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R: Orbit 250 apogee



Hi All !

We want to compute  the improvement of the (S+N)/N ratio having two
receivers the first one with an overall
SYSTEM NOISE FIGURE = 1 dB and an other one having an overall
SYSTEM NOISE FIGURE= 5 dB
The antenna is the same and the sky temperature at 2400 MHz= 30 kelvin
The question is:
Is the improvement of (S+N)/N ratio in dB equal to the difference of
 SYSTEM NOISE FIGUREs in dB or MUCH MORE ?  Back to numbers.

SYSTEM NOISE FIGURE= 1 dB  ;  Noise Factor F = 10E0,1= 1,25 ;
T= (1,25-1)x290= 75 K

SYSTEM NOISE FIGURE=5 dB   ; Noise Factor F =  10E0,5 = 3,12  ;
T=(3,12-1)x 290 = 627 K

Noise Power Pn1 at receiver output with System Noise Figure =1 dB and Sky
temperature =30 K

Pn1 = KTB = 1,38 x 10E-23 x ( 75+30)x 1  = 1,449 E -21    [W/Hz]

Noise Power Pn2 at receiver output with System Noise Figure = 5 dB and Sky
temperature =30 K

Pn2 = KTB = 1,38 x 10E-23 x (627 +30) x 1 = 9,066E-21   [W/Hz]

Improvement of (S+N)/N = (9,066E-21)/(1,449 E -21) = 6,25 in power factor

and in dB ;   improvement =  10 log     6,25 =  7,96 dB
                                                             10

This computation shows that  Hasan,N0AN is COMPLETELY CORRECT

In this case the improvement of (S+N)/N is greater than the System Noise
Figure difference in dB because the sky temperature of 30 K is very low and
added to the equivalent noise temperature of each system do not
change significantly the ratio of theyr power noise.

The situation is completely different in 2 meters receiving AO10
because the sky temperature in 2 meters is  around 290 K .Please make your
hown calculation.

VERY IMPORTANT ! ! :

Supposing to have an antenna mounted  preamp with a Noise Figure NF=0,8 dB
and its gain is G= 13 dB
The converter Noise Figure NF=5 dB and its gain G=20 dB
The attenuation of coax cable between preamplifier is 3 dB at 2400 MHz

This situation do not means that my overall NF of the system or
SYSTEM NOISE FIGURE is 0,8 dB but it is  MUCH MORE .
Back to numbars :

Preamp, First stage NF1 = 0,8 dB ;  F1= 1,2
Preamp, First stage gain G1= 13 dB = 20
Cable loss,second stage = 3 dB  ; F2= 2
Cable loss  G2=0,5  in power ratio
Converter,thyrd stage, NF3 = 5 dB ; F3= 3,16

SYSTEM NOISE FIGURE  NFtot calculation

                        F2-1       F3-1
NFtot = F1+ ------ + ----------  =
                        G1       G1 xG2

                2-1        3,16-1
= 1,2 +  -------- + ---------- =  1,47   = 1,66 dB
                20          20x0,5

All of this means that having an execellent preamp with very low NF but
having a cable with high loss and a converter with high NF is vastig time
because our 0,8 dB NF increase at 1,66 dB
In order to reduce this effect and not to ruins the low preamp NF it is
necessary to perform all or partially the following task

1) Increase the preamplifier gain
2) Reduce the cable loss
3) Reduce the converter NF

To be considered that  any increase of the converter gain  affects only the
gain G4 of the 4° stage of the chain and it have a little effect in
improving the overall noise figure of the system.

With all of this in mind it is possible to predict with calculation the
performance of our receiving system
in receiving signals from the AO40  S2 band and also compare different
system on paper without to go up and down from the antenna tower.

73 de i8CVS Domenico


----- Original Message -----
From: hasan schiers <schiers@netins.net>
To: Estes Wayne-W10191 <W10191@motorola.com>; <amsat-bb@AMSAT.Org>
Sent: Thursday, May 17, 2001 1:48 AM
Subject: Re: [amsat-bb] Orbit 250 apogee


> N.B. The comments below are an attempt to address Wayne's perception that
> AO40's downlink signal is "disappointing". I'm not sure if it is or isn't.
> What I wrote below is an attempt to explain why it may seem that the
> downlink signal is not what one would like. Please take it in the light it
> is offered. I'm not trashing the surplus downconverters...I think they
> were/are a great find....but they leave some important work to be done,
and
> if it isn't, it may lead to conclusions about AO40 that are not
> "reasonable".
>
>
>
> Wayne said,
>
> "I'm still disappointed in the downlink signal
> strength of AO40.  For me, the AO40 downlink is less readable than what I
> got from AO13 with much less RX antenna gain."
>
>
> (I bet you had a MUCH better system NF on 2m for AO13, than you have for
> AO40 on Mode S!)
>
> Are we comparing apples and oranges?  From what I've been reading on the
> list, the NF of your downconverter is around 5 dB (or worse, depending on
> mods done or not done) . My "guess" is, if that is accurate, your
threshold
> signal is poorer by at least 7 dB than it should be. A 5 dB NF is just
> totally inadequate for weak signal work. I'm betting you had a much better
> NF on AO-13 mode B.
>
> Again, what I glean from reading the reports here, and the discussions on
> the #amsat chat channel is the following:
>
> 1. System NF of 1 to 1.5 dB, certainly less than 2 dB.
> 2. Rx antenna of at least 20 dBi
> 3. Squint < 10 degrees.
> 4. No buildings or trees to impede line of site
>
> produces VERY good signals, nothing to be disappointed about. The problem
> appears to be largely squint induced. Pointing angles are very poor much
of
> the time, and they expose "compromise" systems. A downconverter with a 5
dB
> or greater noise figure,  without a preamp is just asking for lousy
signals,
> unless everything else is super-optimal, including the squint. There are
> also significant obstruction losses on 2.4 gig that are minimal by
> comparision at 2m (but still there).
>
> If one compares the MDS (minimum discernable signal), of a 5 dB NF system
to
> a 1 dB NF system, I think it will become quite apparent what the problem
is.
> A 5 dB or greater NF is like having a 7 dB attenuator on the antenna.
> Unfortunately, getting a 1 dB NF ain't cheap. There are two choices....big
> bucks for a top of the line downconverter, like the UEK-3000, or fairly
big
> bucks for a low NF preamp, ahead of the downconverter. (Or ...a humongous
> antenna that you have trouble pointing)
>
> I may end up sharing your assessment, but I hope not. I'll be using a low
> system NF setup along with a 26 dBi gain parabolic section dish (HyperLink
> Technology). If that doesn't produce a signal to noise ratio greater than
10
> dB at squint angles less than 5 degrees, then I will be disappointed. The
> antenna I'm describing is 39 inches wide and about 15 inches high. I don't
> consider it "big", but it has substantial gain.
>
> I just don't see any way to fairly compare the ease of AO-13 (and AO-10
for
> that matter), on Mode B with AO-40 Mode S. I'm willing to bet however,
that
> a BBQ linear antenna with an antenna mounted preamp into these surplus
> downconverters will produce impressive signals. Expecting a 5 dB NF
> converter to produce decent signals without a gigantic antenna, is, to my
> mind, unrealistic.
>
> I'm basing this "assessment" on comments on the #amsat chat group, and an
> article I wrote for Amsat Jounal when AO-10 was launched. The article
> examined the effects of antenna gain, receive system noise figure,
feedline
> loss, etc. on receive threshold improvements. It is downright SCARY how
much
> you can improve your ability to hear by paying STRICT attention to the
> overall SYSTEM NOISE FIGURE. This is precisely what the surplus
> downconverters (with no preamp) do NOT accomplish and may explain why some
> people are disappointed.
>
> Even if you are hearing AO-40 at an acceptable level with the 5 dB NF
> downconverters, you have no idea what you're missing. I don't have the
> figures or equations in front of me at the moment, but I would be
astounded
> if your receive threshold improvement was not  at least 7 dB, buy putting
a
> low noise preamp in front of those surplus downconverters.
>
>  (Note: improving the system noise figure will improve your ability to
hear
> weak signals by MORE than the difference in the two noise figures. That is
> why I "guessed" that a 4 dB improvement in NF, will result in a 7 dB
> improvement in receive threshold.)
>
> (Note 2: The 5 dB NF for the downconverters being discussed is NOT the
> system noise figure. The loss in the coax to the radio at 2m must be added
> in, as well as the noise contribution of the front end of the radio. As a
> result, the system noise figure of the downconverter setups could well be
6
> or 7 dB, especially if the gain of the converter is insufficient to
overcome
> the noise contribution of the 2m feedline to the radio).
>
> Compare the predicted S/N ratio of a 1.5 dB system NF to a 7 dB system NF
> and you are talking nearly 10 dB improvement in weak signal reception., as
I
> recall. I wonder how AO-13 would have sounded with a 10 dB attenutator at
> the antenna? That's precisely what a mediocre NF system behaves like for
> Mode S on AO40.
>
> I apologize if my remarks offended anyone, that was not my intention. I
also
> apologize for the length of the post.
>
> 73
> hasan schiers, NØAN
> schiers@netins.net
>
>
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>










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