gain of the preamp, you don't set the noise figure out at the preamp
properly.  10 or 12 dB gain , just isn't good enough. The noise figure
following the preamp, which is a composite of coax loss and the noise figure
of the radio,  is divided by the GAIN OF THE preceding  PREAMP...if that
value isn't high enough, you won't "overcome" the poor noise figure of the
radio and its preceding coax out to the preamp/converter.

Experiment:

Set the NF of the preamp and converter at 2.5 dB or 5 dB...the values that
we might be seeing from the surplus converters. Set the gain for only 12 dB.
Run out your calculations. Now...change the gain to 30 dB, leave the NF set
at 2.5 dB. Compare the two values of MDS.

Next: Do the same experiment with antenna gains from 18 dBi to 26 dBi linear
polarization and a preamp with a NF of 1 dB. Set gain to 12 dB...run it out.
Now set it to 30 dB...run it out....

==============================================================

The noise figure of the first preamp is VERY important, of course...but so
is its GAIN. So please, don't tell people to ignore the gain...anything less
than 20 dB is not going to be anywhere near as good as it could be. (if just
a downconverter).

In practical terms, the antenna gain is going to be somewhere between 18 and
26 dBi. That will NOT make up for a poor NF front end.

Please see my comments interspersed in your text below.

73
hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Greg Dolkas" <greg@core.rose.hp.com>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, May 25, 2001 3:11 PM
Subject: [amsat-bb] AO-40 spreadsheet


> Wow, this spreadsheet is truly enlightening.  There's nothing better
> than a good model to quickly teach how things work.
>
> I notice that the ability to hear the transponder is VERY dependent on
> the antenna gain, and relatively independent on the gain of the preamp.


It is most dependent on the NOISE FIGURE of the first RF stage, assuming any
kind of decent antenna at all. Unless your antenna is absurdly large, both
the NF and GAIN of the 1st RF stage is REALLY IMPORTANT.



> For example, there appears to be no benefit to having a preamp gain of
> more than 10-15 db.  Same for the Converter.  Don't waste your money on
> super high gain components, just get a really good antenna.

Very bad advice...it's NOISE FIGURE and GAIN of the 1st RF STAGE that is
SUPER CRITICAL!

Look at the numbers for realistic antennas and you will see that the NF and
GAIN of the preamp are VERY, VERY important. You are minimizing their impact
and I think you are in error. Again, what is a practical gain figure for the
Mode S antenna? Very few are running as much as 26 dBi (a 39 inch dish).
Most are using helices, and various BBQ dishes with gains from 18 to 22 dBi.
Combine that with a mediorcre front-end in the preamp/downconverter and you
have nothing but disappointment in your future.

>
> So, my question:  Given the high dependence on antenna gain, besides
> renting Arecibo for the weekend, what will give the most gain for the
> buck?  I'm starting with an 18"x36" grill antenna with a dipole+hat feed.
>
>   Increasing the size of the grill to about 30" x 36"
>   Replacing the feed with a 2 1/2 turn helix
>   covering the grill with a metal screen
>   eliminating the 3-odd feet of RG-8 coax between feed and preamp
>     (i.e. mount the preamp at the feed)
>
> I think the answer to this is "none of the above".  If I've got this
> right, none of these really increases the gain of the antenna itself,
> just makes it pick up more of the signal and noise around it.  The
> big gain will come by getting an antenna that has a sharper pattern,
> which will pick up the same signal and less background noise.  Sure,
> improving the reflector and feed helps, but putting the same feed on
> a wider grill mostly increases S+N, not just S.

You don't have it right at all. Until you understand the relationship
between preamp noise figure/gain to MDS, you will think it's all about the
antenna...and in practical terms, that's just not true. Set the antenna gain
at a "practical" value (21 dBi) and begin playing with the NF and GAIN of
the preamp/downconverter and you will see that you "missed the boat" a bit.

>
> Basically, getting a good signal from a mediocre antenna is hard.
> Making a really good antenna, of course is hard too.  Where's the
> sweet spot?

Here's the sweet, and readily attainable spot, Greg:


Antenna > 21 dBi Linear....BBQ antennas with this performance are not
absurdly expensive. My HyperLink 39" is spec'd at 26 dBi and I paid full
retail for it of 165 bucks. (I got it for wireless internet)

Coax Loss From Antenna to Preamp:  no more than .5 db, hopefully just couple
tenths of a dB.

Preamp/Downconverter: NF < 1 dB, Gain 30 dB if a downconverter.15 dB if a
preamp  feeding a downconverter. Preamp cost: 150 bucks. NF: .6 dB, Gain >
20 dB

In other words, if you are using just a downconverter, the NF should be less
than 1 dB and it's gain should be 25 to 30 dB.

If you are using a preamp before the downconverter, then the preamp should
have a NF less than 1 dB and a gain of at least 15 dB. The downconverter
will have a NF of 3 to 9 dB if it's a surplus unit, and a gain of maybe 15
more dB.

======================================================================

Bottom Line: (there is no free lunch)

System Noise Figure : < 1 dB  (the result of low preamp NF AND at least 20
dB gain)
Antenna: > 20 dBi if possible.

Do this, and you will be VERY HAPPY.

Now...go back and play...the spreadsheet summarizes years of well understood
relationships among antenna, coax loss, preamp NF and GAIN and path loss.
When you understand how to play with those variables properly, you will no
longer pooh pooh both the gain and the NF of the first stage. They are NOT
PRACTICALLY overcome by any reasonable antenna  you could buy or are likely
to build.

With so many people disappointed in their surplus downconverter performance,
because of silly claims made before any objective analysis was done, I'm
quite worried that assertions like yours about the gain of the first preamp
being unimportant, will cause a similar series of disappointments that could
have been easily avoided.

I's sorry for being so blunt, but your advice is "unsound".

73...

hasan


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