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Re: AO-40 spreadsheet



Domenico, Hasan, others:

Domenico makes a very good suggestion that you use noise measurements from
the sun to test the performance of your 2.4G receiving system.  Eddy
Jespers reported where you can find an article that shows the method for
making solar G/T measurements.  I recommend that you read that article,
then download the Excel spreadsheet [g-t.xls] that I wrote based on this
article which will enable you to analyze your system noise temperature [or
NF] by making sun noise observations:
http://www.setileague.org/software/spreadsh.htm

When you have done that you have made your first radio astronomy
observation, by the way!  A neat QSL for doing this is available from the
SETI-League for this accomplishment.  See:
http://www.setileague.org/awards/qslcard.htm

Domenico also refers you to Bob Atkin's, KA1GT, article "Noise Temperature,
Antenna Temperature and Sun Noise"  published in The ARRL UHF/MICROWAVE
EXPERIMENTER'S MANUAL.  Bob's articles are good reading on the subject,
however there is an error in Bob's noise figure formula on page 7-57.  It
will give you fits if you are using it to evaluate your system.  This
article was originally published in QST in the 1980's and the error has
escaped correction and been copied often.  The problem lies in the third
coax cable loss expression T(L3-1).  It should be: T*L1*L2/G1*G2*(L3-1).  

I have a noise figure/MDS Excel spreadsheet [cascade.xls] that computes
system noise temp/NF for you:  http://www.setileague.org/software/spreadsh.htm

Finally, I suspect that readers may want to know what the minimum system
{dish gain, NF} will detect sun noise on 2.4G:  

I ran g-t.xls and found that a G/T = 1.0 produced an observed sun noise 1.5
dB over the "cold" sky background.  
An antenna gain of 19 dBi [18 inch dish] and a NF of 0.8 dB will produce a
G/T of 0.9.  
If you have a high NF system you will need a much larger dish to see sun
noise:  
A NF of 3 dB and a 4-foot dish [27.8 dBi] produces a G/T of 1.8 and you see
2.6 dB of sun noise.  
I found that NF> 5dB didn't see much sun noise even using a 4-foot dish!

By using g-t.xls you will see that the combination of antenna gain and
system NF produces good performance.  But certainly we know this from just
reading the mail on the bb.  You now have a "bag of tools" to analyze your
system.

I'm burning daylight as I have a rotor and antennas to put up.
Later,
Ed





>From: "i8cvs" <domenico.i8cvs@tin.it>
>Hello Hasan,
>
>Why dont add the received sun noise as a reference of our receiving system ?
>Measuring  sun/sky and ground/sky noise as described in Bob Atkins,KA1GT
>article "Noise Temperature,Antenna Temperature and Sun Noise" is very easy
>and this article has been published in The ARRL UHF/MICROWAVE
>EXPERIMENTER'S MANUAL
>
>My system sun/sky (S+N)/N is 5 dB at 2400 MHz  and under this conditions i
>get the AO40 transponder noise at apogee with at least  3 dB of (S+N)/N or 0
>dB of S/N but normally much more.
>If i get a sun noise (S+N)/N=3 dB i must be happy because any increase of
>antenna gain or any reduction of the overall receiver noise figure is not
>necessary.
>
>If any of my friend do receive the same sun noise he should be happy to
>because under this conditions the ability of my system on receiving weak
>AO40 signals cannot be improved.
>
>If tomorrow i get less of 5 dB of sun/sky noise i must be unhappy and i need
>to investigate if  the solar flux is decreased or something is wrong with my
>antenna or receiver.
>
>Sun noise test is very simple and is widely used by EME operators to
>establish for reference if they are in condition to receive their own echo
>off the moon or not only by using an AC voltmeter directly connected to the
>receiver audio output.
>
>73 de i8CVS Domenico
>
>
>----- Original Message -----
>From: hasan schiers <schiers@netins.net>
>To: Gary Mayfield <gary_mayfield@hotmail.com>; <fspinner@hotmail.com>;
><amsat-bb@AMSAT.Org>
>Sent: Sunday, May 27, 2001 3:29 PM
>Subject: Re: [amsat-bb] 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. 


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