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Re: My preamp coming out...



The only relavent measurment is the ratio between signal and noise. If this
ratio increases when the preamp is added it is accomplising something. The
goal is to increase the singal to noise ratio with minimal gain. Gain only
increases the susceptability of the receiver to overload.

The antenna receives noise from the earth (290 K), sky (variable), the
coaxial cable ahead of the preamp and the preamp itself. Antenna sidelobes
pick up thermal noise from the ground and nearby structures so there is
usually another 20-30 K unless an oversized reflector is used. The sum
should be a low as possible, but once you get to 112% of the sky
temperature, further decreases will add only 0.5 dB to the receiver
sensitivity.

Band    Avg. Sky Temp.
10 m    14,000
 2 m       550
70 cm        64
13 cm         5
 5 cm        3
 3 cm        3

The following is the approximate noise temperature for various noise figures
(NF). The noise figure used should include the loss of any coaxial cable in
front of the preamplifier.

NF    Temp.
3.0    290
2.0    170
1.5    120
1.0     75
0.8     59
0.6     43
0.5     35
0.4     28
0.3     21

The noise temperature of the receiver and coax following the preamp are also
relavent but their contribution is reduced by the gain of the preamp. T = T1
+ T2/G, where T, T1 and T2 are noise temperatures in Kelvins and G is the
dimensionless preamplifier gain. Note that the gain must be converted from
decibels to absolute power gain (1 dB = 1.26, 2 dB = 1.58, 3 dB = 2, etc.).

73,

John
KD6OZH

----- Original Message -----
From: "Luc Leblanc VE2DWE" <lucleblanc6@videotron.ca>
To: <amsat-bb@amsat.org>
Sent: Wednesday, March 22, 2006 22:46 UTC
Subject: Re: [amsat-bb] My preamp coming out...


> On 22 Mar 2006 at 15:56, hasan schiers wrote:
>
> > Luc, (if I understand your post correctly)
> >
> > 1. An S-meter change is not the appropriate measure of the effectiveness
of
> > a preamp.
> >
> > 2. The fact that your S-meter went full scale both at the antenna and at
the
> > radio means nothing. Absolutely nothing (other than it has gain).
> >
> > 3. The preamp was designed to have a low noise figure (let's say < 1
dB).
> > When you mount it at the antenna you get the benefit of that low noise
> > figure. When you mount it at the radio, you add the loss of the coax to
the
> > noise figure of the preamp. Typically, you might see 2.5 dB loss in your
> > feedline at 70 cm. for normal lengths of coax (80'). Now this nice fancy
low
> > noise preamp has a noise figure of 3.5 dB (which stinks), due to the
> > additional loss of 2.5 dB from the coax in front of it.
> >
> > Unless you have VERY low loss in your feedline run, the preamp belongs
at
> > the antenna.
> >
> > For a proper analysis of this issue, it would benefit everyone to look
at
> > Gene's satellite link budget analysis spreadsheet, which evaluates the
> > effects of antenna gain, sky noise, preamp noise figure and gain,
feedline
> > losses, receive bandwidth and front end noise figure of the radio. Just
a
> > few minutes manipulating the values in the spreadsheet and seeing their
> > effect on receive sensitivity will demonstrate convincingly WHY a preamp
> > belongs at the feedpoint, and WHY preamp gain is of only minimal
importance
> > compared to noise figure.
> >
> > Noise figure (and overall system noise temperature) is the ONE variable
that
> > absolutely must be paid attention to, when it comes to weak signal work.
> > Assessments based on S-meter readings are worthless and worse, often
> > misleading the casual satellite operator.
> >
> > If a person has any interest at all in learning about how your satellite
> > receive system works, and how to optimize it, Gene's spreadsheet
(Excel), is
> > an indispensible and informative tool.
> >
> > 73,
> >
> > ....hasan, N0AN
----
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